Investigating the divergence between Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) in defining drought over different climate zones of India.

Drought quantification is carried out by indicators like (1) Standardized Precipitation Index (SPI) and (2) Standardized Precipitation Evapotranspiration Index (SPEI), which encompass the historical characteristics of meteorological variants (precipitation and temperature). The behavioral pattern of these variables has changed significantly due to the recent changes in the climate, posing a question of the mutual agreement among SPI and SPEI in defining drought over regions with different climatic characteristics. India is chosen as the study area owing to the long term data availability, diverse climatic zones (from tropical monsoon belts to dry arid regions), and increasing drought likelihood in the mainland area. Daily, gridded precipitation (0.25º × 0.25º) and temperature (1º × 1º) data, from January 1951 to December 2013, was utilized for the calculation of SPI and SPEI. While the average annual precipitation in India is consistently declining, the yearly average temperature exhibits three distinct trends (decreasing from 1951 to 1975, neutral from 1975 to 1990, increasing abruptly from 1990 onwards). Such variations in the trend behavior are replicated in the increasing divergence of the two indices, represented as percentage area under drought. A robust regional divergence between the indices is detected from east to west, highlighting the arid and semi-arid regions as hotspots of significant deviations.  SPEI showcases an overall increasing drought hazard in India since the 1970s in terms of frequency, magnitude, and duration, when compared to SPI.