Role of Spring–Summer Drought in the Dynamics of Aridization of Arid Lands in European Russia

The relationship of changes in annual moisture with spring-summer and summer moisture in twelve federal subjects in the south of European Russia in 1901–2018 was analyzed using the Standardized-Precipitation Evapotranspiration Index (SPEI). The revealed close relationship between the annual SPEI and the aridity index justifies the usage of the SPEI to assess changes in annual moisture conditions. It was found that changes in moisture in May–July were associated with fluctuations in annual moisture more significantly than those in summer moisture. Changes in the seasonal moisture in the Volga, Trans-Volga and Northwestern Caspian regions explained 50–65% of the variability of annual moisture, while the contribution of the seasonal moisture varied from 30 to 60% in other federal subjects of the south of European Russia. The comparison of the aridization phases identified in each federal subject and their systematization by the duration showed that the change in the frequency of spring-summer and summer droughts affects the formation of alternating multi-decadal periods of increasing/decreasing climate aridization in the study area. Two-four dry periods and two-three wet periods are typical for most of the considered federal subjects. It was revealed that drier conditions of the spring-summer and summer periods were observed at the beginning and middle of the 20th century in the south of European Russia. The increasing seasonal moisture in the second half of the 20^th century was replaced by a drier phase at the beginning of the 21st century. It was shown that droughts during dry periods are a reliable indicator of a dry year: spring-summer droughts in Stavropol krai and summer droughts in Volgograd oblast and Krasnodar krai.

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