Introducing Normalized Surface-adjusted Precipitation Index (NSPI) for regional drought assessment

Drought is a natural event, but its frequency and severity are increasingly influenced by human activity and climate change. In the current Anthropocene era, human-induced changes to the hydrological cycle combined with natural climate variability are reshaping how droughts develop and persist. Droughts often result from complex interactions between atmospheric conditions and land surface processes, which affect how water and energy move through the environment. These interactions disrupt rainfall patterns and can lead to more frequent and intense climate extremes. Climate conditions, such as reduced rainfall, initiate meteorological drought, while catchment characteristics like soil type or vegetation cover affect how well land can retain moisture. However, traditional drought indices, such as the Standardized Precipitation Index (SPI), often fail to capture these combined effects. To address this, the study introduces the Normalized Surface-adjusted Precipitation Index (NSPI), which merges climate data with surface characteristics at the catchment scale. By incorporating moisture availability, energy inputs, and moisture transport efficiency, NSPI better represents how droughts evolve. The index uses data from remote sensing, reanalysis datasets, land surface models, and ground-based observations. Trends show rising wind speeds and surface temperatures, which reduce relative humidity and lower rainfall potential. NSPI accounts for these changes through a Composite Variable derived using Principal Component Analysis and performs well across multiple timescales. The inclusion of a "loss term" enhances its accuracy, aligning drought patterns more closely with satellite-derived vegetation data (NDVI). Applied to Mahanadi River Basin, NSPI reveals more detailed spatial drought patterns than SPI, offering a stronger foundation for early warning systems.