The Upper Mzingwane sub-catchment in Zimbabwe is increasingly exposed to extreme climatic events, particularly recurrent droughts. These conditions threaten water security for agriculture, mining, and domestic use, yet hydrological monitoring in the basin remains sparse, limiting informed management. This study applied the Soil and Water Assessment Tool (SWAT) to investigate water balance dynamics in a semi-arid, data-scarce sub-catchment. Calibration and validation of the model was done for the period 1993–2016 using the SUFI-2 method within SWAT-CUP, drawing on land use, climate, soil, and streamflow data. Key parameters influencing water responses included the runoff curve number (CN2), the groundwater threshold for discharge (GWQMN), and the groundwater revap coefficient (REVAPMN.gw). Model evaluation showed good performance (R2 = 0.81, NSE = 0.68, PBIAS = 4.1%). Results indicated that expansion of cultivated land and other land use shifts, combined with climate variability, have reshaped the hydrological cycle. Surface runoff rose by 10% between 1990 and 2000, raising flood risks, but declined by 16% by 2020, reflecting longer dry periods. At the same time, reductions in groundwater recharge and high evapotranspiration intensified water scarcity during droughts. These findings highlight the urgent need to strengthen hydrological monitoring and adopt climate-resilient water management approaches in semi-arid regions of Africa.