PREDICTION OF STREAM FLOWS IN MAI-NEFHI RESERVOIR BY HYDROLOGIC MODELLING USING SOIL AND WATER ASSESSMENT TOOL (SWAT)

The yield of any surface water source mainly depends on inflow of the reservoir and that impacts the availability of drinking water for the cities in future. Assessment of long-term impacts of projected changes in reservoir level is critical to address the challenges in protecting surface water reserves. In other words, to deal with water management issues, one must analyse and quantify changes of flow taking place within the area and causes. Obviously, this analysis must be carried out on a watershed basis because all these processes are taking place within individual micro watersheds. The development of remote sensing and GIS techniques has allowed the use of spatially and physically based hydrologic models to simulate as simply and realistically as possible the functioning of watershed systems. The main objective of this study is to model the hydrology of the Mai-Nefhi catchment area in order to establish the water balance and monthly stream inflow to the Mai-Nefhi dam. The Mai-Nefhi reservoir is one of the main sources of water to Asmara, the capital city of Eritrea. The Soil and Water Assessment Tool (SWAT) interface which works in the ArcGIS software platform was used to delineate the basin and sub-basins and estimate the streamflow. The calibration was carried out successfully, for the period 1972-1980, using SWAT-CUP SUFI-2 (Sequential Uncertainty Fitting version 2) program for a combined optimization-uncertainty analysis. The validation of the model was checked for the period 19811986 and the values of streamflow are nearly equal to the observed streamflow data. The hydrological analysis shows that the resulting changes on the annual streamflow volume is not significant because of the prevailing short rainy months.