INTEGRATED HYDRO-ECONOMIC ASSESSMENT OF GROUNDWATER WELLFIELD DEVELOPMENT USING COUPLED MODFLOW-WEAP MODELING: CASE STUDY OF KIGALI, RWANDA

Rapid urbanization and population growth have substantially increased municipal water demand in Kigali, Rwanda, creating an urgent need for sustainable and economically efficient groundwater development strategies. This study presents an integrated hydro-economic framework for optimizing groundwater wellfield development in the shallow alluvial and weathered rock aquifers of the Nyabarongo River basin. A three-dimensional MODFLOW groundwater flow model was integrated with the Water Evaluation and Planning (WEAP) system to evaluate the hydraulic performance and economic feasibility of five groundwater development scenarios (32, 28, 24, 20, and 18 production wells) designed to satisfy a target water demand of 40,500 m³/day. The groundwater model was calibrated against observed hydraulic heads, achieving excellent agreement (RMSE = 1.42 m; R² = 0.96). Simulation results revealed substantial differences in hydraulic performance and project economics among the investigated scenarios. Riverbank filtration contributed approximately 65–78% of the total abstraction, significantly mitigating regional groundwater drawdown. The hydro-economic evaluation, incorporating capital expenditure (CAPEX), operational expenditure (OPEX), and net present value (NPV) over a 20-year planning horizon, identified the 20-well configuration as the optimal development strategy. This configuration maintained individual well drawdowns below 4.5 m while minimizing capital investment and long-term energy consumption. In contrast, the 32- and 28-well configurations experienced pronounced hydraulic interference and diminishing marginal yields, whereas the 18-well scenario imposed higher pumping stresses and operational risks. The proposed integrated hydro-economic framework provides a practical and transferable decision-support tool for optimizing groundwater development in river–aquifer systems, offering valuable guidance for sustainable municipal water supply planning in sub-Saharan Africa.