GROUNDWATER LEVEL CHARACTERIZATION OF GEMETO WELL FIELD IN HAWASSA LAKE CATCHMENT

Abstract

Lake Hawassa catchment is located in the central Ethiopian rift valley. The area of the study is estimated to be 415km2 .The study was intended to obtain a better understanding of the groundwater level characterization the small groundwater basin within the larger Hawassa Lake basin by applying numerical flow modeling with flow system conceptual viewpoint. In order to simplify the computation and for the purpose of analysis, the basin was modeled using two conceptual hydrogeological models. The study was set better approach for the defining of the two conceptual models which was transferred to the numerical model that has capability to imitate the field data with relatively good accuracy. This conceptual model’s simulated heads contour map shows that the general hydraulic gradient in the basin pursues the surface topography and the gradient is towards Lake Hawassa. The simulated hydraulic conductivity value varies in the category from 3x10-5m/s to 85x10-5 m/s as we go to the lake from Gemeto well field with in the model domain. From the simulated pumping rate scenario the aquifer under study responded as per the calibration using the measured data, and 1hour per day and 300 day per year of pumping it was used in the model, the 1st scenario started with 749.5 l/s and 1.3 times current pumping rate for the second, 1.4 times current pumping for the 3rd, 1.5 times current pumping rate for the 4th 1.8 times current pumping rate for the 5th finally doubled for the final scenario with 1499 l/s pumping rate for the capture zone. From the 6th scenario pumping rate the amount of water abstracted from the aquifer at 1499 l/s pumping rate is 5396.4m 3 /day or 1618920m 3 /year. From the above point of view and the potable water scarcity of Hawassa town and villages with in Lake Hawassa catchment the Gemeto well field Aquifer will provide Sufficient Potable water if protected from pollution. Thus, the model should be applied with caution for detail groundwater management use. The results should be interpreted and used considering all the limitations and drawbacks related to the input parameters. To improve the calibration values, groundwater level and abstraction rates should be monitored at a set of monitoring wells that are properly distributed in the area. In order to minimize measuring error, the observation wells have to be fitted with automatic water level measuring device. Once an improved steady-state model is developed and the necessary data are obtained, a transient model for the pumping scenario should be made for better predictions of pumping effect and for better recharge simulation, as recharge and groundwater outflow are strongly time dependent. However, for a better accuracy it needs further investigation as it has impact on the model simulation. For further groundwater resource development plans in Gemeto well field, it is important to take into account the balance between the groundwater recharge and the intended abstraction rates for domestic water supply to ensure the sustainability of the resource in the basin