Assessment of Meteorological Drought Frequency using the Standardized Precipitation Index (SP) for Climate Change Adaptation Planning in the Lower Xebanghiang watershed, Savannakhet Province

Abstract

This study aims to assess the frequency and characteristics of future meteorological droughts in the Lower Xebang Hiang Watershed, Savannakhet Province, using the Standardized Precipitation Index (SPI). Monthly rainfall data were derived from the Geophysical Fluid Dynamics Laboratory–Earth System Model version 4 (GFDL-ESM4), a Global Climate Model (GCM) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) framework, under three Shared Socioeconomic Pathway (SSP) emission scenarios: SSP1-2.6, SSP2-4.5, and SSP5-8.5. Raw GCM outputs were downscaled using the Dynamical Downscaling technique to compute drought frequencies at 1-month (SPI-1), 3-month (SPI-3), and 6-month (SPI-6) accumulation periods for the Near Future (2023–2050) and Far Future (2051–2100) periods.

Results indicate that the highest drought frequency across all indices occurs during the Near Future under the SSP5-8.5 scenario, with SPI-1 reaching 6.55% and SPI-3 reaching 4.17%. The dominant characteristic of projected droughts is short-term variability, as SPI-1 consistently exceeds SPI-3 across all scenarios, with a maximum ratio of 2.37:1 under SSP1-2.6 in the Near Future. However, in the Far Future under SSP2-4.5, this ratio converges toward 1:1 (1.33:1), indicating a tendency for short-term droughts to intensify and transition into cumulative medium-term droughts over the long run. Consequently, the basin requires immediate flash drought response measures in the near term and long-term structural planning to address increasingly prolonged drought durations. These findings provide a critical scientific foundation for establishing effective and sustainable drought-risk policy measures and water resource management planning in the Lower Xebang Hiang Watershed.