Redefining Standardized Precipitation Evapotranspiration Index (SPEI) for Changing Climate

Behavioral pattern of temperature and precipitation govern the characteristics (intensity, frequency, and duration) of meteorological drought. The present meteorological drought indices like the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) function on a historical inter-comparison of these meteorological variables. However, previous studies have highlighted that significant changes in the climate have caused spell characteristics (wet spells and dry spells) to evolve as an essential variable in analyzing the drought scenario of a region. Thus, modification in the definition of present meteorological indices is required in order to capture the signature of changing spell characteristics in drought computation. To illustrate these, India is chosen as the study area, owing to the availability of long-term meteorological data and frequent occurrences of meteorological droughts in the mainland. Long term, gridded (0.25° × 0.25°), precipitation and temperature data from January 1951 to December 2013 were used to evaluate SPEI, which is redefined as Redefined Standardized Precipitation Evapotranspiration Index (RSPEI) to incorporate two additional variables: (1) fraction of wet spells and (2) fraction of length of the maximum dry spell, to account for the changing spell characteristics. To understand the utility of RSPEI, we alter the spell characteristics of four wet years (1957, 1977, 1978 and 1990), without changing the total rainfall and temperature data, for a random grid (18°45'N × 78°45'E), and generate a synthetic time series for that grid. The results indicate that newly developed RSPEI highlights the drought events that occur once highly concentrated rainfall events recede, which SPEI misses out. Further, the divergence between SPEI and RSPEI, observed in the natural time series, is attributed to the distribution of rainfall within a month. Thus, RSPEI does not alter the inherent drought capturing characteristics of SPEI, but adds more value to it, thereby representing a better version of reality.