{"paper_id":"1bf1a68c-d46e-491a-a69d-e80edbccd1f9","body_text":"Nile floods reveal Ancient Egypt's pattern of revolts | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Social Sciences - Article Nile floods reveal Ancient Egypt's pattern of revolts James Stagge, Irenee Felix Munyejuru, Joseph Morgan, Francis Ludlow, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6968703/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The Nile River’s annual flood sustained Egyptian agriculture for millennia, overflowing its banks and enabling gravity-fed irrigation in a desert climate 1,2 . Consequently, poor flood years could produce severe agricultural failures, which are hypothesized to have promoted civil unrest during the famous Ptolemaic era (305–30 BCE) 3–5 . Here we show, using a novel two-dimensional hydraulic model of the Egyptian Nile, that even moderate flow reductions produced large declines in feasibly irrigated area, but with marked regional variation. These impacts were more frequent in the Thebaid region, where minor flow reductions would exclude much of the agricultural land from irrigation. By contrast, irrigation in Middle Egypt would remain stable under moderate flow reductions but faced catastrophic losses during rare, extreme droughts. These findings align with historical records of societal unrest and help explain the Thebaid’s repeated role in originating revolt movements. Further, our findings contextualize the human impact of the many major volcanic eruptions during the Ptolemaic period, which would have produced multi-year catastrophic agricultural failures across the entire region 6 . This work demonstrates how external climate shocks likely cascaded through societies dependent on floodplain agriculture and highlights the vulnerability of historical administrative systems to environmental stress. Scientific community and society/Social sciences/History Earth and environmental sciences/Hydrology Scientific community and society/Water resources Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Main text The Nile is the world’s longest river, originating upstream of Lake Victoria in east-central Africa and draining into the Mediterranean Sea. Its total catchment area is approximately 3 million km², spanning a wide variety of topographies and vegetation covers along its length 7 . Despite the diversity in climate and land cover in the upstream section, the final section in modern Egypt is relatively homogeneous, with no major tributaries and limited direct precipitation. Consequently, flood behavior in Egypt is almost entirely driven by incoming streamflow, allowing the Egyptian segment of the Nile to be modeled as a discrete system, with flow below the First Cataract at Aswan treated as the upstream boundary condition. The Egyptian Nile has a predictable and dramatic flood season, occurring from late summer into early fall (July-October), supplied primarily by monsoon precipitation in the Ethiopian highlands 7,8 . Rainfall in Ethiopia drives a large seasonal flood pulse that travels down the Blue Nile (Fig. 1b), contributing approximately 70% of the typical flood peak measured at the Egyptian border 7,9 . The White Nile originates further south in the mountains of Rwanda, Burundi, and the Ugandan plateau, traveling a much longer distance through multiple lakes and the Sudd wetland before joining the Blue Nile at Khartoum (Fig. 1b). Similar to the Blue Nile, most flow in the White Nile derives from rainfall in upstream headwaters. However, unlike the Blue Nile headwaters, the White Nile experiences two wet seasons per year associated with the passage of the Intertropical Convergence Zone and is further attenuated by routing through numerous lakes and the Sudd wetland, upstream of the Sobat River (Fig. 1b). Nearly half of the White Nile’s annual volume is lost through the Sudd (Arabic for “barrier” or “obstruction”) via evapotranspiration and infiltration, leaving a nearly continuous baseflow that represents approximately 10% of the seasonal flood peak downstream 8–10 . The remaining 20% of the flood peak, after accounting for the Blue and White Nile branches, comes from the Atbara River, the last major Nile tributary (Fig. 1b), with its summer flood peak also driven by monsoon rainfall in the Ethiopian highlands. The predictable cycle of late summer and early fall floodwaters sustained Egyptian agriculture and society for millennia 1,7 . Until the late 1800s, the Nile flowed in a near natural state, unimpeded by modern dams with large storage. Egyptians practiced a form of irrigation (flood recession agriculture), using drainage canals to direct floodwaters into adjacent fields by gravity. Water would then pool and seep into the soil as floodwaters receded, allowing the planting of wheat and other crops in winter for an April or May harvest, before the next flood season 1,11 . Because irrigation was gravity-fed, and thus based on the maximum height of the Nile flood, a direct relationship existed between Nile depth at its seasonal peak and the maximum area feasibly irrigated by lateral canals. Without large reservoirs providing interannual storage, flood season flow variations dictated the extent of arable land along the main Nile channel. The Nile entering Egypt flowed naturally until 1902, when the Aswan Low Dam was completed. This was followed by several enlargements of the Low Dam over the ensuing decades, additional reservoirs built on the Blue and White Nile, and the construction of a larger Aswan High Dam 12 . The Aswan dams, in particular, controlled the annual flood to provide interannual storage, reducing irrigation failure risk during multi-year droughts. Diversion dams (termed ‘barrages’) were constructed along the Egyptian Nile between 1843 and 1930, primarily to control lateral flow into irrigation canals, rather than serving as major storage 7 . For our study, we assume the Nile entering Egypt to be near-natural at Aswan prior to 1902 and upstream of Aswan until the mid-20 th century. With harvest expectations in pre-modern Egypt heavily dependent upon the extent of irrigated land, itself determined by peak Nile summer flow 1,2,13 , effective floodwater management was crucial not only for food security but also state revenues, with taxes often paid in grain. Studies of historical Nile flood disruptions have focused on the drivers of precipitation in the Ethiopian highlands during the summer monsoon season. Interannual variation in monsoon precipitation is related to the position of the Intertropical Convergence Zone (ITCZ), which in turn is related to large-scale climate teleconnections like the Indian Ocean Dipole and the El Niño-Southern Oscillation (ENSO) 14–17 . Explosive volcanic eruptions that inject sulfate aerosols into the stratosphere can also suppress Ethiopian monsoon rainfall for up to 2 years by limiting the northward migration of the ITCZ, particularly when aerosols are concentrated in the Northern Hemisphere extra-tropics, also potentially causing global cooling of 1.0-1.5° C that can further suppress precipitation 3 . An extreme example of the volcanic impact on Ethiopian precipitation occurred during the Ptolemaic era (305–30 BCE), when a ‘volcanic quartet’ of four major eruptions occurred during the 11-year period between 168 and 158 BCE. Modelling suggests a protracted period of cooler temperatures, reduced precipitation in the Ethiopian highlands, and reduced Nile River flow in the aftermath 3,4,6 . The period suffered marked political instability, with impacts registering in correspondence between government officials discussing agricultural shortfalls and food security concerns 6 . The great eruption of Alaska’s Okmok volcano in 43 BCE can now also be convincingly posited as triggering a known period of major socioeconomic stress and poor Nile flooding in Egypt 5 . Beyond these cases, ice-cores reveal the Ptolemaic era to have experienced multiple substantial explosive eruptions 18 , but an important gap remains 19 when inferring causality between modelled post-volcanic reductions in Ethiopian precipitation 3 , contemporaneous Egyptian records of agricultural shortfalls, and the cascade of societal responses, including civil unrest. The relationship between flow and inundated area is highly nonlinear, given the Nile’s wide and irregular floodplain, making it challenging to estimate reductions to irrigated land area from rainfall reductions in the distant headwaters. Moreover, sensitivity to flow disruptions varies along the Nile’s length due to topography within the floodplain. Our objective is therefore to simulate flood behavior and maximum inundated area for a range of natural flow rates, representing the range of possible flood years from exceptionally dry to wet. Using these flood simulations, we then quantify the risk of disruptions to Egyptian agriculture, organized by the administrative regions of Ptolemaic society. To simulate flood inundation during the Ptolemaic period, we reconstructed the Egyptian Nile and its floodplain using a combination of modern LiDAR-derived elevation data and bathymetric measurements of the Nile itself. These were validated against survey measurements from the late 1800s and early 1900s to ensure the model captured historical conditions. We then simulated the predictable seasonal flow patterns of the natural Nile using a two-dimensional unsteady flow hydraulic model. To our knowledge, the large spatial extent, fine spatial resolution, and focus on pre-modern conditions of this two-dimensional (2D) hydraulic model is unprecedented 20,21 and offers a detailed spatially explicit estimate of how irrigation capacity was affected by flood variability prior to the construction of modern reservoirs, providing insight into the geography of civil unrest in the Ptolemaic era. Results 2.1. Flood model and validation The Nile flood model, extending from modern Aswan to Cairo (Fig. 1a), was developed as a 2D unsteady flow hydraulic model using HEC-RAS v6.5 (Hydrologic Engineering Center-River Analysis System) 22 . A 2D unsteady flow simulation was necessary to accurately represent Nile flood pulse behavior as it overtops its channel and inundates the wide, flat floodplain, with flow traveling in multiple directions. Floodplain elevations were derived from the remotely sensed and bias-corrected MeritDEM dataset, which has a spatial resolution of approximately 90 meters 23 . Because remote sensing cannot effectively penetrate water surfaces, river channel bed elevations were based on existing one-dimensional flood models representing modern conditions 24 . Channel bathymetry was assumed to be triangular, connecting the modern centerline elevation 24 to the water’s edge elevations obtained from remote sensing 23 . This triangular channel assumption is reasonable both as a simplified geometric model of pre-measurement conditions and based on channel surveys from 1904 that show a broadly triangular channel 25 . Modern centerline elevations were validated against the 1904 survey to confirm the stability of channel bathymetry through time. We chose to limit the model to the area upstream of the Nile delta due to the meandering, erosion, and sedimentation of the delta 26,27 . By contrast, our region of focus, upstream of Cairo, has remained relatively stable over the modern period 28,29 , with low sedimentation rates and largely fixed channels over the last 3,000 years 30,31 , though some locations show migration 32 . Our model also included the Bahr Yussef channel, an expanded natural channel which was active during the Ptolemaic period and transported water from the west bank of the Nile near Asyut to the Fayum 33,34 (Fig 1). Model simulations used over 1.14 million computational cells with a spatial resolution of approximately 50x50m within the Nile river channel, transitioning to a coarser 250 meter square resolution in the floodplain. Post-processing produced finer resolution of the flood inundated area through spatial interpolation. This level of spatial detail is appropriate for modeling such an extensive floodplain and given the uncertainties inherent in reconstructing historical channel geometry. Typical flows were based on monthly mean flow records measured at Aswan and Dongola, upstream of the reservoir created by the Aswan dam, from 1869 to 1958 35 . The record at Aswan was adjusted to approximate the near-natural flow record by re-inserting upstream irrigation withdrawals 35 . A composite 90-year record of annual maxima was then constructed as the annual maxima of the two sites, generally shifting from Aswan to the upstream Dongola in the early 1900s following the Aswan low dam construction. We chose to end the record in 1958, prior to completion of the Roseires Dam on the Blue Nile, which coincided with a significant decrease in annual maximum flow. Annual maximum flows between 1869 and 1958 were stable, showed no significant trends, and followed a normal distribution with a mean of 9,030 m 3 /s and standard deviation of 1,550 m 3 /s (Fig. E1). We therefore chose to simulate years with annual peak flows between 5,000 to 13,000 m 3 /s, corresponding to approximately the 0.5% to 99.5% percentiles, or return periods of once per 200 years, on average. Historical analysis showed consistent monthly flow percentiles across varying peak flows, allowing us to assume the same seasonal proportions for all simulated years, scaled to each respective peak. Simulations were run with a three-hour timestep beginning with a one-year warmup, followed by the target simulation year (Fig. E2). Proxies show the climate of north Africa was significantly wetter during the so-called African Humid Period, between approximately 12,000 and 4,000 BCE 36,37 . Our results are therefore not representative for this period. However, the Nile watershed has remained relatively consistent with regard to precipitation over the last 3-4 thousand years 37–40 . Climate fluctuations during the last two millennia have been smaller than those in prior millennia, with slightly warmer and drier conditions during the Medieval Climate Anomaly (950-1250 CE) and slightly wetter than average conditions during the Little Ice Age, especially 1325-1470 CE 37,39–42 . By not considering the late 20 th century, this model avoids more extreme temperature increases due to climate change 43 . Our simulation of peak flows between 5,000 and 13,000 m3/s is therefore deemed a reasonable range for the Ptolemaic and surrounding centuries. The model’s ability to replicate Ptolemaic-era flooding was validated by overlaying a database of 395 georeferenced Ptolemaic sites, including cities, villages, monuments, fortresses, and stations, usually positioned on local high points to minimize inundation risk (Figs. 2 and E3). In a typical flood year, with a peak inflow of 9,000 m 3 /s entering the upstream Nile, nearly all sites remain above or outside the flooded area in our model. It is important to note that the inundated area represents the maximum temporary extent of floodwaters, which do not always occur simultaneously and subsequently infiltrate into the soil or evaporate. The close alignment of flood extents around urban centers and key landmarks is consistent with historical depictions of the Nile in flood, such as the Mosaic of Palestrina 44 . Modern net irrigated area in the Nile River Valley is 8,600 km 2 45 , whereas estimates of pre-Industrial irrigated area in Middle Egypt are closer to 2,000 km 2 , though with significant uncertainty due to challenges in georeferencing hand-drawn French maps created in the early 1800s 2,15 . Our estimate for Middle Egypt and the Thebaid total flood irrigated area during a typical year is 6,400 km 2 (3,800 and 2,600 km 2 respectively, Fig. 5a). This estimate is lower than modern agricultural area and higher (but similar to) historical estimates of Middle Egypt, lending greater confidence in the inundated area estimates presented here. Confidence is strongest for the relative effects of flow disruptions on inundated area, even if some bias remains in the absolute area estimates. 