Assessment of spatial water balance of wami river basin (Tanzania) using invest water yield model

Water resources planning and management in Wami River Basin is to a large extent not based on how much water resources is a available in the basin but rother is driven by water resources demand by different users. This is due to lack sufficient knowledge of estimated spatial distribution of water balance components (Hutchrantz, 1997). Such situation poses difficulties for sustainable water resources development planning and management at catchment, national, regional and international platforms. The objective of this research was to assess spatial distribution of water balance components (actual evapotranspiration and water yield). The study deployed InVEST model developed by Natural Capital Project in USA to calculate and approximate water yield and actual evapotranspiration for each raster cell in a catchment. The model inputs are annual raster maps for precipitation, reference evapotranspiration land use land cover, soil depth and plant available water content at annual time scale. ArView 3.2a; Arc GIS 9.2 and ILWIS techniques have been used to process input data for running the InVEST model. The study has shown that average annual water yield in Wami River Basin is 1,782×〖10〗^6 m^3. Accuracy of water yield estimation was checked by observation of catchment outlet annual flows and it has been found to be 90% of the observed water yield. Further, the research shows that there is high water yield in forested areas as compared to non forested areas. This information is very useful guide to stakeholders as a basis for decision making for efficient utilization and management of land and water resources at catchment level.