Landcover dynamics and hydological functioning of wetlands in the Usangu plains in Tanzania

In the Usangu Plains of the Upper Great Ruaha River Catchment in Tanzania, the hydrology of the wetlands and the downstream flow regime changes in response to changes in land use and cover was investigated through analysis of remotely sensed images and modelling. A hydrological model for the Eastern wetland that accounts for the water balance was developed. This model was used to evaluate the hydrology of the Eastern wetland in response to changes in land use and cover and the amount of inflows into the Eastern wetland required to maintain a specified outflow downstream of the wetland. A small wetland locally called Ifushiro located in the upstream of the Eastern wetland was also investigated to evaluate its contribution to dry season flows. The analysis involved a detailed hydrometric monitoring and modelling using Visual MODFLOW software. The Ifushiro wetland was shown to have no contribution to dry season flow, since much of its water was lost through evaporation. The hydrology of the Eastern wetland was shown to be modulated by the changes in land use and cover on the upstream. Since 1958. increasing diversions of water has caused average dry season inflows to the Eastern wetland to decrease from approximately 15.0 mV to 4.3 mV. This has led to a reduction in the average minimum dry season surface area of the wetland from approximately 160 km2 to 93 km2. Since the early 1990s the decrease in dry season water-levels within the wetland has resulted in prolonged periods of zero flow in the Great Ruaha River, with severe consequences for the ecology of the Ruaha National Park. The wetland model enabled calculation of the inflows required to maintain specified discharges. To maintain a flow of 0.5 m’s'1, as the minimum required flow for maintenance offishiii habitat and the current ecology of the Park, requires an average dry season inflow of approximately 7.0 mV into the Eastern wetland in the dry season. The results from this research demonstrate the value of combining different research methods/approaches and the use of simple models to examine system functioning to assist decision-making.

The International Water Management Institute (IWMI) and the RIPARWIN (Raising Irrigation Productivity and Releasing Water for Inter-sectoral Needs) Project, and the Soil-Water Management Research Group (SWMRG) of Sokoine University of Agriculture