2.2. Nile Flood Sensitivity During the flood season, the flood pulse enters the upstream boundary near Aswan and gradually exceeds the capacity of the Nile channel, filling the adjacent floodplain. This overtopping typically began in July, first near the Qena bend (Fig. 3a) and the Bahr Yussef (Fig. 3b). These regions generally have lower channel slopes, whereas the remainder of the Thebaid (a key political center in Egyptian civilization) is characterized by steeper gradients and a narrower river valley. Middle Egypt, situated downstream between modern Asyut and Cairo, generally contains the widest floodplain and shallowest slope. Once floodwaters exceed the channel capacity in these key areas, water tends to flow parallel to the main channel, following natural depressions, forming secondary channels, and gradually expanding to fill the floodplain. As the seasonal pulse recedes, water is lost through evapotranspiration and infiltration. Two distinct spatial patterns of Nile sensitivity are evident, with the Upper and Middle Thebaid being more sensitive to smaller, more common flow reductions, while the Qena bend and Middle Egypt require more severe droughts to produce major disruptions of irrigable area (Fig. 4). In the more sensitive Upper and Middle Thebaid, the percent change in flooded area rapidly decreases during drought years (Fig. 4). For years with peak flows of 7,000 m 3 /s, inundated area in the upper and middle Thebaid decreased by 63.3% and 67.9%, respectively, relative to a typical year with a peak discharge of 9,000 m 3 /s. Based on historical recurrence (Fig. E1), a 7,000 m 3 /s event would be classified by the US Drought Monitor as only a “moderate drought”, relatively common events that occur in approximately one of ten years. In more severe droughts, when the flood peak fails to surpass 7,000 m 3 /s, most of the Upper and Middle Thebaid remain dry, with water confined within the immediate Nile banks (Fig. 3, Figs. E8 and E10). In addition to smaller inundated area, drought would also have likely decreased the duration of the flood season, limiting infiltration even in inundated areas. Our model thus reveals the Upper and Middle Thebaid as being agriculturally sensitive even to minor drought years, but incrementally more extreme droughts would have had negligible additional effect because the Nile would largely remain in its banks, leaving most of the regions’ agricultural lands dry below this threshold. This pattern of drought sensitivity is different from the Qena Bend and Middle Egypt, which are more resilient to moderate drought years, but produce increasingly more catastrophic losses of agricultural area during the most extreme droughts, which occur once every 50-100 years on average based on 19 th and early 20 th century recurrence (Fig. 4). The topography of these regions is such that both exhibit relatively little loss of inundated area at flow rates down to approximately 7,000 m 3 /s, but beyond which losses become increasingly extreme (Fig. 4). Inundated area for both regions is cut approximately in half for floods of 6,000 m 3 /s and the Nile remains largely within its banks below 5,000 m 3 /s (Fig. 4). For extreme droughts of 5,000 m 3 /s, only the flat, low-lying areas around Koptos would be inundated in the Qena bend, while only secondary channels and the Bahr Yussef would be inundated in Middle Egypt (Fig. 3). Under normal circumstances, Middle Egypt has approximately 1.5 times more inundated area, and thus agricultural capacity (~1,000 km 2 ), than the entire Thebaid, defined as the Upper Thebaid, Qena bend, and Middle Thebaid (Fig. 5a). Because the Upper and Middle Thebaid are very sensitive to moderate droughts, inundated area in the Thebaid decreases by approximately 1,380 km 2 (53.3%) for years with flood peaks of 7,000 m 3 /s. This represents a relatively frequent drought, occurring once every 5-10 years, but a major loss of crop area for the region. Middle Egypt would have remained relatively buffered from these frequent and smaller drought years, producing a gap in lost agricultural area between 7,000 and 8,000 m 3 /s (Fig. 5b). However, for more extreme droughts below 7,000 m 3 /s, inundated area even in Middle Egypt would have dropped precipitously (Fig. 5). The difference between a moderate and extreme drought, 7,000 and 5,000 m 3 /s respectively, would have been a loss of nearly 2,400 km 2 of irrigated land, leaving only 14% of a typical year’s agricultural capacity (Fig. 5). This would likely have been devastating for crop yield, but would occur rarely, in less than 1% of years, taken over a long period. Additionally, because both the Thebaid and Middle Egypt rely on the same Nile flows, the Thebaid would experience this extreme loss of agricultural area simultaneously. The smaller Fayum “oasis” area is not directly adjacent to the main stem of the Nile (Fig. 1a), and receives Nile water through the Bahr Yussef channel, which in turn fills Lake Moeris. During the Ptolemaic period, Lake Moeris was a freshwater lake 34 . Results for the Fayum are included here for completeness but should be interpreted cautiously. Simulations of Fayum irrigable area are complicated by Lake Moeris, which provided some inter-annual storage to buffer drought years 33 , partially decoupling Fayum agriculture from specific modelled flood magnitudes. It should be noted that during the later Ptolemaic period management of the Fayum entered a state of mismanagement and disrepair 34 . For these reasons, and the Fayum’s relatively smaller area, we do not further discuss the Fayum. Nonetheless, the vulnerabilities described above offer a new context for the great efforts made by Ptolemaic kings to develop semi-Nile-independent agricultural regions such as the Fayum and repeated costly wars conducted to secure territories in the Eastern Mediterranean that were conspicuously (given our results) capable of rain-fed agriculture 4,33,46 . Discussion Our study developed a novel 2D hydraulic flood model of the natural Nile River by integrating historical records, modern bathymetry measurements, and remotely sensed topographic data. The primary objective was to simulate the maximum extent of feasibly irrigated land in Ptolemaic Egypt under gravity-fed conditions, to reveal the scale and geography of agricultural vulnerability to Nile flood disruptions. These simulations represent a theoretical maximum inundation extent, acknowledging that now-absent man-made structures may have redirected or limited flooding patterns. Based on historical validation, the models’ results appear robust, despite inherent uncertainties in reconstructing floodplain behavior prior to georeferenced measurements. Hydraulic simulation focused on the floodplain between Aswan and modern Cairo, excluding the Nile delta, where erosion and sedimentation complicate modelling by causing channel meandering 26,27,47,48 . Elsewhere, channels have remained relatively stable over the last 2 to 3 millennia 31 , particularly at our model’s spatial scale (50-200 m grid), allowing for reasonable approximation. The Ptolemaic period climate was not dramatically different from that of the late 1800s-early 1900s, allowing estimates of disruption frequency and severity (Figs. 4 and 5). Our finding that most Ptolemaic villages and towns remained beyond the characteristic flood extent further validates the model’s ability to accurately capture flood behavior. Simulations indicate non-linearities and specific thresholds in agricultural vulnerability which vary by region. Moderate droughts would frequently and significantly have disrupted agriculture in the Thebaid. Notably, the regional administrative capital at Ptolemais Hermeiou, in the sensitive Middle Thebaid, would have lost much of its nearby agricultural capacity, even during relatively minor droughts (Fig. E8). The Qena bend would have remained the exception to Thebaid agricultural disruptions. Middle Egypt, with a larger baseline agricultural capacity, would have remained relatively unaffected by moderate droughts. However, during extreme droughts, inundated area would have decreased dramatically, with losses of up to 2,400 km 2 , leaving only 14% of a typical year’s irrigated area (Figs 4 and 5). These Middle Egypt failures would have occurred simultaneously with losses in the Thebaid, leading to potential disaster in the absence of coping measures. With the Thebaid likely accustomed to frequent and possibly multi-year reductions in irrigated land, the Ptolemaic government and broader Egyptian society doubtless developed such measures. At a state level, grain importation was important 4,49 . During what were likely harsh (volcanically triggered) drought conditions in the 240s BCE, the state imported food at great expense, though this was considered an extreme measure and the effort later used as to glorify the quality of Ptolemaic rule 6,46 . However, external warfare, for which the period is famous, would have complicated grain importation. Internally, larger temple estates often secured and managed land parcels throughout Egypt, thus allowing parts of the Thebaid to supplement poor harvest with production from Middle Egypt or the Qena bend 49 . Internal administrative lapses or civil unrest could, however, impact internal grain transport and other emergency measures, leaving the Thebaid particularly vulnerable. In the most extreme years, agricultural production from Middle Egypt would have decreased precipitously, compounding Thebaid failures. Under these conditions, even a well-functioning state would have struggled to prevent widespread socioeconomic stress. Recent work has revealed a repeated temporal association between internal revolt and ice-core-based dates of explosive volcanism known from polar ice-cores, with a causal link posited between volcanically induced monsoon and related Nile flood disruption, suppressed agricultural production, and the resulting political-psychological impacts in a religious context wherein the Ptolemaic rulers (having taken the mantle of Pharaoh) were deemed responsible for ensuring sufficient Nile flooding 4,5 . However, an environmental context has not been offered for the particular geography of these revolts, most of which (e.g. revolts in 207/206-186, 132, 91, and 88-85 BCE), originated in the Thebaid before spreading downstream to Middle Egypt. While this pattern can be partially explained by the geographic and cultural distance from the Ptolemaic capital of Alexandria, the Greeks established a secondary administrative center at Ptolemais Hermeiou in 310 BCE to help manage the region (Fig. 1). Our results now suggest that the Thebaid’s unique susceptibility to low flow years, even during periods when Middle Egypt remained productive, created a chronic vulnerability that predisposed this region to revolt by fueling public discontent towards the Ptolemaic regime. This contributed to events like the Great Theban Revolt (207/206-186 BCE), which followed a candidate tropical explosive eruption in c.209 BCE 4,49 . This revolt, in which the Ptolemies lost control of much of southern Egypt to one or more native claimants to title of Pharaoh, began at Apollonopolis Megale, a particularly drought susceptible area (Fig. E10). Amid popular revolt and drought, the temples at Diospolis Megale (Thebes) and Apollonopolis Megale (Edfu) managed the political response, acting as cultural symbols of kingship and buffering against major drought failures as the largest landowners 49 . The impacts of further cases of known hydroclimate stress for the region can also now be better understood. This includes the “volcanic quartet” sequence of four large and closely timed eruptions between 168 and 158 BCE 3,4 . Climate simulations of an eruption scaled to match the estimated climate forcing from the first and largest eruption in 168 BCE suggest annual mean flow decreases of 29% and 38% across the Nile