Water, water everywhere and not a streamflow gauge in sight: Estimating freshwater inflows in an ungauged watershed at the Big Boggy National Wildlife Refuge, USA

Abstract Bays and estuaries often rely on freshwater inflows to maintain the salinity balance necessary to sustain their fisheries. Reduced freshwater inflows, particularly during summer seasons, can be detrimental to the health of these systems. Despite an extensive network of streamflow gauges in the U.S., many coastal watersheds remain ungauged. Our objective was to identify a way to quickly build a water budget for an ungauged watershed, using limited data, to determine its freshwater inflow contributions to an estuary. We developed and tested this method for Big Boggy Creek, which flows into East Matagorda Bay (EMB), Texas. Streamflow into and out of Big Boggy Creek was quantified at key upstream and downstream points. Over the study period, we found average monthly freshwater inflows of 1,897 megaliters (ML). Historic and predicted freshwater inflows over a longer time period were calculated using a data-driven model, based on our water budget. Historic monthly inflows for this model averaged 2,000 ML from 1953 to 2019 for Big Boggy Creek. Predicted inflows followed three climate scenarios outlined by the Intergovernmental Panel on Climate Change. Using these findings, we developed an inflow decision tool to assist resource managers in assessing future freshwater inflows into EMB. We show two inflow improvement actions for EMB that can be informed by using this tool—purchasing water from the local river authority and increasing the effective watershed. The approaches developed in this study can be applied to similarly ungauged watersheds to budget, model, and predict their freshwater inflow contributions to estuaries.