Projections of future hydrologic drought in a reservoir-regulated region: the role of climate change and reservoir operation

Abstract. Future hydrological droughts in reservoir-regulated regions remain unclear due to the complex interactions between climate change and reservoir operation. Existing studies usually make simple empirical assumptions about historical reservoir operation patterns to explore the role of climate change and reservoir operation on hydrological drought without even considering the role of optimal reservoir operation policies. Here, we take the upper Hanjiang River basin (UHRB) in China as a typical example to project its future hydrological drought evolutions using various standard streamflow indices (i.e., SSI-1, SSI-3, and SSI-12) and to quantify the role of each relevant factor. A new LSTM+Reservoir that combines a long and short-term memory (LSTM)-based hydrological model with a physics-guided LSTM reservoir model is used to perform future projections using the meteorological outputs of five bias-corrected global climate models (GCMs) under three shared socioeconomic CMIP6 pathways (SSP126, SSP370, and SSP585). The results indicate that future climate change over the UHRB tends to reduce natural streamflow and exacerbate hydrological droughts, especially in the far-future period (2071–2100) under the SSP585 scenario. The operation of Ankang reservoir can mitigate drought frequency, duration, and severity for short-term SSI-1 and SSI-3 but fails for long-term SSI-12. Additionally, optimal reservoir operating policies that aim to maximize hydropower generation and pow generation guarantee rate can well reconcile the trade-off between short-term hydrological drought and hydropower benefits, which underscores the necessity of future reservoir operation improvements.