Temporal and spatial variation of drought in the Xilingol grassland based on the standardized precipitation index and standardized precipitation evapotranspiration index

As the main terrestrial ecosystem, grasslands have an important impact on global climate change. It is of great scientific significance to study the temporal and spatial distribution of grassland drought and explore the drought index with high applicability for drought monitoring and evaluation in this ecosystem. This study analyzed the spatial and temporal distribution characteristics of annual and seasonal meteorological droughts and the trends, mutations, and annual drought frequencies using the drought indices of the standardized precipitation index (SPI) and standardized precipitation evapotranspiration index (SPEI), as well as the Mann-Kendall test, based on the monthly precipitation and monthly mean temperature data of the Xilingol League 9 meteorological stations from 1990 to 2019. The results showed that: 1) Both indexes identified an increasing trend of drought in the region, and the SPEI was more significant than SPI (P < 0.05). In spring, SPI showed a wetting trend, whereas SPEI showed a drought trend. Both indexes showed a significant drought trend in summer (P < 0.05) and showed a wetness trend in autumn and winter. 2) The drought range detected by SPEI was larger than that detected by SPI, and the drought was the most severe from the central to western part of the Xilingol League. 3) SPEI detected more severe drought events than the SPI. 4) There were no mutation points in either index. In general, SPEI has a better applicability than SPI in evaluating drought conditions in the Xilingol League.