SALAME ABDI AHMED2026-02-032024-08-19https://etd.hu.edu.et/handle/123456789/523Quantifying 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 rolesenSWATMODFLOWsurface watergroundwaterrechargeriver-groundwater interactionThesis