Temporal Trends and Future Projections of Reference Evapotranspiration under Different Climate Change Scenarios in the Parbati Watershed

This study assessed the temporal trends and future projections of reference evapotranspiration (ET₀) in the Parbati watershed under different climate change scenarios. Future maximum and minimum temperature data were obtained from the CORDEX regional climate database for the CMIP5-based RCP2.6, RCP4.5 and RCP8.5 scenarios. Mean air temperature was calculated from the maximum and minimum temperature values, and reference evapotranspiration was estimated using the Hargreaves–Samani method. The projected period was divided into three future time slices: FP1 (2027–2042), FP2 (2046–2064) and FP3 (2082–2100). Monthly temporal trends were evaluated using the non-parametric Mann–Kendall test, while Sen’s slope estimator was applied to quantify the magnitude and direction of change. The results showed considerable seasonal and scenario-dependent variability in projected ET₀. Most monthly trends were statistically non-significant at the 5% significance level, indicating that interannual variability remained dominant during several months. Under RCP2.6, mixed trends occurred during FP1, predominantly increasing tendencies were observed during FP2, and decreasing tendencies dominated FP3. RCP4.5 displayed the clearest recurring increase, with June showing a statistically significant rising trend in all three future periods. Significant decreasing trends were also observed under RCP4.5 in October during FP1 and April during FP2. Under RCP8.5, significant declines occurred in September during FP1 and January during FP3, whereas May showed a significant increase during FP3. Overall, the findings indicate that future reference evapotranspiration in the Parbati watershed may not change uniformly with increasing climate forcing. Instead, its response is expected to vary among months, scenarios and future periods. The results provide useful information for irrigation planning, soil-moisture management, watershed conservation and climate-resilient water-resource management.