GIS BASED MODELING OF GROUNDWATER RECHARGE AND HYDROCHEMICAL INVESTIGATION OF MAY-ADRASHA CATCHMENT, NORTHERN ETHIOPIA

Abstract

The May-Adrasha watershed is situated in Northern part of Ethiopia, Tigray regional state with area coverage of 135.50 krrr', Geographically, it is bounded between 13°59°-14°08° N latitude and 38°12°-38°2i' E longitudes. The area is characterized by semi-arid climate with mono-modal and erratic nature of rainfall. The mean annual value of this rainfall accounts for 995 mm. However, it's temporal and spatial variation, not understood. Hence, the main objective of the present work was to investigate the hydrological and hydro-chemical situation. of the watershed to improve the groundwater resources management. A detail and intensive field investigation were conducted in the watershed. Field survey indicated that study area is covered by volcanic, Adigrate sandstone and other geological units. The land-use/land-cover types in watershed include cultivated, bush, forest land and settlement area. Clay, sandy clay, and clay loams are the dominant soil typesBoth WetSpass model and chloride ion mass balance methods were applied to determine the mean annual recharge of May-Adrasha watershed. The result of WetSpass model and chloride ion mass balance method indicated 6% and 5.7% of annual precipitation recharge. In addition, the WetSpass model simulation indicated that 80% of the precipitation in the watershed is lost through evapotranspiration while the rest 14% becomes surface runoff. The hydro-chemical investigation was understood by analyzing thirty groundwater samples using random and well distributed selecting system for major and minor ionic concentrations and insitu measurements of EC, temperature and PH. The groundwater has been evaluated using the interpretation of the Aquachem analysis results. Accordingly, the dominant water types in the catchment are mainly Ca-Mg-HC03-Cl (36.67%), Ca Mg-Ca-HC03-Cl (30%), Mg-Ca-Cl-HC03 (16.67%) and Ca­ Mg-HC03 (16.67%) The study area is characterized by low groundwater recharge due to the presence of high evapotranspiration rate and surface runoff associated with high temperature, dry wind and .