basin in the first two post-eruption years, respectively 4 . Given a typical peak discharge of 9,000 m³/s, this would result in consecutive years with flows at or below 6,000 m³/s, creating back-to-back catastrophic flood failures for Middle Egypt and the Thebaid, reducing irrigated land to only 25% of normal levels. Two or more such consecutive failures would have presented extraordinary challenges. Indeed, these register abundantly in the 160s BCE, with internal revolts and the near conquest of Egypt by the rival Seleucid empire, an event only prevented by self-interested Roman intervention. The three subsequent smaller extratropical eruptions showed more moderate simulated reductions in discharge (~10%) and would have primarily affected the Thebaid. This combination of Egypt-wide catastrophe with continued Thebaid-centric failures provides a reasonable hypothesis as to a climatic driver pushing Thebaid revolts against the Ptolemaic government in these years. Overall, this study demonstrates how droughts within the Nile watershed translated nonlinearly into regional agricultural failures. The findings align closely with historical records of contemporaneous Ptolemaic sites and highlight the particular vulnerability of the Thebaid, which now assists in explaining the need for enhanced administrative oversight and the region’s repeated role as the origin of revolts. Simulations of known volcanic eruptions during this era imply multi-year agricultural failures affecting nearly all of Egypt, which would have had potentially devastating consequences. Contemporaneous sources explicitly reference drought and a lack of irrigation throughout this period. The hydraulic flood model developed here provides a valuable tool for investigating natural flood behavior along the Egyptian Nile over the past two millennia, as well as better contextualizing the human history of the period, prior to the construction of major reservoirs. Declarations 8. Acknowledgements This research has been financially supported by the US National Science Foundation Dynamics of Coupled Natural and Human Systems Program (grant no. 1824770) and the Yale Planetary Solutions (YPS) Grant Program. Organizational support was provided by the Byrd Polar and Climate Research Center and the Yale Peabody Museum of Natural History. 9. Author Contributions 10. JHS: conceptualization, methodology, formal analysis, visualization, writing (original draft). IFM: data curation, resources. JM: data curation, resources. FML: conceptualization, resources. SM: resources. 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Samir, Y., Moussa, A. M. & El-Badry, H. M. Hydrodynamic Study of Nile River usig 1D Model. Civil Engineering Research Magazine Civil Engineering Department Al-Azhar University 41 , 313–325 (2019). Willcocks, W. The Nile in 1904. https://library.si.edu/digital-library/book/nilein190400will (1904). Hassan, F. A., Hamdan, M. A., Flower, R. J., Shallaly, N. A. & Ebrahem, E. Holocene alluvial history and archaeological significance of the Nile floodplain in the Saqqara-Memphis region, Egypt. Quaternary Science Reviews 176 , 51–70 (2017). Pennington, B. T., Sturt, F., Wilson, P., Rowland, J. & Brown, A. G. The fluvial evolution of the Holocene Nile Delta. Quaternary Science Reviews 170 , 212–231 (2017). Ahmed, A. F. & Fahmy, W. A. Long-Term Morphological Changes in the Nile River since High Aswan Dam Construction to Year 2010. Nile Basin Water Science & Engineering Journal 7 , (2014). Hekal, N. Evaluation of the equilibrium of the River Nile morphological changes throughout “Assuit-Delta Barrages” reach. Water Science 32 , 230–240 (2018). Macklin, M. G. et al. A new model of river dynamics, hydroclimatic change and human settlement in the Nile Valley derived from meta-analysis of the Holocene fluvial archive. Quaternary Science Reviews 130 , 109–123 (2015). Peeters, J. et al. Shift away from Nile incision at Luxor ~4,000 years ago impacted ancient Egyptian landscapes. Nat. Geosci. 17 , 645–653 (2024). Hillier, J. K., Bunbury, J. M. & Graham, A. Monuments on a migrating Nile. Journal of Archaeological Science 34 , 1011–1015 (2007). Römer, C. The Nile in the Fayum. in The Nile: Natural and Cultural Landscape in Egypt (eds. Willems, H. & Dahms, J.-M.) vol. 36 171 (2017). Thompson, D. J. Irrigation and drainage in the early Ptolemaic Fayyum. in PROCEEDINGS-BRITISH ACADEMY vol. 96 107–122 (OXFORD UNIVERSITY PRESS INC., 1999). Hurst, H. E. & Phillips, P. The Nile Basin: Ten-Day Mean and Monthly Mean Gauge Readings...-v. 4. Ten-Day Mean and Monthly Mean Discharges . vol. 4 (1960). Kröpelin, S. et al. Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Science 320 , 765–768 (2008). Verschuren, D. et al. Half-precessional dynamics of monsoon rainfall near the East African Equator. Nature 462 , 637–641 (2009). Blanchet, C. L., Frank, M. & Schouten, S. Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene. PLOS ONE 9 , e115958 (2014). Jaeschke, A. et al. Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi). Front. Earth Sci. 8 , (2020). Nash, D. J. et al. African hydroclimatic variability during the last 2000 years. Quaternary Science Reviews 154 , 1–22 (2016). Kondrashov, D., Feliks, Y. & Ghil, M. Oscillatory modes of extended Nile River records (A.D. 622–1922). Geophys. Res. Lett. 32 , L10702 (2005). Revel, M. et al. 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments. Quaternary Science Reviews 130 , 200–221 (2015). Nicholson, S. E. et al. Temperature variability over Africa during the last 2000 years. The Holocene 23 , 1085–1094 (2013). Meyboom, P. G. The Nile Mosaic of Palestrina: Early Evidence of Egyptian Religion in Italy . vol. 121 (Brill, 2015). Attia, F. A. R., Allam, M. N. & Amer, A. W. A Hydrologic Budget Analysis for the Nile Valley in Egypt. Ground Water 24 , 453–459 (1986). Austin, M. M. The Hellenistic World from Alexander to the Roman Conquest: A Selection of Ancient Sources in Translation . (Cambridge University Press, 2006). El Bastawesy, M., Gebremichael, E., Sultan, M., Attwa, M. & Sahour, H. Tracing Holocene channels and landforms of the Nile Delta through integration of early elevation, geophysical, and sediment core data. The Holocene 30 , 1129–1141 (2020). Stanley, J.-D. Egypt’s Nile Delta in Late 4000 Years BP: Altered Flood Levels and Sedimentation, with Archaeological Implications. Journal of Coastal Research 35 , 1036 (2019). Manning, J. G. Land and Power in Ptolemaic Egypt : The Structure of Land Tenure . (Cambridge University Press, Cambridge, UK ; New York, 2003). Methods 6.1. Hydraulic Model All hydraulic modeling was performed using the US Army Corps of Engineers (USACE) HEC-RAS model 1 . HEC-RAS is a one- or two-dimensional hydraulic modeling platform widely applied for flood inundation analysis on major waterways in the U.S. and internationally. 6.2. Floodplain Topography Floodplain elevations, outside of the main Nile channel, were based on the Multi-Error-Removed Improved-Terrain Digital Elevation Model (MERIT-DEM) 2 . MERIT-DEM is a highly accurate 3 global digital elevation model with a spatial resolution of 3 arcseconds, or approximately 80-90 m in Egypt. It is designed specifically for hydrologic modeling and merges multiple remote sensing datasets, including NASA SRTM3, JAXA AW3d-20m and Viewfinder Panorama's DEM 2 . In the data merging process, MERIT-DEM corrects for dataset bias, stripe noise, speckle noise, and tree height bias. As a result, MERIT-DEM has a vertical accuracy of 2 meters or better over 58% of the global land area covered by the product, a significant improvement over prior methods 2 . 6.3. River channel geometry Although MERIT-DEM effectively captures floodplain elevations, satellite-based elevation products cannot penetrate deep or occluded water surfaces like those of the Nile River. To address this limitation, we combined MERIT-DEM data with river bathymetry from a published one-dimensional hydraulic model of the modern Nile between Aswan and Cairo (Samir, Moussa, and El-Badry 2019). The river centerline was manually digitized from this study (Samir, Moussa, and El-Badry 2019). River right and left banks were delineated using areas classified as permanent water in the JRC’s Global Surface Water Explorer 4 . Nile cross-sections were assumed to be triangular, derived by connecting the river centerline to bank elevations. River surveys from 1904 5 were used to validate this triangular assumption and to confirm there was minimal changes in centerline elevation through time. Additional channel validation was performed using modern reach-specific studies 6 . No dams were included in the hydraulic model. The Bahr Yussef channel was generated by digitizing the channel centerline from the Global Surface Water Explorer 4 . This modified natural channel was assumed to be trapezoidal, with a 30m bottom width, 2:1 side slopes, and a 50 m maximum top width. These assumptions align with surveyed measures of the modern Bahr Yussef 7 . Bathymetry of both the Nile and Bahr Yussef channels was then interpolated and merged with MERIT-DEM data, with river channel bathymetry always taking precedence. 6.4. Land cover, infiltration, and evapotranspiration Surface roughness estimates are necessary to estimate momentum loss. Land cover was estimated based on MDA’s BaseVue 2013 land cover data 8 . BaseVue has a 30m spatial resolution and is derived from Landsat 8 remotely sensed measurements. BaseVue uses 13 land cover classes, though only a subset appeared within the model extent. To approximate pre-modern land cover, urban areas were assumed equivalent to the most dominant surrounding grid cells, generally becoming agricultural land. Manning’s roughness values of 0.035 were used for grassland and agricultural land, while roughness values of 0.1, 0.03, 0.025, and 0.07 were used for shrub/scrub, open water, barren rock, and wetlands, respectively. These values are approximately the mid-points of those recommended by the US Army Corps of Engineers 9 , adjusted to accommodate the slightly different land cover classes used in the BaseVue dataset. In order to generate realistic behavior during the flood recession, the hydraulic model includes soil infiltration and evapotranspiration losses to the atmosphere. Infiltration was calculated using the Deficit and Constant loss method 1 , with maximum percolation rates of 2.5 mm/hr assumed for grassland and agricultural land, and 5.5 mm/hr assumed for shrub/scrub and barren rock. These rates are reasonable given prior estimates of vertical hydraulic conductivity for the Nile 10 . No infiltration was assumed within the main Nile river channel. The evapotranspiration rate was based on monthly mean temperature and applied equally across the study area. Evapotranspiration rates peak at 0.509 mm/hr in June and fall to 0.152 mm/hr in December. These values are within typical ranges assumed for the region 11 .Flood inundated area was generally not sensitive to infiltration or evapotranspiration rates because these processes only cleared standing water following the flood season. Because we simulated individual years, there was no hold-over storage on the land and thus negligible effects from infiltration or evapotranspiration. 6.5. Flow Data Representative flows were based on mean monthly discharge measurements at the Aswan and Dongola gauging stations for each year between 1869 and 1958 12 . These gauge observations were adjusted in the original study to account for upstream irrigation withdrawals, creating the best estimate of a near-natural record for this period. From this, we compiled a 90-year composite dataset by combining the Aswan and Dongola records, always using the larger of the two values, and generally using the downstream Aswan gauge prior to the early 1900s and then shifting to the upstream Dongola gauge after the completion and raising of the Aswan low dam. The time series ends in 1958 to avoid the influence of the Roseires Dam on the Blue Nile, which coincided with a significant decrease in annual maximum flow. During the period, assumed to be represented of natural flows, annual maxima were stable, with no long-term trends (Fig. E1). The data follow a normal distribution, with a mean of 9,030 m 3 /s and standard deviation of 1,550 m 3 /s. We therefore chose to simulate representative near-natural peak flows ranging from 5,000 to 13,000 m 3 /s, capturing percentiles from 0.5% to 99.5%, or approximately a 1-in-200 year event. Historical analysis showed consistent monthly flow percentiles across varying peak flows, allowing us to assume the same seasonal proportions for all simulated years, scaled to each respective peak (Fig. E2). We did not consider any direct rainfall within the model. This is a reasonable assumption given the relative paucity of rainfall in Egypt relative to the incoming flow at Aswan 11 . 6.6. Hydraulic Simulation All simulations used the diffusive wave equation for the conservation of momentum. The computational mesh for flow modeling was approximately 50 m within the Nile river channel and transitioned smoothly to a 250 m spatial resolution in the floodplain, creating a computational mesh of 1.14 million cells. Upstream boundary conditions were assumed equal to flow entering the study area at Aswan, while downstream boundary conditions assumed normal depth with a friction slope of 0.00006 m/m, matching the Nile’s slope at Cairo 5 . Normal depth was computed separately for each computational cell face along the downstream boundary. Simulations included a warmup period of 1,000 m 3 /s constant flow for a year, followed by the year to be simulated (Fig. E2). This ensured the model reached steady state prior to the flood simulation. The computational interval was 3 hours, with output reported daily. 