Modified standardized precipitation index for skewed hydro-meteorological data

The Standardized Precipitation Index (SPI), due to its mean-based standardization and implicit symmetry assumption, may not always represent precipitation conditions accurately when distributions are skewed. Under such circumstances, wet or dry classifications may occasionally diverge from the underlying hydrological signal. To address this potential limitation, this study introduces the Modified Standardized Precipitation Index (M-SPI), which replaces the mean with the median as the central reference, aiming to better reflect the statistical structure of precipitation series, particularly where skewness is evident. While both indices employ the same drought classification scheme, M-SPI allows the standardized distribution to shift according to the direction of skewness. For comparative evaluation of SPI and M-SPI, annual total precipitation data from seven meteorological stations in Türkiye for the period 1966–2020 were utilized. In addition, to illustrate index behavior across different accumulation windows, 3-, 6- and 12-month applications were performed for three representative stations. The results indicate that both indices produce comparable outcomes for near-symmetric series (e.g., Çorum), whereas M-SPI tends to reveal relatively longer dry periods in Aydın and Burdur, and more extended wet periods in Edirne and Rize. Overall, the findings suggest that M-SPI provides smoother drought–wetness transitions and may offer a more stable representation of persistence, particularly in Mediterranean, semi-arid and humid climates. Accordingly, M-SPI can be considered a flexible alternative to SPI in regions where precipitation distributions are significantly skewed.