QUANTIFYING SURFACE AND GROUND WATER AVAILABILITY OF THE MEKI RIVER, CENTRAL RIFT VALLEY LAKES BASIN, ETHIOPIA

Abstract

Quantifying surface water (SW) and groundwater (GW) availability is crucial for effective water resource management. This study assesses SW and GW in the Meki River sub-basin, central Rift Valley, Ethiopia, by SWAT and MODFLOW models. Integrating SWAT and MODFLOW. This study analyzed hydrological dynamics and groundwater resources for a period 2000 to 2020. SWAT divided the watershed into 18 sub-basins and 86 Hydrological Response Units (HRUs), simulating over 21 years with a three-year warm-up period. The SWAT model, calibrated and validated for 2000–2013, successfully simulated hydrological processes. Model performance was robust, with R² values of 0.76 and 0.85 and NSE values of 0.61 and 0.74, following 1000 simulations during calibration and validation. Critical parameters influencing streamflow included CN2, SOL_K, and GWQMN. Using SWAT-derived GW recharge and evapotranspiration, the MODFLOW-NWT model simulated groundwater flow. Calibration with PEST ensured accuracy, achieving a strong correlation (R² = 0.9922) between observed and simulated groundwater levels across 62 piezometers. Error metrics (RMSE = 9.46 m, MAE = 7.22 m) confirmed model accuracy. Spatial analyses showed heterogeneous groundwater flow influenced by local conditions and SW interactions. River-aquifer interactions revealed significant groundwater discharge to rivers, with daily discharge (91,198.128 m³/day) exceeding recharge (24,866.406 m³/day). The steady-state model showed balanced inflows and outflows, with recharge and river discharge being major inputs. This calibrated model offers a solid framework for managing groundwater resources in the Meki River sub-basin, supporting sustainable water management and planning. Groundwater flow primarily moved from the western escarpment towards the Tora-Koshe-Dugda ridge, influenced by varying hydraulic conductivity. The steady-state model balanced inflows and outflows (40.947 Mm³/year), with recharge (23.5 Mm³/year) and river contributions (9.1 Mm³/year) as key inputs. Evapotranspiration, river discharge, and extraction also played significant roles