Estimation of standardized precipitation indexes using the extended generalized gamma distribution

Abstract

The Drôme river basin is located in southern France, where in recent decades the agricultural sector has undergone recurrent periods of low water availability and severe water scarcity. The Standardized Precipitation Index (SPI) has been used to characterize drought in terms of frequency, intensity and duration. In this research, the use of the extended generalized gamma distribution is proposed to compute the SPI and to identify seasonal drought in the Drôme river basin based on the analysis of historical rainfall data from the meteorological station in Montélimar, France. The parameters were numerically estimated using the maximum likelihood method. The goodness-of-fit test of the model was based on the Kolmogorov-Smirnov statistic, computed with a parametric bootstrap technique. The SPIs were computed yearly and for the spring-summer and summer irrigation seasons. The extended generalized gamma distribution presented a good fit to observed rainfall frequencies. The development of SPI indexes enabled verifying a negative trend for seasonal rainfall in spring and summer over the last two decades in the region under study.