6.7. Inundated Area and Regionalization Regions used for inundated area calculations were based on administrative areas of Ptolemaic Egypt. These included the Upper Thebaid (Aswan to Esna), Qena Bend (Esna to Nag Hammadi), Middle Thebaid (Nag Hammadi to Assiut/Abydos), Middle Egypt (Assiut to modern Cairo), and the Fayum. While hydraulic simulations used computational cells with spatial resolutions between 50 and 250 meters, post-processing was performed in HEC-RAS to interpolate the land surface elevation, producing inundated area estimates with a gridded resolution of 25x25 meters. Inundated area was quantified by calculating the proportion of time each 25x25 meter grid cell was submerged during the simulation year (excluding the warmup). A cell was defined as being inundated for a simulation if it was covered by water for at least 5% of the simulation period, approximately 18 days from March to March. This approximately represents the peak of the flood season in simulations. In sensitivity tests, the results were robust to this threshold. Only areas outside the main channel and permanent water bodies were included in the inundated area analysis. These areas would not be fit for agricultural practices. Grid cells submerged for 95% or more of the year were excluded to avoid overestimating the inundated area. 6.8. Ptolemaic Sites The online data repository Trismegistos Places 13 was employed in collating a preliminary list of settlements attested more than once in historical texts of the Ptolemaic Period. The Trismegistos Places database aggregates topographical data attested in the published documentary record of Greco-Roman Egypt 13 . Only a fraction of these sites can be positively identified with modern toponyms. Trismegistos relies upon regional surveys performed by ancient historians identifying modern settlements with ancient toponyms, not all of which agree in individual cases. Following the selection of 395 individual places, each point was manually geolocated using Google Earth and converted to a point shapefile with an associated attribute table. In cases of disagreement between identifications in scholarly literature, a location was selected on the merits of the respective arguments. A small number of additional identifications were made and included in this topographical dataset. The distribution of confirmed identifications is geographically discontinuous across our study area, clustering in the Fayum, Middle Egypt, and the Qena bend. Fortunately, the major population centers (Gr. metropoleis ) of individual districts of Egypt can be identified in nearly every case due to preservation of this knowledge from the ancient period through to the present day in historical and geographical literature. Furthermore, population estimates for individual districts and larger subdivisions of ancient Egypt have been generated through the analysis of archaeological surveys and administrative texts such as census documents, permitting general observations on population density. By contrast, smaller villages can typically be identified in two circumstances: (1) the ancient Egyptian toponym has been preserved as its primary referent through the Medieval and Modern Periods, and (2) archaeological remains of the ancient town survive. The former is typical of Middle Egypt, while the latter is characteristic of the Fayum. Because only a fraction of these ancient villages can be positively identified with modern settlements, and because the nature of the Nile floodplain restricts the limits of permanent settlements and encourages continuity of occupation, it can be assumed with some degree of confidence that many modern settlements that have not yet been identified with an ancient toponym do in fact correspond with these ancient places. The primary hindrance to this endeavor is the historical tendency to rename villages with Greek and later Arabic toponyms that bear no resemblance to the ancient Egyptian equivalents. Nevertheless, the distribution of positive identifications within each region is relatively homogenous and facilitates general observations on the relationship between settlement concentrations and floodplain topography, particularly the contrasting relationship between settlement density and floodplain topography in the Thebaid and Middle Egypt. 6.9. Data Availability The Hec-Ras model and all output is provided via an open-access repository (Figshare). The Hec-Ras model and all relevant data are stored in a folder labeled “Nile flood model”, while output is still in a folder labeled “Output”. 6.10. Code Availability All code used to analyze the Hec-Ras output is available in the same open-access repository (Figshare) in a separate folder titled “Code”. 7. Methods Section References 1. Hydrologic Engineering Center. HEC-RAS 2D Modeling User’s Manual . (U.S. Army Corps of Engineers, Davis CA, 2021). 2. Yamazaki, D. et al. A high-accuracy map of global terrain elevations. Geophysical Research Letters 44 , 5844–5853 (2017). 3. Uuemaa, E., Ahi, S., Montibeller, B., Muru, M. & Kmoch, A. Vertical Accuracy of Freely Available Global Digital Elevation Models (ASTER, AW3D30, MERIT, TanDEM-X, SRTM, and NASADEM). Remote Sensing 12 , 3482 (2020). 4. Pekel, J.-F., Cottam, A., Gorelick, N. & Belward, A. S. High-resolution mapping of global surface water and its long-term changes. Nature 540 , 418–422 (2016). 5. Willcocks, W. The Nile in 1904. https://library.si.edu/digital-library/book/nilein190400will (1904). 6. Ahmed, A. F. & Fahmy, W. A. Long-Term Morphological Changes in the Nile River since High Aswan Dam Construction to Year 2010. Nile Basin Water Science & Engineering Journal 7 , (2014). 7. Awad, B. S. R. et al. Bahr Yousef meandering canal management – Hydraulic and morphological assessment. Water Science 39 , 28–41 (2025). 8. MDA Information Systems US. World Land Cover 30m BaseVue 2013. (2013). 9. U.S. Army Corps of Engineers. Creating Land Cover, Manning’s N Values, And % Impervious Layers . (2021). 10. Attia, F. A. R., Allam, M. N. & Amer, A. W. A Hydrologic Budget Analysis for the Nile Valley in Egypt. Ground Water 24 , 453–459 (1986). 11. Sutcliffe, J. V. The Hydrology of the Nile Basin. The Nile 335–364 (2009) doi:10.1007/978-1-4020-9726-3_17. 12. Hurst, H. E. & Phillips, P. The Nile Basin: Ten-Day Mean and Monthly Mean Gauge Readings...-v. 4. Ten-Day Mean and Monthly Mean Discharges . vol. 4 (1960). 13. H. Verreth, A survey of toponyms in Egypt in the Graeco-Roman period (Trismegistos Online Publications, 2), Leuven: Trismegistos Online Publications, 1253. www.trismegistos.org/geo/index.php Additional Declarations There is NO Competing Interest. Supplementary Files Qenaflood.mp4 Qena flood video StaggeNileFloodingExtendedData.docx Extended Data file Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-6968703\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Social Sciences - Article\",\"associatedPublications\":[],\"authors\":[{\"id\":476433823,\"identity\":\"a83605ce-7643-4f16-b3c7-e82aec23426d\",\"order_by\":0,\"name\":\"James Stagge\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYBADHjb2BjCDsYGgWjYIJcPHc4BELTZyEglEapGf3/z45ZcKoMMkHx/+zMNgI7vhAAEtBsfYzKxlzgC1SKelSfMwpBkT1sLGYGYs2QbSkmPGzMNwOJGgFvk29m/Gkv9ADjtjDHTYf8JaGI7xGD/82ADUIsFjAHTYAcJaDI7llDEzHJPgYeNJS5OcY5BsPJOgw5qPb/74o8bGXr798OEPbyrsZPsIOgwYM0D3SMAsJawcBJg//iBO4SgYBaNgFIxUAABIZDmIL9octAAAAABJRU5ErkJggg==\",\"orcid\":\"https://orcid.org/0000-0002-3667-2904\",\"institution\":\"The Ohio State University\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"James\",\"middleName\":\"\",\"lastName\":\"Stagge\",\"suffix\":\"\"},{\"id\":476433824,\"identity\":\"050164c0-fd18-4d7c-9671-b3fe1af4426a\",\"order_by\":1,\"name\":\"Irenee Felix Munyejuru\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"The Ohio State University, Department of Civil, Environmental and Geodetic Engineering\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Irenee\",\"middleName\":\"Felix\",\"lastName\":\"Munyejuru\",\"suffix\":\"\"},{\"id\":476433825,\"identity\":\"c76f1f35-9d05-46b1-a2a1-43ebc6e4d439\",\"order_by\":2,\"name\":\"Joseph Morgan\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"University of Oklahoma, Department of Classics and Letters\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Joseph\",\"middleName\":\"\",\"lastName\":\"Morgan\",\"suffix\":\"\"},{\"id\":476433826,\"identity\":\"db0337e0-bb6f-42e6-b30f-3376c2d9d057\",\"order_by\":3,\"name\":\"Francis Ludlow\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0003-0008-0314\",\"institution\":\"Trinity College Dublin\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Francis\",\"middleName\":\"\",\"lastName\":\"Ludlow\",\"suffix\":\"\"},{\"id\":476433827,\"identity\":\"0cebb331-06e0-4714-8700-162a878a4e75\",\"order_by\":4,\"name\":\"Selga Medenieks\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Trinity College Dublin, Department of History\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Selga\",\"middleName\":\"\",\"lastName\":\"Medenieks\",\"suffix\":\"\"},{\"id\":476433828,\"identity\":\"b9c542f9-ae72-4d1a-b406-1e26ef9ef1d8\",\"order_by\":5,\"name\":\"Joseph Manning\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-4454-198X\",\"institution\":\"Yale University\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Joseph\",\"middleName\":\"\",\"lastName\":\"Manning\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2025-06-24 20:15:08\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-6968703/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-6968703/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":85451172,\"identity\":\"683b2895-d497-4a55-8699-84f8158980a5\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:49:27\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":1171356,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eStudy area showing (a) the flood model extent, with regions used for flood inundation calculations shown in color. Relevant Ptolemaic places in (a) are labeled first using their former Greek name, followed by the modern name in italics for clarity. The entire Nile watershed, shown in (b), indicates major subwatersheds as grey shaded regions, with the flood model extent shown by the overlayed rectangle.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/e48bc07cf1f7a6f165b91a78.png\"},{\"id\":85451170,\"identity\":\"88c3cb82-d108-4dd1-8037-0232eb1d53d7\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:49:27\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":761378,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eMaximum inundated extent during a typical flood year, when peak flow equals 9,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s. Ptolemaic towns and villages are labeled in black. This confirms that major Ptolemaic era places were constructed outside of commonly inundated areas.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/facbbceb43daab7401817e07.png\"},{\"id\":85451723,\"identity\":\"94407235-e3d3-41dd-bd74-0ec59f56bc03\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:57:27\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":2028727,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eMaximum inundated area ranging from extremely poor (5,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s) to typical flood peaks (9,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s) for the (a) Qena bend and (b) portions of Middle Egypt. Flood magnitudes are shown above each figure panel. Ptolemaic-era towns and points of interest are shown as black dots, with labels for larger cities. Exceptionally high flood peaks above 9,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s are not shown. Similar figures for other regions are shown in the Extended Data.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/57e4794c1011750fe20b7cfb.png\"},{\"id\":85451977,\"identity\":\"96857781-1bbd-4afb-bb83-9f527cf4961d\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 05:05:27\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":151146,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eChange in inundated area (%) relative to a typical year, defines as a peak flow of 9,000 m3/s. Colors are equivalent to Fig. 1. Shaded bands show progressively more extreme events, corresponding to the 20%, 10%, and 1% percentiles, or an average return frequency of 5, 10, and 100 years, repectively.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/006bf825fc31ad6c14336fa4.png\"},{\"id\":85451724,\"identity\":\"84271e51-e015-4c9d-8e24-3b013b58a797\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:57:27\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":149154,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eInundated area (a) by region and (b) the change in inundated area relative to a typical year, represented by 9,000 m3/s. Colors are simlar to Figs. 1 and 4, with the three subregions of the Thebaid summed together. As in Fig. 4, shaded bands show progressively more extreme events, corresponding to the 20%, 10%, and 1% percentiles, or an average return frequency of 5, 10, and 100 years, repectively.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/3fda0ccccc1c2efd0e4cd78d.png\"},{\"id\":86170824,\"identity\":\"241782dd-5753-4c11-817d-7a5dfbdd0e7a\",\"added_by\":\"auto\",\"created_at\":\"2025-07-07 14:23:48\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":4677133,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/f7f14629-2166-48e4-9bed-83bcb6e270b7.pdf\"},{\"id\":85451176,\"identity\":\"ef610834-fe8b-4dee-9828-7c9fbc4eba62\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:49:27\",\"extension\":\"mp4\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":4006334,\"visible\":true,\"origin\":\"\",\"legend\":\"Qena flood video\",\"description\":\"\",\"filename\":\"Qenaflood.mp4\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/bc40a3278e962b89f71ab190.mp4\"},{\"id\":85451177,\"identity\":\"40b39eab-bd9a-4704-b1d1-5cf54d0d324b\",\"added_by\":\"auto\",\"created_at\":\"2025-06-26 04:49:27\",\"extension\":\"docx\",\"order_by\":2,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":6670406,\"visible\":true,\"origin\":\"\",\"legend\":\"Extended Data file\",\"description\":\"\",\"filename\":\"StaggeNileFloodingExtendedData.