Future hydrological drought changes over the upper Yellow River basin: The role of climate change, land cover change and reservoir operation

The upper Yellow River Basin (UYRB) provides more than 50 % of freshwater for the Yellow River. Recent research reveals continuous climatic wetting and vegetation greening over the UYRB in a warming future, but future hydrological drought changes remain unclear due to complex interactions between climate change, land cover change and reservoir operations. Here we project hydrological drought changes at different global warming levels and quantify contributions from each driving factor. The Conjunctive Surface-Subsurface Process eco-hydrological model is coupled with a reservoir module, and the new CSSPv2 + Reservoir model is used to perform future projections driven by bias-corrected meteorological forcings from 11 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The CSSPv2 + Reservoir model simulates monthly streamflow over the UYRB reasonably well with a Kling-Gupta efficiency of 0.75. The increasing precipitation during dry seasons reduces hydrological drought duration by 9 %/12 %/19 % at the 1.5 degrees C/2.0 degrees C/4.0 degrees C warming level, while its impact on drought severity is limited. The dramatic increase of leaf area index at the 4.0 degrees C warming level robustly exacerbates hydrological drought severity to 28 % by intensifying evapotranspiration. The reservoir operation, designed to reduce seasonality in monthly releases to ensure stable hydropower generation, further decreases drought duration, but increases drought frequency and severity to 63 %/48 %/108 % and 9 %/11 %/32 % at the 1.5 degrees C/2.0 degrees C/4.0 degrees C warming levels respectively. A mitigated operation rule which stores less water during wet seasons, however, can reduce hydrological drought severity and duration significantly. Thus, future reservoir operations should pay more attention to the trade-off between hydropower generation and hydrological drought mitigation.