THE PERIODICITY OF DROUGHTS IN THE GRAIN-GROWING AREAS OF THE USSR

On the basis of a chronology of droughts in the grain-growing regions of the USSR, compiled from historical sources and the more recent period of instrumental observations, the author shows that the occurrence of drought may be regarded as a random process characterized as a quasi-stationary Poisson series of events. Most of the grain regions in the USSR follow this regularity. In the case of simultaneous drought throughout the principal grain zone of the USSR, comprising the European part and the Midland region (West Siberia and Kazakhstan), the sequence of events assumes a nonstationary character. Statistical parameters are developed for the time series of drought years and nondrought years, and are compared with extreme values in grain-yield fluctuations. Over the last 140 years, drought occurred in 76 years in at least one of the three major grain regions of the USSR (European part, West Siberia, Kazakhstan) while 64 years had no drought and corresponding normal or higher yields. A catastrophic drought affecting all three grain regions simultaneously is virtually (95%) certain to occur at least once in 20 years.