A stochastic assessment of basin hydrological dynamics for evaluating low flow probability and environmental water availability in subtropical climate.

Understanding basin hydrological dynamics is essential, as climate change-induced extreme events and water overutilization reduce water resource availability. This study shows that integrating environmental flow (E-flow), dependable flow, drought quantification, and low-flow approaches under climate change provides a comprehensive framework for assessing reliable water resources in subtropical catchments. Applying the Tennant method, flow-duration-curve-integrated probability models, a hybrid composite drought index (CDI), multiple climate change indicators, and low-flow frequency methods, the study finds pronounced seasonal and spatial variability in E-flows. Inflows frequently fail to meet ecological thresholds, particularly in the Kangsabati upper catchment (KBUC). According to the E-flow analysis, the healthy river flow and minimum survival flow should be 44.67 and 22.34 m3/s, respectively, during the wet season in KBUC, whereas observed inflows are only 14.46 and 6.88 m3/s. Integrating flow duration curves with probability models effectively captures hydrological extremes. The Log-normal model [R2 = 0.994 in KBUC and 0.990 in Konar upper catchment (KRUC)] and GEV model are reliable for monsoon periods, while GEV (R2 = 0.997 in KBUC) and Log Pearson-III (R2 = 0.602 in KRUC) are suitable for lean periods. Such deterioration has occurred due to rainfall uncertainty, as found in drought indices and multi-dimensional rainfall trend assessment. The hybrid CDI approach improves drought assessment accuracy (78-90%), with rainfall, soil moisture, and land surface temperature identified as key drivers. Drought intensity has increased since the late 1990s, peaking between 2006 and 2010 due to monsoon variability and prolonged dry spells. Statistical models indicate declining low-flow conditions and increased vulnerability to extreme drought, underscoring the need for adaptive water management. Here, drought variability and low-flow availability are linked to regional rainfall characteristics. This study found that rainfall has declined over the long term, with a partial recovery after 2011, corresponding to reduced drought severity. Conclusively, climate change amplifies hydrological extremes and drives instability in KBUC, while KRUC remains relatively stable.