UMERICAL GROUND WATER FLOW MODELING OF ALAMATA SUB-BASIN IN THE RAYA VALLEY (NORTHERN ETHIOPIA

Abstract

Alamata sub-basin which is found in the Raya Valley is located in the southern part of Tigray Regional State of the Northern Ethiopia. The area covers 51 % of the Raya valley with area coverage of 1292.57km 2. The sub-basin water resource is used for irrigation and domestic water supply and is covered by tertiary volcanic in western and eastern highlands and alluvial deposit in the valley fill. The main purpose of this project work is to make a quantitative estimation for sustainable use of groundwater resource for domestic, live stock and agricultural supply. To achieve this objective careful understanding of hydrology, hydrogeology and dynamics of groundwater flow in and around the study area is mandatory. As numerical groundwater flow models represent the simplification of complex natural systems, different parameters were assembled into conceptual model to represent the complex natural system in a simplified form. The conceptual model was put into the numeric model to examine system response. Numerical groundwater flow was simulated in the model by the finite-difference method using MODFLOW, 1996 (McDonald and Harabaugh, 1988). The finite difference grid consisted of 1 layer, 113 rows and 97 columns. Two dimensional profile model was developed considering the system to be under steady state condition and assuming flow system view point. Model calibration was carried out by trial and error calibration method using groundwater contours constructed from heads collected in 14 wells. The calibration showed that about 100% of simulated heads were within the calibration target and the overall root mean square error for simulated hydraulic heads is about 4.24m. The poor fit at some points was due to numerous limitations associated with the model. Model sensitivity analysis was conducted by considering the horizontal hydraulic conductivity and recharge because they are sensitive. A change in hydraulic conductivity by -55%,-45%,-25% and 55%, 45% 25% resulted in root mean square Arba Minch Universty xiii Alamata sub-basin ground water modeling (RMS) 76 . 31 , 53.7, 25. 43 24.5, 45. 6 and 51 respectively. A change in recharge by the same amount as the horizontal hydraulic conductivity RMS head changes by 53.3, 40.52, 23, 22.17, 39. 9, and 47.8. As the model is intended to study the response of the hydrologic system the following four scenarios were assigned: In the first and second scenarios which apply for the sub-basin and, withdrawals were increased by 25 and 50% of the average existing withdrawals. In the third and forth scenarios, 25% and 50% reduction in recharge is applied. The effects of the scenarios are evaluated with respect to change induced groundwater heads to the steady state simulated heads. Accordingly, an increase of withdrawals by 25% over the whole area resulted in an average decline of steady state water level with minimum decline of 3.747m at well 084 and a maximum decline of 7.74 at well PZ3. Again an increase of withdrawals by 50% results with a minimum drawdown of 7.985m at well 084 and a maximum of 31 . 472m at well PZ3. On the other way, reduction of recharge by 25% results with a minimum decline of 20.333m at well 8H21 and a maximum decline of 32.485m at well PZ3. A reduction of 50% recharge resulted with a complete drying of well PZ3.