Combined drought index for drought monitoring and severity assessment under future climate and land use changes

This study aimed to derive the Combined Drought Index (CDI) for the Prom-Choen-Upper Phong River Basin, which was based on the Principal Component Analysis (PCA) and the combination of Standardized Precipitation Evapotranspiration Index (SPEI), Standardized Streamflow Index (SSFI), and Standardized Water Supply-Demand Index (SSDI). Future climate data from CanESM5, MPI-ESM1-2-LR, and NESM3 under SSP245 and SSP585, and future land use were used for drought monitoring and severity assessment. The analysis showed that the increasing trend of future mean annual rainfall under SSP245 was lower than SSP585. The maximum and minimum annual average temperature have a smaller increase under SSP245 than under SSP585. In 2075, compared to 2019, the obvious change was the decrease in paddy field of 158.3 km², and the increase in urban area of 156.6 km². For 2000-2023, the CDI values under SSP245 and SSP585 were found to be -0.24 and -0.16, respectively. The projected CDIs during 2024-2100 under SSP245 and SSP585, which varied between -0.14-0.89 and 0.05-0.36, respectively, were expected to increase compared to past-present period. This is due to the increase in future rainfall, which could lead to severe flooding rather than drought. The CDI was then used to determine the drought hazard, in which the very high category was observed in most of the sub-districts within Khon Kaen and Chaiyaphum Provinces, and may continue to increase in the future, especially under SSP585. The results provided insights for forecasting drought severity and identification of drought-prone areas, whereas measures suitable for drought management could be identified.