The Effect of Papyrus Wetlands on Flow Regulation in a Tropical River Catchment

Papyrus wetlands, native to central and eastern Africa and attaining heights up to 5 to 6 m, occur as rooted or floating marshes in riverine and lacustrine landscapes. This study assessed and quantified the impacts of these papyrus wetlands on historical baseflow and quickflow, and future flood and low flows in the Mpologoma catchment in Uganda. By estimating the Wetland Specific Impact (metric of increase or decrease in flow to wetland area) for a 30-year historical period and indices of flow duration, magnitude, and frequency for the future period, it was possible to demonstrate the role that papyrus-dominated floodplain wetlands play in regulating river flow at the catchment scale. Assessment over the historic period shows that wetlands strongly attenuate quickflow while moderately enhancing baseflow. They play a moderating role in most months, except for the first dry season (June and July), due to the reversal of flows between wetlands and rivers that often occurs during this period. Annual estimates show that wetlands are four times better at regulating quickflow than baseflow. Examination of changes at 2 and 4°C global warming levels (GWLs) indicate that wetlands will play critical roles in mitigating flood risks, with a lesser role in supporting low flows. Wetlands are predicted to lower future mean flood magnitude by 5.2 and 7.8% at GWL2 and GWL4, respectively, as well as halving the average number of flood events in a year, irrespective of the warming level. However, the mean flood duration is expected to increase by 77.8% (56 days) and 80.3% (49 days), respectively. This study highlights the need for the conservation and protection of papyrus-dominated wetlands.