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6968703/v1/9f6d7b999ca33cf75d160038.docx\"}],\"financialInterests\":\"There is \\u003cb\\u003eNO\\u003c/b\\u003e Competing Interest.\",\"formattedTitle\":\"Nile floods reveal Ancient Egypt's pattern of revolts\",\"fulltext\":[{\"header\":\"Main text\",\"content\":\"\\u003cp\\u003eThe Nile is the world’s longest river, originating upstream of Lake Victoria in east-central Africa and draining into the Mediterranean Sea. Its total catchment area is approximately 3 million km², spanning a wide variety of topographies and vegetation covers along its length \\u003csup\\u003e7\\u003c/sup\\u003e. Despite the diversity in climate and land cover in the upstream section, the final section in modern Egypt is relatively homogeneous, with no major tributaries and limited direct precipitation. Consequently, flood behavior in Egypt is almost entirely driven by incoming streamflow, allowing the Egyptian segment of the Nile to be modeled as a discrete system, with flow below the First Cataract at Aswan treated as the upstream boundary condition.\\u003c/p\\u003e\\n\\u003cp\\u003eThe Egyptian Nile has a predictable and dramatic flood season, occurring from late summer into early fall (July-October), supplied primarily by monsoon precipitation in the Ethiopian highlands \\u003csup\\u003e7,8\\u003c/sup\\u003e. Rainfall in Ethiopia drives a large seasonal flood pulse that travels down the Blue Nile (Fig. 1b), contributing approximately 70% of the typical flood peak measured at the Egyptian border \\u003csup\\u003e7,9\\u003c/sup\\u003e. The White Nile originates further south in the mountains of Rwanda, Burundi, and the Ugandan plateau, traveling a much longer distance through multiple lakes and the Sudd wetland before joining the Blue Nile at Khartoum (Fig. 1b). Similar to the Blue Nile, most flow in the White Nile derives from rainfall in upstream headwaters. However, unlike the Blue Nile headwaters, the White Nile experiences two wet seasons per year associated with the passage of the Intertropical Convergence Zone and is further attenuated by routing through numerous lakes and the Sudd wetland, upstream of the Sobat River (Fig. 1b). Nearly half of the White Nile’s annual volume is lost through the Sudd (Arabic for “barrier” or “obstruction”) via evapotranspiration and infiltration, leaving a nearly continuous baseflow that represents approximately 10% of the seasonal flood peak downstream \\u003csup\\u003e8–10\\u003c/sup\\u003e. The remaining 20% of the flood peak, after accounting for the Blue and White Nile branches, comes from the Atbara River, the last major Nile tributary (Fig. 1b), with its summer flood peak also driven by monsoon rainfall in the Ethiopian highlands.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe predictable cycle of late summer and early fall floodwaters sustained Egyptian agriculture and society for millennia \\u003csup\\u003e1,7\\u003c/sup\\u003e. Until the late 1800s, the Nile flowed in a near natural state, unimpeded by modern dams with large storage. Egyptians practiced a form of irrigation (flood recession agriculture), using drainage canals to direct floodwaters into adjacent fields by gravity. Water would then pool and seep into the soil as floodwaters receded, allowing the planting of wheat and other crops in winter for an April or May harvest, before the next flood season \\u003csup\\u003e1,11\\u003c/sup\\u003e. Because irrigation was gravity-fed, and thus based on the maximum height of the Nile flood, a direct relationship existed between Nile depth at its seasonal peak and the maximum area feasibly irrigated by lateral canals. Without large reservoirs providing interannual storage, flood season flow variations dictated the extent of arable land along the main Nile channel.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe Nile entering Egypt flowed naturally until 1902, when the Aswan Low Dam was completed. This was followed by several enlargements of the Low Dam over the ensuing decades, additional reservoirs built on the Blue and White Nile, and the construction of a larger Aswan High Dam \\u003csup\\u003e12\\u003c/sup\\u003e. The Aswan dams, in particular, controlled the annual flood to provide interannual storage, reducing irrigation failure risk during multi-year droughts. Diversion dams (termed ‘barrages’) were constructed along the Egyptian Nile between 1843 and 1930, primarily to control lateral flow into irrigation canals, rather than serving as major storage \\u003csup\\u003e7\\u003c/sup\\u003e. For our study, we assume the Nile entering Egypt to be near-natural at Aswan prior to 1902 and upstream of Aswan until the mid-20\\u003csup\\u003eth\\u003c/sup\\u003e century.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eWith harvest expectations in pre-modern Egypt heavily dependent upon the extent of irrigated land, itself determined by peak Nile summer flow\\u0026nbsp;\\u003csup\\u003e1,2,13\\u003c/sup\\u003e, effective floodwater management was crucial not only for food security but also state revenues, with taxes often paid in grain.\\u0026nbsp;Studies of historical Nile flood disruptions have focused on the drivers of precipitation in the Ethiopian highlands during the summer monsoon season. Interannual variation in monsoon precipitation is related to the position of the Intertropical Convergence Zone (ITCZ), which in turn is related to large-scale climate teleconnections like the Indian Ocean Dipole and the El Niño-Southern Oscillation (ENSO)\\u0026nbsp;\\u003csup\\u003e14–17\\u003c/sup\\u003e. Explosive volcanic eruptions that inject sulfate aerosols into the stratosphere can also suppress Ethiopian monsoon rainfall for up to 2 years by limiting the northward migration of the ITCZ, particularly when aerosols are concentrated in the Northern Hemisphere extra-tropics, also potentially causing global cooling of 1.0-1.5° C that can further suppress precipitation\\u0026nbsp;\\u003csup\\u003e3\\u003c/sup\\u003e. An extreme example of the volcanic impact on Ethiopian precipitation occurred during the Ptolemaic era (305–30 BCE), when a ‘volcanic quartet’ of four major eruptions occurred during the 11-year period between 168 and 158 BCE. Modelling suggests a protracted period of cooler temperatures, reduced precipitation in the Ethiopian highlands, and reduced Nile River flow in the aftermath\\u0026nbsp;\\u003csup\\u003e3,4,6\\u003c/sup\\u003e. The period suffered marked political instability, with impacts registering in correspondence between government officials discussing agricultural shortfalls and food security concerns\\u0026nbsp;\\u003csup\\u003e6\\u003c/sup\\u003e.\\u0026nbsp;The great eruption of Alaska’s Okmok volcano in 43 BCE can now also be convincingly posited as triggering a known period of major socioeconomic stress and poor Nile flooding in Egypt \\u003csup\\u003e5\\u003c/sup\\u003e.\\u003c/p\\u003e\\n\\u003cp\\u003eBeyond these cases, ice-cores reveal the Ptolemaic era to have experienced multiple substantial explosive eruptions\\u0026nbsp;\\u003csup\\u003e18\\u003c/sup\\u003e, but an important gap remains\\u0026nbsp;\\u003csup\\u003e19\\u003c/sup\\u003e when inferring causality between modelled post-volcanic reductions in Ethiopian precipitation\\u003csup\\u003e3\\u003c/sup\\u003e , contemporaneous Egyptian records of agricultural shortfalls, and the cascade of societal responses, including civil unrest. The relationship between flow and inundated area is highly nonlinear, given the Nile’s wide and irregular floodplain, making it challenging to estimate reductions to irrigated land area from rainfall reductions in the distant headwaters. Moreover, sensitivity to flow disruptions varies along the Nile’s length due to topography within the floodplain. Our objective is therefore to simulate flood behavior and maximum inundated area for a range of natural flow rates, representing the range of possible flood years from exceptionally dry to wet.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eUsing these flood simulations, we then quantify the risk of disruptions to Egyptian agriculture, organized by the administrative regions of Ptolemaic society. To simulate flood inundation during the Ptolemaic period, we reconstructed the Egyptian Nile and its floodplain using a combination of modern LiDAR-derived elevation data and bathymetric measurements of the Nile itself. These were validated against survey measurements from the late 1800s and early 1900s to ensure the model captured historical conditions. We then simulated the predictable seasonal flow patterns of the natural Nile using a two-dimensional unsteady flow hydraulic model. To our knowledge, the large spatial extent, fine spatial resolution, and focus on pre-modern conditions of this two-dimensional (2D) hydraulic model is unprecedented\\u0026nbsp;\\u003csup\\u003e20,21\\u0026nbsp;\\u003c/sup\\u003eand offers a detailed spatially explicit estimate of how irrigation capacity was affected by flood variability prior to the construction of modern reservoirs, providing insight into the geography of civil unrest in the Ptolemaic era.\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"Results\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e2.1.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Flood model and validation\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe Nile flood model, extending from modern Aswan to Cairo (Fig. 1a), was developed as a 2D unsteady flow hydraulic model using HEC-RAS v6.5 (Hydrologic Engineering Center-River Analysis System)\\u003csup\\u003e22\\u003c/sup\\u003e. A 2D unsteady flow simulation was necessary to accurately represent Nile flood pulse behavior as it overtops its channel and inundates the wide, flat floodplain, with flow traveling in multiple directions. Floodplain elevations were derived from the remotely sensed and bias-corrected MeritDEM dataset, which has a spatial resolution of approximately 90 meters \\u003csup\\u003e23\\u003c/sup\\u003e. Because remote sensing cannot effectively penetrate water surfaces, river channel bed elevations were based on existing one-dimensional flood models representing modern conditions \\u003csup\\u003e24\\u003c/sup\\u003e. Channel bathymetry was assumed to be triangular, connecting the modern centerline elevation\\u003csup\\u003e24\\u003c/sup\\u003e to the water\\u0026rsquo;s edge elevations obtained from remote sensing \\u003csup\\u003e23\\u003c/sup\\u003e. This triangular channel assumption is reasonable both as a simplified geometric model of pre-measurement conditions and based on channel surveys from 1904 that show a broadly triangular channel \\u003csup\\u003e25\\u003c/sup\\u003e. Modern centerline elevations were validated against the 1904 survey to confirm the stability of channel bathymetry through time. We chose to limit the model to the area upstream of the Nile delta due to the meandering, erosion, and sedimentation of the delta \\u003csup\\u003e26,27\\u003c/sup\\u003e. By contrast, our region of focus, upstream of Cairo, has remained relatively stable over the modern period \\u003csup\\u003e28,29\\u003c/sup\\u003e, with low sedimentation rates and largely fixed channels over the last 3,000 years \\u003csup\\u003e30,31\\u003c/sup\\u003e, though some locations show migration \\u003csup\\u003e32\\u003c/sup\\u003e. Our model also included the Bahr Yussef channel, an expanded natural channel which was active during the Ptolemaic period and transported water from the west bank of the Nile near Asyut to the Fayum \\u003csup\\u003e33,34\\u003c/sup\\u003e\\u0026nbsp; (Fig 1). Model simulations used over 1.14 million computational cells with a spatial resolution of approximately 50x50m within the Nile river channel, transitioning to a coarser 250 meter square resolution in the floodplain. Post-processing produced finer resolution of the flood inundated area through spatial interpolation. This level of spatial detail is appropriate for modeling such an extensive floodplain and given the uncertainties inherent in reconstructing historical channel geometry.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eTypical flows were based on monthly mean flow records measured at Aswan and Dongola, upstream of the reservoir created by the Aswan dam, from 1869 to 1958 \\u003csup\\u003e35\\u003c/sup\\u003e. The record at Aswan was adjusted to approximate the near-natural flow record by re-inserting upstream irrigation withdrawals \\u003csup\\u003e35\\u003c/sup\\u003e. A composite 90-year record of annual maxima was then constructed as the annual maxima of the two sites, generally shifting from Aswan to the upstream Dongola in the early 1900s following the Aswan low dam construction. We chose to end the record in 1958, prior to completion of the Roseires Dam on the Blue Nile, which coincided with a significant decrease in annual maximum flow. Annual maximum flows between 1869 and 1958 were stable, showed no significant trends, and followed a normal distribution with a mean of 9,030 m\\u003csup\\u003e3\\u003c/sup\\u003e/s and standard deviation of 1,550 m\\u003csup\\u003e3\\u003c/sup\\u003e/s (Fig. E1). We therefore chose to simulate years with annual peak flows between 5,000 to 13,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, corresponding to approximately the 0.5% to 99.5% percentiles, or return periods of once per 200 years, on average. Historical analysis showed consistent monthly flow percentiles across varying peak flows, allowing us to assume the same seasonal proportions for all simulated years, scaled to each respective peak. Simulations were run with a three-hour timestep beginning with a one-year warmup, followed by the target simulation year (Fig. E2).