*2.4. Drought Frequency*

Drought frequency is defined as the number of droughts that exceeds a certain risk threshold per unit time. In this study, the drought frequency was defined as a ratio of the total number of drought months with different grades to the number of the total months of the study period (totally 468 months). For example, from 1990 to 2018, the number of months with SSMI lower than −1.5 for each grid cell was 10, and the frequency of extreme drought was 10/468, or 2.13%. The spatial frequency of droughts with different grades was calculated. Subsequently, the spatial frequency of different drought levels in spring (March–May), summer (June–August), autumn (September–November), and winter (December–February) were calculated to discuss the seasonal dynamics of soil drought frequency.
