Simulation-Optimization Modeling for the Management ofGroundwater Resources in Modjo River Catchment, MainEthiopia rift

Abstract

In this study, steady state groundwater flow model was constructed and calibrated to analyzegroundwater budget and ground flow system. Pumping scenarios were done to investigate theresponse of the system under increased groundwater pumping rates. The main inflows are inflowfrom lakes and recharge from precipitation. An outflow from the system is through pumpingwell, subsurface outflow from the catchment, groundwater flow to lakes and base flow. Theresponse of the system under increased groundwater pumping rates and decreased recharges ratewere investigated using calibrated groundwater heads and fluxes as a base line. Increasing thepumping rate considerably changed the groundwater dynamics and fluxes. Increasinggroundwater pumping rate by 10% resulted in a slight decline in groundwater levels and baseflow. However, 50% increase in pumping rate causes groundwater level decline, reduction ofgroundwater subsurface out flow from the catchment and base flow by 6.37 m, 5.6 % and 27 %respectively compared to calibrated steady pumping model value. The decrease in groundwaterrecharge by 25 % result in maximum decline of the water level by 24.18 m and minimum waterlevel decline by 1.57 m at around vicinity of lakes, decrease of subsurface groundwater outflowby 7.21% and base flow by 22.99%. Under optimum steady pumping condition, the totalabstraction rate is reduced to 200,093 m3/day. In conclusion, the excessive groundwater pumpingrate to meet growing water demand and altered recharge due land use and climate changes isexpected to disrupt hydrological system and groundwater budgets in study area.The physico–chemical results were compared with the Ethiopian and World Health Organization(WHO) standards for drinking and public health, in order to have an overview of thegroundwater quality in study area. According to the overall assessment of the area, almost all theparameters analyzed are below the desirable limits of WHO except Katila and Mudasenkele..Using GIS interpolation methods with Arc GIS 10.3.1, spatial distribution maps of pH, TDS, EC,HCO3, F, Ca, Na, and K, RSC, SAR, TH, Na% were produced. Most of the groundwatersamples’ fall within excellent and good water class and suitable for irrigation purpose. However, different industries exist in study area used chemicals different from mojor cations and anions.Thus, these chemicals need to be investigated in future.