\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eProxies show the climate of north Africa was significantly wetter during the so-called African Humid Period, between approximately 12,000 and 4,000 BCE \\u003csup\\u003e36,37\\u003c/sup\\u003e. Our results are therefore not representative for this period. However, the Nile watershed has remained relatively consistent with regard to precipitation over the last 3-4 thousand years \\u003csup\\u003e37\\u0026ndash;40\\u003c/sup\\u003e. Climate fluctuations during the last two millennia have been smaller than those in prior millennia, with slightly warmer and drier conditions during the Medieval Climate Anomaly (950-1250 CE) and slightly wetter than average conditions during the Little Ice Age, especially 1325-1470 CE \\u003csup\\u003e37,39\\u0026ndash;42\\u003c/sup\\u003e. By not considering the late 20\\u003csup\\u003eth\\u003c/sup\\u003e century, this model avoids more extreme temperature increases due to climate change \\u003csup\\u003e43\\u003c/sup\\u003e. Our simulation of peak flows between 5,000 and 13,000 m3/s is therefore deemed a reasonable range for the Ptolemaic and surrounding centuries.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe model\\u0026rsquo;s ability to replicate Ptolemaic-era flooding was validated by overlaying a database of 395 georeferenced Ptolemaic sites, including cities, villages, monuments, fortresses, and stations, usually positioned on local high points to minimize inundation risk (Figs. 2 and E3). In a typical flood year, with a peak inflow of 9,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s entering the upstream Nile, nearly all sites remain above or outside the flooded area in our model. It is important to note that the inundated area represents the maximum temporary extent of floodwaters, which do not always occur simultaneously and subsequently infiltrate into the soil or evaporate. The close alignment of flood extents around urban centers and key landmarks is consistent with historical depictions of the Nile in flood, such as the Mosaic of Palestrina\\u003csup\\u003e44\\u003c/sup\\u003e. Modern net irrigated area in the Nile River Valley is 8,600 km\\u003csup\\u003e2\\u003c/sup\\u003e \\u003csup\\u003e45\\u003c/sup\\u003e, whereas estimates of pre-Industrial irrigated area in Middle Egypt are closer to 2,000 km\\u003csup\\u003e2\\u003c/sup\\u003e, though with significant uncertainty due to challenges in georeferencing hand-drawn French maps created in the early 1800s \\u003csup\\u003e2,15\\u003c/sup\\u003e. Our estimate for Middle Egypt and the Thebaid total flood irrigated area during a typical year is 6,400 km\\u003csup\\u003e2\\u003c/sup\\u003e (3,800 and 2,600 km\\u003csup\\u003e2\\u003c/sup\\u003e respectively, Fig. 5a). This estimate is lower than modern agricultural area and higher (but similar to) historical estimates of Middle Egypt, lending greater confidence in the inundated area estimates presented here. Confidence is strongest for the relative effects of flow disruptions on inundated area, even if some bias remains in the absolute area estimates.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e2.2.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Nile Flood Sensitivity\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eDuring the flood season, the flood pulse enters the upstream boundary near Aswan and gradually exceeds the capacity of the Nile channel, filling the adjacent floodplain. This overtopping typically began in July, first near the Qena bend (Fig. 3a) and the Bahr Yussef (Fig. 3b). These regions generally have lower channel slopes, whereas the remainder of the Thebaid (a key political center in Egyptian civilization) is characterized by steeper gradients and a narrower river valley. Middle Egypt, situated downstream between modern Asyut and Cairo, generally contains the widest floodplain and shallowest slope. Once floodwaters exceed the channel capacity in these key areas, water tends to flow parallel to the main channel, following natural depressions, forming secondary channels, and gradually expanding to fill the floodplain. As the seasonal pulse recedes, water is lost through evapotranspiration and infiltration.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eTwo distinct spatial patterns of Nile sensitivity are evident, with the Upper and Middle Thebaid being more sensitive to smaller, more common flow reductions, while the Qena bend and Middle Egypt require more severe droughts to produce major disruptions of irrigable area (Fig. 4). In the more sensitive Upper and Middle Thebaid, the percent change in flooded area rapidly decreases during drought years (Fig. 4). For years with peak flows of 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, inundated area in the upper and middle Thebaid decreased by 63.3% and 67.9%, respectively, relative to a typical year with a peak discharge of 9,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s. Based on historical recurrence (Fig. E1), a 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s event would be classified by the US Drought Monitor as only a \\u0026ldquo;moderate drought\\u0026rdquo;, relatively common events that occur in approximately one of ten years. In more severe droughts, when the flood peak fails to surpass 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, most of the Upper and Middle Thebaid remain dry, with water confined within the immediate Nile banks (Fig. 3, Figs. E8 and E10). In addition to smaller inundated area, drought would also have likely decreased the duration of the flood season, limiting infiltration even in inundated areas. Our model thus reveals the Upper and Middle Thebaid as being agriculturally sensitive even to minor drought years, but incrementally more extreme droughts would have had negligible additional effect because the Nile would largely remain in its banks, leaving most of the \\u0026nbsp;regions\\u0026rsquo; agricultural lands dry below this threshold. \\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThis pattern of drought sensitivity is different from the Qena Bend and Middle Egypt, which are more resilient to moderate drought years, but produce increasingly more catastrophic losses of agricultural area during the most extreme droughts, which occur once every 50-100 years on average based on 19\\u003csup\\u003eth\\u003c/sup\\u003e and early 20\\u003csup\\u003eth\\u003c/sup\\u003e century recurrence (Fig. 4). The topography of these regions is such that both exhibit relatively little loss of inundated area at flow rates down to approximately 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, but beyond which losses become increasingly extreme (Fig. 4). Inundated area for both regions is cut approximately in half for floods of 6,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s and the Nile remains largely within its banks below 5,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s (Fig. 4). For extreme droughts of 5,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, only the flat, low-lying areas around Koptos would be inundated in the Qena bend, while only secondary channels and the Bahr Yussef would be inundated in Middle Egypt (Fig. 3).\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eUnder normal circumstances, Middle Egypt has approximately 1.5 times more inundated area, and thus agricultural capacity (~1,000 km\\u003csup\\u003e2\\u003c/sup\\u003e), than the entire Thebaid, defined as the Upper Thebaid, Qena bend, and Middle Thebaid (Fig. 5a). Because the Upper and Middle Thebaid are very sensitive to moderate droughts, inundated area in the Thebaid decreases by approximately 1,380 km\\u003csup\\u003e2\\u003c/sup\\u003e (53.3%) for years with flood peaks of 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s. This represents a relatively frequent drought, occurring once every 5-10 years, but a major loss of crop area for the region. Middle Egypt would have remained relatively buffered from these frequent and smaller drought years, producing a gap in lost agricultural area between 7,000 and 8,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s (Fig. 5b). However, for more extreme droughts below 7,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, inundated area even in Middle Egypt would have dropped precipitously (Fig. 5). The difference between a moderate and extreme drought, 7,000 and 5,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s respectively, would have been a loss of nearly 2,400 km\\u003csup\\u003e2\\u003c/sup\\u003e of irrigated land, leaving only 14% of a typical year\\u0026rsquo;s agricultural capacity (Fig. 5). This would likely have been devastating for crop yield, but would occur rarely, in less than 1% of years, taken over a long period. Additionally, because both the Thebaid and Middle Egypt rely on the same Nile flows, the Thebaid would experience this extreme loss of agricultural area simultaneously.\\u003c/p\\u003e\\n\\u003cp\\u003eThe smaller Fayum \\u0026ldquo;oasis\\u0026rdquo; area is not directly adjacent to the main stem of the Nile (Fig. 1a), and receives Nile water through the Bahr Yussef channel, which in turn fills Lake Moeris. During the Ptolemaic period, Lake Moeris was a freshwater lake \\u003csup\\u003e34\\u003c/sup\\u003e. Results for the Fayum are included here for completeness but should be interpreted cautiously. Simulations of Fayum irrigable area are complicated by Lake Moeris, which provided some inter-annual storage to buffer drought years \\u003csup\\u003e33\\u003c/sup\\u003e, partially decoupling Fayum agriculture from specific modelled flood magnitudes. It should be noted that during the later Ptolemaic period management of the Fayum entered a state of mismanagement and disrepair \\u003csup\\u003e34\\u003c/sup\\u003e. For these reasons, and the Fayum\\u0026rsquo;s relatively smaller area, we do not further discuss the Fayum. Nonetheless, the vulnerabilities described above offer a new context for the great efforts made by Ptolemaic kings to develop semi-Nile-independent agricultural regions such as the Fayum and repeated costly wars conducted to secure territories in the Eastern Mediterranean that were conspicuously (given our results) capable of rain-fed agriculture \\u003csup\\u003e4,33,46\\u003c/sup\\u003e.\\u003c/p\\u003e\"},{\"header\":\"Discussion\",\"content\":\"\\u003cp\\u003eOur study developed a novel 2D hydraulic flood model of the natural Nile River by integrating historical records, modern bathymetry measurements, and remotely sensed topographic data. The primary objective was to simulate the maximum extent of feasibly irrigated land in Ptolemaic Egypt under gravity-fed conditions, to reveal the scale and geography of agricultural vulnerability to Nile flood disruptions. These simulations represent a theoretical maximum inundation extent, acknowledging that now-absent man-made structures may have redirected or limited flooding patterns. Based on historical validation, the models’ results appear robust, despite inherent uncertainties in reconstructing floodplain behavior prior to georeferenced measurements. Hydraulic simulation focused on the floodplain between Aswan and modern Cairo, excluding the Nile delta, where erosion and sedimentation complicate modelling by causing channel meandering \\u003csup\\u003e26,27,47,48\\u003c/sup\\u003e. Elsewhere, channels have remained relatively stable over the last 2 to 3 millennia \\u003csup\\u003e31\\u003c/sup\\u003e, particularly at our model’s spatial scale (50-200 m grid), allowing for reasonable approximation. The Ptolemaic period climate was not dramatically different from that of the late 1800s-early 1900s, allowing estimates of disruption frequency and severity (Figs. 4 and 5). Our finding that most Ptolemaic villages and towns remained beyond the characteristic flood extent further validates the model’s ability to accurately capture flood behavior.\\u003c/p\\u003e\\n\\u003cp\\u003eSimulations indicate non-linearities and specific thresholds in agricultural vulnerability which vary by region. Moderate droughts would frequently and significantly have disrupted agriculture in the Thebaid. Notably, the regional administrative capital at Ptolemais Hermeiou, in the sensitive Middle Thebaid, would have lost much of its nearby agricultural capacity, even during relatively minor droughts (Fig. E8). The Qena bend would have remained the exception to Thebaid agricultural disruptions. Middle Egypt, with a larger baseline agricultural capacity, would have remained relatively unaffected by moderate droughts. However, during extreme droughts, inundated area would have decreased dramatically, with losses of up to 2,400 km\\u003csup\\u003e2\\u003c/sup\\u003e, leaving only 14% of a typical year’s irrigated area (Figs 4 and 5). These Middle Egypt failures would have occurred simultaneously with losses in the Thebaid, leading to potential disaster in the absence of coping measures.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eWith the Thebaid likely accustomed to frequent and possibly multi-year reductions in irrigated land, the Ptolemaic government and broader Egyptian society doubtless developed such measures. At a state level, grain importation was important \\u003csup\\u003e4,49\\u003c/sup\\u003e. During what were likely harsh (volcanically triggered) drought conditions in the 240s BCE, the state imported food at great expense, though this was considered an extreme measure and the effort later used as to glorify the quality of Ptolemaic rule \\u003csup\\u003e6,46\\u003c/sup\\u003e. However, external warfare, for which the period is famous, would have complicated grain importation. Internally, larger temple estates often secured and managed land parcels throughout Egypt, thus allowing parts of the Thebaid to supplement poor harvest with production from Middle Egypt or the Qena bend \\u003csup\\u003e49\\u003c/sup\\u003e. Internal administrative lapses or civil unrest could, however, impact internal grain transport and other emergency measures, leaving the Thebaid particularly vulnerable. In the most extreme years, agricultural production from Middle Egypt would have decreased precipitously, compounding Thebaid failures. Under these conditions, even a well-functioning state would have struggled to prevent widespread socioeconomic stress.