Characterising the Hydrological Regime of a Tropical Papyrus Wetland in the Lake Kyoga Basin, Uganda

Abstract

Papyrus wetlands are predominant in permanently flooded areas of Sub-Saharan Africa, covering approximately 40,000 km2 in East and Central Africa. Previous studies have quantified and valued ecosystem services of these wetlands, but there is still a need to understand the key processes of the wetlands’ hydrology. The study objective was to quantify the seasonal variations in water balance and retention time of a papyrus wetland section. Discharge measurements were carried out to quantify channel flow in upstream and downstream parts of the wetland section. Groundwater fluxes were estimated using borehole triangulation of groundwater levels, while precipitation and other climatic variables for estimating evapotranspiration were monitored using an automatic weather station located 1 km from the study site. The retention time of the wetland section was estimated from tracer experiments and by calculating the volume-discharge ratio. Results show that the water balance of the section is dominated by wetland channel flow, contributing approximately 99.7% of the total inputs, while precipitation and groundwater discharge contribute approximately 0.2% and <0.1%, respectively. Estimated retention time varied between 2 hours and 7 days during periods of high and low flows, respectively. The groundwater gradient showed flow towards the wetland throughout the monitoring period, with average gradients of 0.0074 and 0.0043 on the western and eastern edges of the wetland, respectively. Since wetland channel flow is dominant, the wetland’s hydrology is vulnerable to land cover changes and resultant changes in surface runoff from the upstream catchment. Further research on impacts of land use changes within the upstream catchment on wetland channel flow is recommended.