Restoring the perennial Great Ruaha River using ecohydrology, engineering and governance methods in Tanzania

Abstract

The Great Ruaha River (GRR) in Tanzania was perennial before 1993. Its source, the Usangu wetlands, was also perennial. Since then, the GRR has started drying out during the dry season, with a trend towards earlier and longer periods of drying. This drying process degrades the surrounding ecosystems along the entire length of the GRR, including the Ruaha National Park (RNP) and impacts human livelihoods throughout its course; it also impairs the economy of Tanzania through reduced hydropower generation at the Mtera and Kidatu power plants. The Usangu wetlands dried up in 2000, 2002 and 2005 during the dry season and its areal extent has been shrinking. Intensive livestock grazing and both dry and wet season irrigated agriculture in the Usangu wetlands, were the main reasons for this water crisis. In 2006, the Government of Tanzania moved to address the crisis by removing livestock from the Usangu wetlands, attempting to regulate water use in the GRR catchment and expanding the RNP to include the Usangu wetlands.The present study uses satellite radar altimetry data dated 1992 to 2015 for the Usangu wetlands, verified using data from water level loggers, to assess the effects of these hydrological changes. The combined data sets show that the perennial Usangu wetlands have become re-established, though somewhat diminished in size, and that as a result of eutrophication from fertilizers leached from irrigated fields, 95% of their surface is now covered by floating vegetation, resulting in reduced loss of water by evaporation. This reduction has extended the water flow to the GRR by one to two months. These figures highlight the important role played by the wetlands vegetation in controlling the water budget.The hydrological model of the Usangu wetlands was further developed and verified, and then used to justify the construction of a low-level V-notch weir at the outlet of the Usangu wetlands; by slowing down the outflow from the wetlands, this construction would reduce the period of zero flow in the GRR during the dry season by one to two months. Additional ecohydrological solutions based on the improvement of water governance upstream of the Usangu wetlands, such as controlling rice and horticultural irrigation activities along the length of the GRR, should be integrated with this engineering solution; this would meet the hydrological needs of the water users all along the GRR.