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eRecent work has revealed a repeated temporal association between internal revolt and ice-core-based dates of explosive volcanism known from polar ice-cores, with a causal link posited between volcanically induced monsoon and related Nile flood disruption, suppressed agricultural production, and the resulting political-psychological impacts in a religious context wherein the Ptolemaic rulers (having taken the mantle of Pharaoh) were deemed responsible for ensuring sufficient Nile flooding\\u003csup\\u003e4,5\\u003c/sup\\u003e. However, an environmental context has not been offered for the particular geography of these revolts, most of which (e.g. revolts in 207/206-186, 132, 91, and 88-85 BCE), originated in the Thebaid before spreading downstream to Middle Egypt. While this pattern can be partially explained by the geographic and cultural distance from the Ptolemaic capital of Alexandria, the Greeks established a secondary administrative center at Ptolemais Hermeiou in 310 BCE to help manage the region (Fig. 1). Our results now suggest that the Thebaid’s unique susceptibility to low flow years, even during periods when Middle Egypt remained productive, created a chronic vulnerability that predisposed this region to revolt by fueling public discontent towards the Ptolemaic regime. This contributed to events like the Great Theban Revolt (207/206-186 BCE), which followed a candidate tropical explosive eruption in c.209 BCE\\u003csup\\u003e\\u0026nbsp;4,49\\u003c/sup\\u003e. This revolt, in which the Ptolemies lost control of much of southern Egypt to one or more native claimants to title of Pharaoh, began at Apollonopolis Megale, a particularly drought susceptible area (Fig. E10). Amid popular revolt and drought, the temples at Diospolis Megale (Thebes) and Apollonopolis Megale (Edfu) managed the political response, acting as cultural symbols of kingship and buffering against major drought failures as the largest landowners \\u003csup\\u003e49\\u003c/sup\\u003e.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe impacts of further cases of known hydroclimate stress for the region can also now be better understood. This includes the “volcanic quartet” sequence of four large and closely timed eruptions between 168 and 158 BCE \\u003csup\\u003e3,4\\u003c/sup\\u003e. Climate simulations of an eruption scaled to match the estimated climate forcing from the first and largest eruption in 168 BCE suggest annual mean flow decreases of 29% and 38% across the Nile basin in the first two post-eruption years, respectively \\u003csup\\u003e4\\u003c/sup\\u003e. Given a typical peak discharge of 9,000 m³/s, this would result in consecutive years with flows at or below 6,000 m³/s, creating back-to-back catastrophic flood failures for Middle Egypt and the Thebaid, reducing irrigated land to only 25% of normal levels. Two or more such consecutive failures would have presented extraordinary challenges. Indeed, these register abundantly in the 160s BCE, with internal revolts and the near conquest of Egypt by the rival Seleucid empire, an event only prevented by self-interested Roman intervention. The three subsequent smaller extratropical eruptions showed more moderate simulated reductions in discharge (~10%) and would have primarily affected the Thebaid. This combination of Egypt-wide catastrophe with continued Thebaid-centric failures provides a reasonable hypothesis as to a climatic driver pushing Thebaid revolts against the Ptolemaic government in these years.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eOverall, this study demonstrates how droughts within the Nile watershed translated nonlinearly into regional agricultural failures. The findings align closely with historical records of contemporaneous Ptolemaic sites and highlight the particular vulnerability of the Thebaid, which now assists in explaining the need for enhanced administrative oversight and the region’s repeated role as the origin of revolts. Simulations of known volcanic eruptions during this era imply multi-year agricultural failures affecting nearly all of Egypt, which would have had potentially devastating consequences. Contemporaneous sources explicitly reference drought and a lack of irrigation throughout this period. The hydraulic flood model developed here provides a valuable tool for investigating natural flood behavior along the Egyptian Nile over the past two millennia, as well as better contextualizing the human history of the period, prior to the construction of major reservoirs.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e8. \\u0026nbsp; Acknowledgements\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis research has been financially supported by the US National Science Foundation Dynamics of Coupled Natural and Human Systems Program (grant no. 1824770) and the Yale Planetary Solutions (YPS) Grant Program. Organizational support was provided by the Byrd Polar and Climate Research Center and the Yale Peabody Museum of Natural History.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e9.\\u0026nbsp; \\u0026nbsp;Author Contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e10.\\u0026nbsp;JHS: conceptualization, methodology, formal analysis, visualization, writing (original draft). IFM: data curation, resources. JM: data curation, resources. FML: conceptualization, resources. SM: resources. JGM: conceptualization, resources. All co-authors contributed to review and editing of the text.\\u003cbr\\u003e\\u0026nbsp;\\u003cbr\\u003e\\u003cstrong\\u003e5. Competing Interests Statement\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eNone of the authors has any competing interests.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e11.\\u0026nbsp;Additional Information\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eExtended Data is available for this paper.\\u003cbr\\u003e\\u0026nbsp;\\u003cbr\\u003e\\u0026nbsp;Correspondence and requests for materials should be addressed to James H. Stagge (stagge.11@osu.edu\\u003c/a\\u003e).\\u003c/p\\u003e\\n\\u003cp\\u003eReprints and permissions information is available at www.nature.com/reprints.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eWillcocks, W. \\u003cem\\u003eEgyptian Irrigation\\u003c/em\\u003e. (London, Spon, 1913).\\u003c/li\\u003e\\n\\u003cli\\u003eWillems, H., Creylman, H., De Laet, V. \\u0026amp; Verstraeten, G. 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A Hydrologic Budget Analysis for the Nile Valley in Egypt. \\u003cem\\u003eGround Water\\u003c/em\\u003e\\u003cstrong\\u003e24\\u003c/strong\\u003e, 453\\u0026ndash;459 (1986).\\u003c/li\\u003e\\n\\u003cli\\u003e Austin, M. M. \\u003cem\\u003eThe Hellenistic World from Alexander to the Roman Conquest: A Selection of Ancient Sources in Translation\\u003c/em\\u003e. (Cambridge University Press, 2006).\\u003c/li\\u003e\\n\\u003cli\\u003e El Bastawesy, M., Gebremichael, E., Sultan, M., Attwa, M. \\u0026amp; Sahour, H. Tracing Holocene channels and landforms of the Nile Delta through integration of early elevation, geophysical, and sediment core data. \\u003cem\\u003eThe Holocene\\u003c/em\\u003e\\u003cstrong\\u003e30\\u003c/strong\\u003e, 1129\\u0026ndash;1141 (2020).\\u003c/li\\u003e\\n\\u003cli\\u003e Stanley, J.-D. Egypt\\u0026rsquo;s Nile Delta in Late 4000 Years BP: Altered Flood Levels and Sedimentation, with Archaeological Implications. \\u003cem\\u003eJournal of Coastal Research\\u003c/em\\u003e\\u003cstrong\\u003e35\\u003c/strong\\u003e, 1036 (2019).\\u003c/li\\u003e\\n\\u003cli\\u003e Manning, J. G. \\u003cem\\u003eLand and Power in Ptolemaic Egypt : The Structure of Land Tenure\\u003c/em\\u003e. (Cambridge University Press, Cambridge, UK ; New York, 2003).\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"},{\"header\":\"Methods\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e6.1.\\u0026nbsp;Hydraulic Model\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAll hydraulic modeling was performed using the US Army Corps of Engineers (USACE) HEC-RAS model \\u003csup\\u003e1\\u003c/sup\\u003e. HEC-RAS is a one- or two-dimensional hydraulic modeling platform widely applied for flood inundation analysis on major waterways in the U.S. and internationally.\\u003c/p\\u003e\\n\\u003cp\\u003e6.2. \\u003cstrong\\u003eFloodplain Topography\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eFloodplain elevations, outside of the main Nile channel, were based on the Multi-Error-Removed Improved-Terrain Digital Elevation Model (MERIT-DEM) \\u0026nbsp;\\u003csup\\u003e2\\u003c/sup\\u003e. MERIT-DEM is a highly accurate \\u003csup\\u003e3\\u003c/sup\\u003e global digital elevation model with a spatial resolution of 3 arcseconds, or approximately 80-90 m in Egypt. It is designed specifically for hydrologic modeling and merges multiple remote sensing datasets, including NASA SRTM3, JAXA AW3d-20m and Viewfinder Panorama's DEM \\u003csup\\u003e2\\u003c/sup\\u003e. In the data merging process, MERIT-DEM corrects for dataset bias, stripe noise, speckle noise, and tree height bias. As a result, MERIT-DEM has a vertical accuracy of 2 meters or better over 58% of the global land area covered by the product, a significant improvement over prior methods \\u003csup\\u003e2\\u003c/sup\\u003e.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e6.3. \\u003cstrong\\u003eRiver channel geometry\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAlthough MERIT-DEM effectively captures floodplain elevations, satellite-based elevation products cannot penetrate deep or occluded water surfaces like those of the Nile River. To address this limitation, we combined MERIT-DEM data with river bathymetry from a published one-dimensional hydraulic model of the modern Nile between Aswan and Cairo (Samir, Moussa, and El-Badry 2019). The river centerline was manually digitized from this study (Samir, Moussa, and El-Badry 2019). River right and left banks were delineated using areas classified as permanent water in the JRC\\u0026rsquo;s Global Surface Water Explorer \\u003csup\\u003e4\\u003c/sup\\u003e. Nile cross-sections were assumed to be triangular, derived by connecting the river centerline to bank elevations. River surveys from 1904 \\u003csup\\u003e5\\u003c/sup\\u003e were used to validate this triangular assumption and to confirm there was minimal changes in centerline elevation through time. Additional channel validation was performed using modern reach-specific studies \\u003csup\\u003e6\\u003c/sup\\u003e. No dams were included in the hydraulic model.\\u003c/p\\u003e\\n\\u003cp\\u003eThe Bahr Yussef channel was generated by digitizing the channel centerline from the Global Surface Water Explorer \\u003csup\\u003e4\\u003c/sup\\u003e. This modified natural channel was assumed to be trapezoidal, with a 30m bottom width, 2:1 side slopes, and a 50 m maximum top width. These assumptions align with surveyed measures of the modern Bahr Yussef \\u003csup\\u003e7\\u003c/sup\\u003e. Bathymetry of both the Nile and Bahr Yussef channels was then interpolated and merged with MERIT-DEM data, with river channel bathymetry always taking precedence.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e6.4. \\u003cstrong\\u003eLand cover, infiltration, and evapotranspiration\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eSurface roughness estimates are necessary to estimate momentum loss. Land cover was estimated based on MDA\\u0026rsquo;s BaseVue 2013 land cover data \\u003csup\\u003e8\\u003c/sup\\u003e. BaseVue has a 30m spatial resolution and is derived from Landsat 8 remotely sensed measurements. BaseVue uses 13 land cover classes, though only a subset appeared within the model extent. To approximate pre-modern land cover, urban areas were assumed equivalent to the most dominant surrounding grid cells, generally becoming agricultural land. Manning\\u0026rsquo;s roughness values of 0.035 were used for grassland and agricultural land, while roughness values of 0.1, 0.03, 0.025, and 0.07 were used for shrub/scrub, open water, barren rock, and wetlands, respectively. These values are approximately the mid-points of those recommended by the US Army Corps of Engineers \\u003csup\\u003e9\\u003c/sup\\u003e, adjusted to accommodate the slightly different land cover classes used in the BaseVue dataset.\\u003c/p\\u003e\\n\\u003cp\\u003eIn order to generate realistic behavior during the flood recession, the hydraulic model includes soil infiltration and evapotranspiration losses to the atmosphere. Infiltration was calculated using the Deficit and Constant loss method \\u003csup\\u003e1\\u003c/sup\\u003e, with maximum percolation rates of 2.5 mm/hr assumed for grassland and agricultural land, and 5.5 mm/hr assumed for shrub/scrub and barren rock. These rates are reasonable given prior estimates of vertical hydraulic conductivity for the Nile \\u003csup\\u003e10\\u003c/sup\\u003e. No infiltration was assumed within the main Nile river channel. The evapotranspiration rate was based on monthly mean temperature and applied equally across the study area. Evapotranspiration rates peak at 0.509 mm/hr in June and fall to 0.152 mm/hr in December. These values are within typical ranges assumed for the region \\u003csup\\u003e11\\u003c/sup\\u003e.Flood inundated area was generally not sensitive to infiltration or evapotranspiration rates because these processes only cleared standing water following the flood season. Because we simulated individual years, there was no hold-over storage on the land and thus negligible effects from infiltration or evapotranspiration.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.5.