Detecting Land Use Change Impacts on Streamflow by Combining Field Data and Water Balance Modelling

ABSTRACT Over the last half‐century, land use changes, including deforestation, urban sprawl, and open‐pit surface mining, have accelerated across the Susurluk Basin in northwestern Türkiye. This study analysed how land use changes, damming and mining activities affected basin hydrology using empirical and analytical methods and the process‐based Water Supply Stress Index Model (WaSSI). The monthly WaSSI water balance model was validated using streamflow data from gaging stations between 1980 and 2005. Two of the eight subbasins exhibited streamflow reductions of about 32%–42%, with mean annual discharge decreasing between 1980–1989 and 1990–2005, primarily due to land use change rather than climate variability. The runoff coefficient (Runoff/Precipitation) dropped from 22% during 1980–1989 to 12% during 1990–2005 in one rural subbasin containing several surface‐mine ponds. Overall, empirical and process‐based modelling indicated that land use dynamics, rather than climate, were responsible for the hydrological change. The monthly WaSSI showed satisfactory performance in subbasins with low human impacts (NSE > 0.50) but considerably lower performance (NSE < 0.20) in highly human‐modified areas. This integrated study concludes that land use activities, especially pond creation for pit mining, were the dominant drivers of hydrological changes in the study area.