WATER ALLOCATION AND FUTURE WATER MANAGEMENT FOR THE KILOMBERO WATERSHED

Abstract

Kilombero watershed experiences water conflicts due to attractive natural resources, fertile land, water resources, extensive grazing land, reliable rains, and Tanzania’s Government policy for agriculture development, which aims to attain National Development Vision by 2025. The Policy set aside the Kilombero Watershed as the target area for agricultural growth, especially in expanding irrigated commercial and subsistence food production areas. Competition over water also affects the recommended minimum environmental streamflow required for sustaining the ecosystem, including different bird species, flora, and fauna in the watershed.This research used Water Evaluation and Planning System (WEAP) as a computer-based Decision Support Systems (DSS) to develop proper water allocation planning, and simulating water allocation for current and future scenario in response to an increased population and irrigation activities. The study’s main objective is to develop water allocation frame work for Kilombero watershed using the Water Evaluation and Planning System Model (WEAP) to minimize water conflict problems. The model is structured to link water demand and supply.The rainfall-runoff modeling was used to evaluate water availability in the catchment and simulating two scenarios (increased population growth rates and irrigation activities) with a current account (1995) and reference period (1996 - 2050) to predict possible impacts on the water balance and water allocation due to varied water demands. Data used for calibration and validation of the model include meteorological data (Precipitation & Temperature, 1995 - 2015) acquired from a global climatic model provided by RBWO, LULC (Landsat 8 OLI/TIRS C2 L1), Water demand data (The Ministry of Water (https://design.maji.go.tz/index.php/Main_Page), Social - Economic data (Tanzania National Bureau of Statistics), and Digital Elevation Model, DEM (SRTM, resolution of 90m). The model performance as shown by statistical indices during calibration (NSE = 0.991, MSE = 0.995 and PBIAS = -2.47%), and validation (NSE = 0.931, MSE = 0.974 and PBIAS = -6.61%) shows that the modal performed well and can be used for the water allocation for different activities in the watershed. The result showed that, for the reference scenario (current situation), the water demands for all the activities within the watershed can be fulfilled. This shows that water conflicts arising in the iv Kilombero watershed are due to insufficient water allocation plans mainly and supply shortages to a lesser extent. However, the supply deficit was observed for the future scenarios of increased population growth (human and livestock) from 2025 and continue to 2050. Also the defict was observed in increased irrigation activities from 2017 and continue to 2050. This situation may lead to the water conflict in all of the future scenarios. Some intervention for water management aspects tested by establishing a water-saving technique to reduce water deficit. The intervention included water reuse rate and demand side management. When the reuse rate and demand-side management evaluated, the average annual water demand was reduced by 29% , and in the increased irrigation activities scenario, the average annual water demand was reduced by 38%. After simulating the demand-side management, the deficit in future scenario 1 (population growth scenario) was observed in 2032 instead of 2025. Also, the deficit in future scenario 2 (increased irrigation activities scenario) was observed in 2026 instead of 2017. The water allocation framework for the Kilombero watershed is strongly recommended because it can play a crucial role in addressing the challenges and needs. However, implementing the framework plans will depend on the system capacity, political will, and understanding of the water allocation framework concepts.