\\u0026nbsp;Flow Data\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eRepresentative flows were based on mean monthly discharge measurements at the Aswan and Dongola gauging stations for each year between 1869 and 1958 \\u003csup\\u003e12\\u003c/sup\\u003e. These gauge observations were adjusted in the original study to account for upstream irrigation withdrawals, creating the best estimate of a near-natural record for this period. From this, we compiled a 90-year composite dataset by combining the Aswan and Dongola records, always using the larger of the two values, and generally using the downstream Aswan gauge prior to the early 1900s and then shifting to the upstream Dongola gauge after the completion and raising of the Aswan low dam. The time series ends in 1958 to avoid the influence of the Roseires Dam on the Blue Nile, which coincided with a significant decrease in annual maximum flow. During the period, assumed to be represented of natural flows, annual maxima were stable, with no long-term trends (Fig. E1). The data follow a normal distribution, with a mean of 9,030 m\\u003csup\\u003e3\\u003c/sup\\u003e/s and standard deviation of 1,550 m\\u003csup\\u003e3\\u003c/sup\\u003e/s. We therefore chose to simulate representative near-natural peak flows ranging from 5,000 to 13,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s, capturing percentiles from 0.5% to 99.5%, or approximately a 1-in-200 year event. Historical analysis showed consistent monthly flow percentiles across varying peak flows, allowing us to assume the same seasonal proportions for all simulated years, scaled to each respective peak (Fig. E2). We did not consider any direct rainfall within the model. This is a reasonable assumption given the relative paucity of rainfall in Egypt relative to the incoming flow at Aswan \\u003csup\\u003e11\\u003c/sup\\u003e.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.6.\\u0026nbsp;Hydraulic Simulation\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAll simulations used the diffusive wave equation for the conservation of momentum. The computational mesh for flow modeling was approximately 50 m within the Nile river channel and transitioned smoothly to a 250 m spatial resolution in the floodplain, creating a computational mesh of 1.14 million cells. Upstream boundary conditions were assumed equal to flow entering the study area at Aswan, while downstream boundary conditions assumed normal depth with a friction slope of 0.00006 m/m, matching the Nile\\u0026rsquo;s slope at Cairo \\u003csup\\u003e5\\u003c/sup\\u003e. Normal depth was computed separately for each computational cell face along the downstream boundary. Simulations included a warmup period of 1,000 m\\u003csup\\u003e3\\u003c/sup\\u003e/s constant flow for a year, followed by the year to be simulated (Fig. E2). This ensured the model reached steady state prior to the flood simulation. The computational interval was 3 hours, with output reported daily.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.7.\\u0026nbsp;Inundated Area and Regionalization\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eRegions used for inundated area calculations were based on administrative areas of Ptolemaic Egypt. These included the Upper Thebaid (Aswan to Esna), Qena Bend (Esna to Nag Hammadi), Middle Thebaid (Nag Hammadi to Assiut/Abydos), Middle Egypt (Assiut to modern Cairo), and the Fayum. While hydraulic simulations used computational cells with spatial resolutions between 50 and 250 meters, post-processing was performed in HEC-RAS to interpolate the land surface elevation, producing inundated area estimates with a gridded resolution of 25x25 meters.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eInundated area was quantified by calculating the proportion of time each 25x25 meter grid cell was submerged during the simulation year (excluding the warmup). A cell was defined as being inundated for a simulation if it was covered by water for at least 5% of the simulation period, approximately 18 days from March to March. This approximately represents the peak of the flood season in simulations. In sensitivity tests, the results were robust to this threshold. Only areas outside the main channel and permanent water bodies were included in the inundated area analysis. These areas would not be fit for agricultural practices. Grid cells submerged for 95% or more of the year were excluded to avoid overestimating the inundated area.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.8.\\u0026nbsp;Ptolemaic Sites\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe online data repository Trismegistos Places\\u003csup\\u003e13\\u003c/sup\\u003e was employed in collating a preliminary list of settlements attested more than once in historical texts of the Ptolemaic Period. The Trismegistos Places database aggregates topographical data attested in the published documentary record of Greco-Roman Egypt\\u003csup\\u003e13\\u003c/sup\\u003e. Only a fraction of these sites can be positively identified with modern toponyms. Trismegistos relies upon regional surveys performed by ancient historians identifying modern settlements with ancient toponyms, not all of which agree in individual cases. Following the selection of 395 individual places, each point was manually geolocated using Google Earth and converted to a point shapefile with an associated attribute table. In cases of disagreement between identifications in scholarly literature, a location was selected on the merits of the respective arguments. A small number of additional identifications were made and included in this topographical dataset.\\u003c/p\\u003e\\n\\u003cp\\u003eThe distribution of confirmed identifications is geographically discontinuous across our study area, clustering in the Fayum, Middle Egypt, and the Qena bend. Fortunately, the major population centers (Gr. \\u003cem\\u003emetropoleis\\u003c/em\\u003e) of individual districts of Egypt can be identified in nearly every case due to preservation of this knowledge from the ancient period through to the present day in historical and geographical literature. Furthermore, population estimates for individual districts and larger subdivisions of ancient Egypt have been generated through the analysis of archaeological surveys and administrative texts such as census documents, permitting general observations on population density. By contrast, smaller villages can typically be identified in two circumstances: (1) the ancient Egyptian toponym has been preserved as its primary referent through the Medieval and Modern Periods, and (2) archaeological remains of the ancient town survive. The former is typical of Middle Egypt, while the latter is characteristic of the Fayum. Because only a fraction of these ancient villages can be positively identified with modern settlements, and because the nature of the Nile floodplain restricts the limits of permanent settlements and encourages continuity of occupation, it can be assumed with some degree of confidence that many modern settlements that have not yet been identified with an ancient toponym do in fact correspond with these ancient places. The primary hindrance to this endeavor is the historical tendency to rename villages with Greek and later Arabic toponyms that bear no resemblance to the ancient Egyptian equivalents. Nevertheless, the distribution of positive identifications within each region is relatively homogenous and facilitates general observations on the relationship between settlement concentrations and floodplain topography, particularly the contrasting relationship between settlement density and floodplain topography in the Thebaid and Middle Egypt.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.9.\\u0026nbsp;Data Availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe Hec-Ras model and all output is provided via an open-access repository (Figshare). The Hec-Ras model and all relevant data are stored in a folder labeled \\u0026ldquo;Nile flood model\\u0026rdquo;, while output is still in a folder labeled \\u0026ldquo;Output\\u0026rdquo;.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e6.10.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Code Availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eAll code used to analyze the Hec-Ras output is available in the same open-access repository (Figshare) in a separate folder titled \\u0026ldquo;Code\\u0026rdquo;.\\u003cbr /\\u003e\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e7.\\u0026nbsp; \\u0026nbsp;Methods Section References\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e1.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Hydrologic Engineering Center. \\u003cem\\u003eHEC-RAS 2D Modeling User\\u0026rsquo;s Manual\\u003c/em\\u003e. (U.S. Army Corps of Engineers, Davis CA, 2021).\\u003c/p\\u003e\\n\\u003cp\\u003e2.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Yamazaki, D. \\u003cem\\u003eet al.\\u003c/em\\u003e A high-accuracy map of global terrain elevations. \\u003cem\\u003eGeophysical Research Letters\\u003c/em\\u003e \\u003cstrong\\u003e44\\u003c/strong\\u003e, 5844\\u0026ndash;5853 (2017).\\u003c/p\\u003e\\n\\u003cp\\u003e3.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Uuemaa, E., Ahi, S., Montibeller, B., Muru, M. \\u0026amp; Kmoch, A. Vertical Accuracy of Freely Available Global Digital Elevation Models (ASTER, AW3D30, MERIT, TanDEM-X, SRTM, and NASADEM). \\u003cem\\u003eRemote Sensing\\u003c/em\\u003e \\u003cstrong\\u003e12\\u003c/strong\\u003e, 3482 (2020).\\u003c/p\\u003e\\n\\u003cp\\u003e4.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Pekel, J.-F., Cottam, A., Gorelick, N. \\u0026amp; Belward, A. S. 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R. \\u003cem\\u003eet al.\\u003c/em\\u003e Bahr Yousef meandering canal management \\u0026ndash; Hydraulic and morphological assessment. \\u003cem\\u003eWater Science\\u003c/em\\u003e \\u003cstrong\\u003e39\\u003c/strong\\u003e, 28\\u0026ndash;41 (2025).\\u003c/p\\u003e\\n\\u003cp\\u003e8.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;MDA Information Systems US. World Land Cover 30m BaseVue 2013. (2013).\\u003c/p\\u003e\\n\\u003cp\\u003e9.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;U.S. Army Corps of Engineers. \\u003cem\\u003eCreating Land Cover, Manning\\u0026rsquo;s N Values, And % Impervious Layers\\u003c/em\\u003e. (2021).\\u003c/p\\u003e\\n\\u003cp\\u003e10.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Attia, F. A. R., Allam, M. N. \\u0026amp; Amer, A. W. A Hydrologic Budget Analysis for the Nile Valley in Egypt. \\u003cem\\u003eGround Water\\u003c/em\\u003e \\u003cstrong\\u003e24\\u003c/strong\\u003e, 453\\u0026ndash;459 (1986).\\u003c/p\\u003e\\n\\u003cp\\u003e11.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Sutcliffe, J. V. The Hydrology of the Nile Basin. \\u003cem\\u003eThe Nile\\u003c/em\\u003e 335\\u0026ndash;364 (2009) doi:10.1007/978-1-4020-9726-3_17.\\u003c/p\\u003e\\n\\u003cp\\u003e12.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Hurst, H. E. \\u0026amp; Phillips, P. \\u003cem\\u003eThe Nile Basin: Ten-Day Mean and Monthly Mean Gauge Readings...-v. 4. Ten-Day Mean and Monthly Mean Discharges\\u003c/em\\u003e. vol. 4 (1960).\\u003c/p\\u003e\\n\\u003cp\\u003e13.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;H. Verreth, \\u003cem\\u003eA survey of toponyms in Egypt in the Graeco-Roman period\\u003c/em\\u003e (Trismegistos Online Publications, 2), Leuven: Trismegistos Online Publications, 1253.\\u0026nbsp;www.trismegistos.org/geo/index.php\\u003c/p\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-6968703/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-6968703/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"The Nile River’s annual flood sustained Egyptian agriculture for millennia, overflowing its banks and enabling gravity-fed irrigation in a desert climate\\u003csup\\u003e1,2\\u003c/sup\\u003e. Consequently, poor flood years could produce severe agricultural failures, which are hypothesized to have promoted civil unrest during the famous Ptolemaic era (305–30 BCE)\\u003csup\\u003e3–5\\u003c/sup\\u003e. Here we show, using a novel two-dimensional hydraulic model of the Egyptian Nile, that even moderate flow reductions produced large declines in feasibly irrigated area, but with marked regional variation. These impacts were more frequent in the Thebaid region, where minor flow reductions would exclude much of the agricultural land from irrigation. By contrast, irrigation in Middle Egypt would remain stable under moderate flow reductions but faced catastrophic losses during rare, extreme droughts. These findings align with historical records of societal unrest and help explain the Thebaid’s repeated role in originating revolt movements. Further, our findings contextualize the human impact of the many major volcanic eruptions during the Ptolemaic period, which would have produced multi-year catastrophic agricultural failures across the entire region \\u003csup\\u003e6\\u003c/sup\\u003e. This work demonstrates how external climate shocks likely cascaded through societies dependent on floodplain agriculture and highlights the vulnerability of historical administrative systems to environmental stress.\",\"manuscriptTitle\":\"Nile floods reveal Ancient Egypt's pattern of revolts\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-06-26 04:49:22\",\"doi\":\"10.21203/rs.3.rs-6968703/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"7fedc855-7825-4503-9128-b421b4c87733\",\"owner\":[],\"postedDate\":\"June 26th, 2025\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":50592935,\"name\":\"Scientific community and society/Social sciences/History\"},{\"id\":50592936,\"name\":\"Earth and environmental sciences/Hydrology\"},{\"id\":50592937,\"name\":\"Scientific community and society/Water resources\"}],\"tags\":[],\"updatedAt\":\"2025-07-07T14:15:39+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2025-06-26 04:49:22\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-6968703\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-6968703\",\"identity\":\"rs-6968703\",\"version\":[\"v1\"]},\"buildId\":\"8U1c8b4HqxoKbykW_rLl7\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}