Individual Impact of Near-Future Climate on Water Balance in the Upper Bengawan Solo Watershed, Indonesia

Abstract

Climate change can alter water availability and its seasonal distribution, making water balance an important indicator for watershed assessment. This study estimates water balance in the Dengkeng Watershed using the Thornthwaite approach under the SSP2-4.5 climate scenario, based on an ensemble of two General Circulation Models (GCMs): EC-Earth3-CC and MPI-ESM1. Two projection conditions were evaluated using the median (P50) and 75th percentile (P75) of rainfall and temperature projections for 2025–2039. At α = 0.05, bias-corrected GCM data from seven rainfall stations showed p-values > 0.05 in the Wilcoxon and Kolmogorov–Smirnov tests, indicating no statistically significant differences from observations. The baseline period showed a clear monsoonal pattern, with strong surpluses during DJF, decreasing water balance in MAM, generally lower values during JJA, and a mixed spatial pattern during SON. Under future conditions, P50 and P75 maintained the monsoonal signal but increased spatial and seasonal variability. Under P50, the most pronounced changes occurred in SON, where all stations except Deles experienced deficits. In contrast, P75 produced more widespread surplus conditions, particularly during JJA and SON, although Bendung Colo and Ngawen remained deficit-prone during several seasons. Overall, P50 tended toward greater deficits during several seasons, whereas P75 produced stronger surplus conditions, highlighting the spatially heterogeneous response of water balance to climate change.