Unsustainable groundwater extraction for domestic and agricultural purposes, particularly crop irrigation, is leading to dramatic reductions in the quantity and quality of groundwater in many developing countries, including Ethiopia. Assessing and predicting groundwater responses to hydraulic stress caused by overexploitation related to anthropogenic activities and climate change are crucial for informing water management decisions. The aim of this study is to develop a three-dimensional steady-state groundwater flow model for the Golina River Sub-Basin to understand the relationship between groundwater recharge and groundwater pumping and their impacts under steady-state conditions from the perspective of groundwater management. The model was created using MODFLOW 6 and discretized into 345 rows and 444 columns with a grid resolution of 100 m by 100 m. The subsurface was modeled as two layers: a clastic alluvial layer overlying a weathered and fractured bedrock. The surface-water divide of the Golina River Sub-Basin was treated as a no-flow boundary. The initial values of horizontal hydraulic conductivity ranged from 0.001 m/day for rhyolite to 27.26 m/day for alluvial deposits. The aquifer recharge rates from the WetSpass model ranged from 1.08 × 10⁻³ to 2.25 × 10⁻⁴ m/day, and the discharge rates from the springs, hand-dug wells, and boreholes were 2.79 × 10⁴ m³/day, known flux boundaries. Sensitivity analysis revealed that the model is very sensitive to hydraulic conductivity, moderately sensitive to aquifer recharge, and less sensitive to groundwater pumping. Calibration was performed to match observed and simulated hydraulic heads of selected wells and achieved a correlation coefficient of 0.998. The calibrated hydraulic conductivity ranged from 1.2 × 10⁻⁴ m/day for rhyolite to 20 m/day for gravel-dominated alluvial deposits. The groundwater flow direction is toward the southeast, and the water balance indicates a negligible difference between the total recharge (207,775.8297 m³/day, which is the water entering the aquifer system) and the total pumped volume (207,775.9373 m³/day, which is the water leaving the aquifer system). The scenario analysis showed that an increase in the pumping rate of 25%, 50%, and 75% would result in a decrease in the hydraulic head by 4.64 m, 10.18 m, and 17.38 m, respectively. A decrease in recharge of 25%, 50%, and 75% would instead result in hydraulic-head declines of 6 m, 15.29 m, and 46.97 m, respectively. Consequently, the findings of this study suggest that decision-makers should prioritize enhancing integrated groundwater management strategies to improve recharge rates within the aquifer system of the study area.

Groundwater is a natural freshwater resource and very important for multipurpose use in the Golina River basin. The determination of groundwater resource sustainability depends on the groundwater quality evaluation for specific uses. The objectives of this research work are: (1) to evaluate the groundwater quality and suitability for irrigation purposes and (2) to study the impact of irrigation activities on groundwater quality in the Golina River Basin. Alluvial deposits, basalts, granite, and rhyolite are the main stratigraphic units in the area. 34 groundwater and 3 river water samples were collected from the study area for water quality analysis purposes. Parameters of pH and EC were measured using WTW Multi 3430 and HCO3- was measured using Hach digital titrator in the field while parameters of Na+, K+, Ca2+, Mg2+, Cl-, SO42-, NO3-, PO4, and F-, were analyzed in the lab using Ion Chromatography (IC) in the Technical University of Darmstadt. Besides these, total dissolved solids (TDS), total hardness (TH), sodium absorption ratio (SAR), magnesium hazards (MH), Soluble Sodium Percentage (SSP)(Na%), Residual sodium carbonate (RSC), Permeability Index (PI) and Kelly´s index (KI) were calculated and interpreted. Based on the result of the analysis, the abundance of cations in groundwater in the study area is arranged as Ca > Na > Mg > K, and the anions are arranged as HCO3- > SO42- > Cl- > NO3- > PO43-. Similarly, the result also showed that the common water types in the area are Ca2+-Mg2+-HCO3- and Mg2+-Ca2+-HCO3-. TDS, TH, EC, SAR, MH, SSP, RSC, PI and KI have respective mean values of 530.45 mg/l, 279 mg/l CaCO3, 657 µS/cm, 1.24, 48%, 26%, 0.23 meq/l, 61% and 0.57. The suitability of the dominant water points in the Golina River Basin for irrigation is moderate in terms of TDS, hard to very hard water for irrigation in terms of hardness, medium salinity for irrigation in terms of salinity hazard and low sodium hazards, suitable for irrigation in terms of magnesium hazards, good and excellent water for irrigation in terms of soluble sodium percentage, safe for irrigation in terms of residual sodium carbonate, good for irrigation in terms of the permeable index, and suitable water for irrigation in terms of Kelly ratio. Few water samples in the Golina River Basin show slight contamination of nitrate from the irrigation activities which needs special attention.