Drought Termination and Amelioration: Its Climatological Probability

Abstract

The preliminary Palmer Drought Severity Index (PDSI) is calculated and disseminated operationally by the National Oceanic and Atmospheric Administration/United States Department of Agriculture's (NOAA/USDA) joint agricultural weather facility. On an operational basis, this index is more aptly described as the Palmer Hydrological Drought Index (PHDI) in order to differentiate it from its hindcast value, the PDSI. Using the PHDI the approximate precipitation required to ameliorate or terminate any ongoing drought was calculated across the United States for various prescribed periods, i.e., 1, 2, 3, and 6 months. The climatological probability of receiving at least the needed precipitation was calculated using the gamma distribution. The probability calculations indicate that in many portions of the country it is quite unlikely that serious drought can be terminated in a since season or even in two seasons. Furthermore, due to the varying climatologies across the country, the probability of ending or ameliorating a drought varies both spatially and temporally in a systematic manner across the United States. The annual precipitation cycle and the probability of receiving substantial excess precipitation above normal (the skewness of the precipitation distribution) are two important characteristics of drought termination and amelioration in the Palmer Drought Model.