Hydrological and Water Quality Characterisation of a Tropical Riverine Wetland: Nabajjuzi-Masaka, Uganda (original) (raw)

Abstract In East Africa where wetlands are becoming essential systems for waste water treatment and potable water supply, hydrological characterisation is crucial to understand the dynamics of their functioning on a spatial and temporal scale. Beside ecosystem services, basic ecological properties such as vegetation structure and nutrient cycling of wetland ecosystems depend on hydrology. Nabajjuzi wetland, in central Uganda, East Africa is internationally recognised as a Ramsar site and currently used for potable water supply in addition to other essential ecological and social economic services to the riparian communities. To contribute to the limited and scanty information of this ecosystem, the study focussed on hydrological and water quality characterisation using a water-nutrient mass balance as the major output. Also, the flow dynamics of Fe and allochthonous suspended solids loading into the wetland were investigated. Results show that the water balance of Nabajjuzi wetland is dominated by surface flow, which is influenced by rainy and dry periods. During peak flow, the estimated hydraulic retention time was found to be 1 month compared to 1.5 months during the low flow period. Also, water loss flux due to evapotranspiration in this region is high and in Nabajjuzi wetland, it is twice the daily rate of abstraction. Analysis of long term flow data also revealed that the current water abstraction rate can potentially result in hydrological stress to the system during extremely low flow periods. However, the wetland N and P loading is very low compared to other urban disturbed wetlands. Consequently, high concentration of Fe and suspended solids loading are the major constraints to the surface water quality hence potable water supply. Generally, there is need for a more comprehensive hydrological-water quality study and careful planning of the future catchment land use strategies. This can enhance management and conservation of the wetland to guarantee the crucial ecosystem services it provides. Keywords: Hydrology, Riverine wetland, Hydrological gradient, Surface-flow, Nutrients, Iron, Mass-balance, Lake Victoria basin.