Projections of drought indices under climate change in the upper Chao Phraya river basin

Abstract

Climate change significantly impacts water resource management by intensifying the variability and frequency of extreme events like droughts. This study aims to understand drought processes and assess climate change impacts on drought in the Upper Chao Phraya River basin. The Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSI) of 3-, 6- and 12-month accumulation periods were estimated for historical from 1985 to 2014, near future (NF) from 2025 to 2054 and far future (FF) from 2070 to 2099 periods. Using GCMs from CMIP6 (MPI-ESM1-2-L, IPSL-CM6A-LR, and GFDL-CM4), bias-corrected via quantile mapping, we found good agreement with observed rainfall data. R2 from 0.61 to 0.70, NSE from 0.56 to 0.68, and RSR from 0.57 to 0.67 were obtained. The corrected rainfall under two climate change scenarios, SSP2-4.5 and SSP5-8.5 was inputted to the Soil and Water Assessment Tool (SWAT) to obtain future flows. The projection of SPI and SSI suggests that meteorological droughts could be more frequent and longer-lasting than hydrological droughts, especially under high-emission scenarios in the NF. Hydrological droughts are expected to be more severe, particularly in the FF under high-emission scenarios. For drought intensity, SPI is projected to show more extreme drought intensity than SSI, notably in the FF under medium-emission and in the NF under high-emission scenarios. In the spatial analysis under SSP2-4.5, SSI occurrences in the NF are more frequent than SPI, especially in the Wang and upper Yom regions, due to reduced rainfall and downstream flow. In the FF under SSP5-8.5, SPI occurrences become more frequent than SSI. Longer accumulation periods result in droughts lasting up to 45 months in the lower Nan and Yom regions. Drought severity and intensity vary across the basin. In the NF, SPI droughts are more severe in the lower regions, while SSI is more severe in the upper Ping. In the FF, SPI shows increased severity in the northeast, with SPI12 indicating more extreme conditions than other accumulation periods.