CONDITION ASSESMENT AND MODELING OF GROUND WATER IN ARBAMINCH AREA '-.' I

Abstract

When the issue of water supply and demand for a certain purpose is raised, the case of the availability of source of water and assessment of its conditions are inseparable considerations. Assessment of conditions of Groundwater in an area may include the water withdrawal rate, its utilization and management, the type of formations from which it is coming, the hydro geological properties, quality conditions of Groundwater, presence of influx (recharge) and more others. The water supply and demand condition of Arba Minch University has been dealt with. The presence of high losses (60-95% of the pumped water) in the distribution system has been detected. The type of aquifers from which this supply is withdrawn has been determined. The area is highly heterogeneous and anisotropic with the transmissivity of aquifer 2 varying between 2 to 532.3m2 per day. The Groundwater flow direction in the area is towards the Lake (Abaya) determined with the aid of water level contour interpolated in the Arc view GIS. A natural scene of higher values of Groundwater constituents and parameters has been observed near Lake Abaya with the general trend of decreasing upstream to the GW flow direction. The higher figures of Potassium and total dissolved salts in An approximate soil-water balance approach to determine Groundwater recharge from rainfall after the necessary accounting of run-off, evapotranspiration and soil moisture storage requirements indicated very little/ no recharge from rainfall in the area. A simplified, homogeneous and anisotropic steady state Groundwater flow model with a single confined aquifer having the varying transmissivity of the actual multi­ layer aquifer system has been developed. The available MODFLOW software called Processing MODDLOW for Windows (PMWIN) has been adopted for model run. Calibrations with few point data of water levels from field measurements at available and selected control points has been done by varying and computing parameters and conditions at a time. Of all the parameters Groundwater level (head) is very sensitive to variations of transmissivity and aerial recharge values. Prediction of the model run was considered for the sustainability of the existing wells and well No-7 (AMU-7) for the current and future population with reference to some head values which could be attained in the wells due to excessive pumping in these wells. With the preset head values in the wells (considered 'undesirable head" for each well) scenarios have been developed for varying combinations of discharge. Well No-7 has been proved to be a good potential well for current and future population.