*3.1. Identification of Variation Characteristics of Soil Drought*

Figure 2 shows that, since 1990, drought and flood disasters occurred alternately in Guangxi, with slight or serious droughts and floods occurring almost every year; additionally, the temporal distribution of different types of droughts and floods is evident from the figure. Serious soil drought occurred every five to six years on average in Guangxi, with multiple droughts observed in 1993, 1998, and 2004. In general, Guangxi experienced many soil droughts during 1990–2018, with the drought duration and intensity being generally heavy. After 2000, the soil droughts in arid areas decreased (Figure 2).

**Figure 2.** Variation curve of soil moisture in Guangxi. (**a**) Changes in SSMI monthly time series. (**b**) Damaged areas affected by soil drought. Damaged area is defined as the ratio between the number of pixels with a certain level of drought and the total number of pixels in this area, and then the ratio is multiplied by the total area of Guangxi to obtain the regional drought damage area.

Further, the SSMI showed evident seasonal characteristics in Guangxi (Figure 3), with the magnitude of variation being the highest in autumn. Serious soil droughts were observed in the autumns of 1992 and 1998, but none have occurred since 2010. However, the SSMI trends in winter and spring were similar. Overall, the seasonal soil moisture dynamics in Guangxi showed similar changes with the interannual dynamics. After 2012, the observed loss of soil moisture in each season was alleviated.

**Figure 3.** Seasonal variations in SSMI in Guangxi.

### *3.2. Statistics of Drought Frequency*

The results of the frequency of slight to extreme drought occurrence (Figure 4) indicated that the frequency of slight droughts in Guangxi was 16.37–34.77%, of which the frequency in central Guangxi was the highest, followed by the southern region. The frequency of slight droughts in most other areas was less than 30%. The frequency of moderate droughts was 10.63–21.84%, while it was less than 18% in most areas. The frequency of severe droughts was less than 5% in most areas, with the lowest frequency being 2.29%. The frequency of extreme droughts was extremely low (0–6%), with the value being less than

2% for most areas. The order of the average frequency of different drought levels (Figure 4) was light drought > (26.84%) > moderate drought (16.15%) > severe drought (6.11%) > extreme drought (2.64%) in Guangxi. Overall, no significant geographical difference was observed in the soil droughts.

**Figure 4.** Frequency of different soil drought levels. (**a**) Slight droughts, (**b**) moderate droughts (**c**) severe droughts, and (**d**) extreme droughts.

Figure 5 shows that slight droughts mostly occurred in autumn and winter, lasting for more than 20 months. During spring, the probability of mild weather in the southwest was higher than that in the southeast. During summer, soil droughts occurred for a smaller number of months. The spatial variation of moderate droughts was similar to that of slight droughts, with soil droughts occurring in autumn and winter. Further, the number of months of severe and extreme droughts was relatively small, and the number of months of sudden droughts in each season was mostly less than five months. The underlying surface in Guangxi is relatively uniform, and the water and heat redistribution of this region do not allow for evident regional drought differences, thereby resulting in no spatial heterogeneity. However, severe and extreme droughts occurred in the least number of months. These trends are significantly important factors that affect crop production and carbon accumulation.

**Figure 5.** Spatial variations in the frequency of drought months in different seasons in Guangxi. (**a**–**d**) Frequency of slight droughts in each season ((**a**–**d**) represent spring, summer, autumn, and winter, respectively); (**e**–**h**) frequency of moderate droughts in each season; (**i–l**) frequency of severe droughts in each season; (**m**–**p**) frequency of extreme droughts in each season.
