IMPACTS OF LAND-USE CHANGES ATTRIBUTED TO DISCHARGE LEVEL OF MAHA OYA RIVER BASIN, SRI LANKA

Nuwan Sameera Rankiralage 

River discharge in tropical regions are driven by both rainfall and land-use changes. This study quantifies how land-cover change and altered rainfall regimes have affected discharge in the Maha Oya River basin, Sri Lanka. Previous studies suggest that the river basin had been subjected to serious erosion due to increased soil digging, clay mining, and high flood vulnerability over the last three decades. This study uses Discharge data from 1992 to 2019 were analysed to identify factors influencing flood hazards and to inform disaster management strategies. Two hydrologic models representing historical (1992–1995) and recent (2016–2019) conditions were developed using the HEC-HMS software, incorporating the Soil Moisture Accounting (SMA) method. Land-use maps for 1990 and 2019 were generated through supervised classification of Landsat imagery, achieving overall accuracies of 92.5% and 96%, with Kappa coefficients of 0.90 and 0.95, respectively. Model calibration and validation yielded Nash–Sutcliffe Efficiency (NSE) values of 0.70 and 0.607 for the historical model, and 0.713 and 0.653 for the recent model. Validated model parameters such as Time of concentration, baseflow, recession and groundwater moisture indicates the impact of anthropogenic activities and land-use land-cover changes on groundwater level and baseflow of the catchment. Comparison of the calibrated parameters of the models further demonstrated the decrease in river discharge over the research period, which was mostly due to land-use changes such as increasing sand mining and agricultural development. The findings reveal a clear hydrological shift from an infiltration- to runoff-dominated system, driven by land-use change and rainfall variability, underscoring the urgent need for soil conservation and sand mining regulation to restore baseflow and enhance flood resilience in the Maha Oya Basin