Spatiotemporal characteristics of drought in a semi-arid grassland over the past 56 years based on the Standardized Precipitation Index

Drought is one of the major natural disasters in northern China. Therefore, monitoring and analyzing the changes of drought indices can provide scientific evidence for disaster assessment and for instituting policies for disaster prevention and mitigation. In this work, we used the gridded precipitation data set of a semi-arid steppe region in the Inner Mongolian Plateau from 1962 to 2017 to calculated the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). By comparing the spatiotemporal distribution of the two indices with the historical drought records, we found that the SPI was more suitable for the drought description in the study area. Based on this, we calculate the SPI at different time scales (annual and seasonal), then depict the spatiotemporal variation of drought during the past 56 years by analyzing the frequency of drought events, drought frequency, and the station frequency ratio. The results indicate that in recent years, the degree of drought in the study area has increased, and the south-central regions are the highest occurrence area for different types of drought events. In addition, light drought occurs mostly in the autumn and is mainly distributed in the central and southern parts of the study area. The most extreme drought events often occur in the summer, and the highest frequency area is located in the western part of the study area. Large-scale light drought occurred in all seasons in the 1990s, while the domain moderate drought and the domain extreme drought occurred in the summers of 2001 and 2010, respectively.