*3.3. Spatial Evolution Characteristics of Two Extremely Severe Soil Droughts*

Two extreme soil drought events, which occurred in 1998 and 2003, were selected from the period 1990–2018 using the drought migration method (Figure 6). The migration direction of drought cores indicated that the two droughts were mainly concentrated in central Guangxi. The 1998 and 2003 soil drought followed a similar northeast to southwest trajectory. The migration paths of the two soil droughts were longer at the initial stage of formation and later extended with the aggravation of drought duration.

**Figure 6.** Spatial migration process of soil droughts. The direction of the black arrow indicates the migration direction of droughts in the next month. The green and red circles indicate the comprehensive drought index of the current month. (**a**) Average SSMI from September 1998 to March 1999, (**b**) spatial migration of the 1998 soil drought from September 1998 to March 1999, (**c**) average SSMI value from November 2003 to April 2004, and (**d**) spatial migration of the 2003 soil drought from November 2003 to April 2004.

#### *3.4. Correlation between Soil Moisture Anomaly and Ocean Surface Temperature*

The soil moisture anomaly is regulated by precipitation, and the main reasons for precipitation differences are caused by anomalies in the ocean temperature [28,29]. Therefore, to understand the importance of atmospheric circulation caused by SST anomaly to soil moisture in Guangxi, we compared the teleconnection between soil moisture and ocean temperature (Figure 7). The spatial variation of the dominant pattern (EOF-1) obtained from EOF analysis was similar to the soil moisture trend during 1990–2018, accounting for 66.9% of the total square covariance of Guangxi. Overall, the PC-1 showed that the soil moisture in Guangxi showed an increasing trend over time. Correlation analysis showed that PC-1 and SST were significantly positively correlated (*p* < 0.05), suggesting that SST might be an important teleconnection factor affecting soil moisture in Guangxi.

**Figure 7.** Relationship between SSMI and sea surface temperature in Guangxi during 1990–2018. (**a**) Spatial trend of soil moisture, (**b**) main transformer mode calculated using an empirical orthogonal function (EOF-1), (**c**) main transformer mode (EOF-1) corresponding to the monthly change in the principal component (PC-1), and (**d**) correlation between ocean temperature and PC-1 (corresponding to EOF-1).

#### **4. Discussion**

As soil moisture plays an important role in drought research, an assimilation data product is a useful alternative method in the absence of long-term consistent soil moisture observational data at the national scale [15,30,31]. In this study, TerraClimate soil moisture product was used to construct the SSMI. The soil drought index derived from the data set was ideal to monitor regional droughts, and it accurately describe the spatiotemporal characteristics of regional soil water addition and loss. Using this index, we observed that the drought types in Guangxi are mainly light drought and moderate drought, and the occurrence of severe and extreme drought is relatively low, which is basically once in five years. Spatially, the occurrence frequency is low in the middle and high in the east and west. The majority of the soil droughts in Guangxi evolved from moderate droughts, and the probability of sudden, short, and strong droughts was low. This provides additional time for early warning and prevention from the beginning of droughts to the beginning of abnormal droughts, which further helps to reduce negative implications of droughts.

According to the different disaster seasons, soil droughts can be divided into spring, summer, autumn and winter droughts [15]. The drought pattern in Guangxi differed under the influence of monsoon circulation and tropical cyclone, and the frequency of autumn droughts was the highest, followed by winter droughts, while that of spring and summer droughts was low. Spring droughts refer to the droughts between March and May [32]. During spring, crops bloom, grow, and develop in Guangxi; moreover, it is the sowing and emergence season for spring plants. Spring precipitation in Guangxi is relatively less, and precipitation less than the usual intensity can cause serious droughts that not only affect summer vegetation productivity, but also cause bad spring sowing conditions and affect the growth and harvest of autumn crops and carbon accumulation. Summer affects vegetation productivity and ecosystem operation [33] and is most vulnerable to monsoon. The frequency of droughts in Guangxi in summer was extremely low, possibly due to the location of Guangxi in the monsoon region. In summer, the strong East Asian monsoon brings in a large water mass from the ocean, and the rain-forming clouds over Guangxi lead to abundant regional precipitation, which improves the soil water content. During autumn, autumn harvest plants mature and overwintering plants sprout and are sown. Autumn droughts occur between September and November. They may not only affect the autumn vegetation productivity of the current year, but also the summer vegetation productivity of the next year [34]. Autumn droughts occur almost once every two years in Guangxi, with slight droughts being more common. The frequency of these droughts was higher in the middle-eastern regions than of that in the western regions. Moreover, the frequency of moderate, severe, and extreme droughts in this season was also significantly higher than of that in other seasons. In addition, autumn is generally characterized by water storage, long-term droughts, and less rain. Subsequently, the reductions in runoff cause insufficient water reserves for water conservancy projects, thereby creating difficulties in using water during winter and spring. Winter droughts occur from December to February of the next year. In Guangxi, winter droughts occur once in two years, with slight droughts having a higher frequency in the central and southwest regions than in in the northeast regions. Overall, the frequency of soil droughts in autumn and winter in Guangxi was relatively high. Among these droughts, most were slight and moderate droughts. These findings suggested that the impact of autumn and winter soil droughts should be considered while assessing the impacts of droughts on regional crop production and ecosystem.

From the perspective of temporal characteristics, the two major soil droughts (1998 and 2003) in Guangxi lasted for more than six months. Every natural phenomenon has its own unique process of formation, occurrence, development, and extinction [15]. For example, floods tend to form quickly and can be formed in a few days or even hours [35]. Hurricanes form relatively faster, probably within hours, minutes, or seconds [36]. Contrastingly, the occurrence and development of soil droughts is much slower (several months and several seasons) [15]. Long-term soil water deficit affects the regional crop production and domestic and ecological water demand [13,30]. In addition, regarding the spatial scale, the two major soil droughts in Guangxi occurred extensively. Most areas in Guangxi are affected by the subtropical monsoon humid climate, which reduces the probability of soil droughts. However, once soil droughts occur, the soil moisture in the entire region is relatively reduced. Some studies predict that although Guangxi is a humid region, several measures to deal with soil water deficit under the background of frequent extreme climate events in the future will assist in reducing losses in agricultural production and other associated economic losses [37].

Regarding variability, as soil droughts are temporary phenomena, they are a direct reflection of the persistent anomalies in atmospheric circulation and major weather systems. The time and intensity of monsoon onset and retreat and the duration of monsoon interruption are directly related to soil drought [38]. The atmospheric circulation anomaly refers to the abnormal changes in the development, mutual configuration and interaction, and intensity and location of some atmospheric circulation systems, all of which directly cause large-scale droughts and floods [39,40]. The anomaly of monsoon circulation implies that the time, position, advance and retreat speed, and intensity of monsoon change considerably compare with those of normal years [41,42], which is often the reason for the frequent occurrence of soil droughts in the monsoon region. The abnormal atmospheric or monsoon circulation results in less precipitation in a certain area compared with the normal conditions. When the degree and duration of low precipitation reach a certain

degree, meteorological droughts occurs [43]. As precipitation is the main source of water supply, meteorological droughts may induce soil droughts [15]. During the early stage of meteorological droughts, the soil moisture content will not decrease immediately due to the regulation and storage of soil moisture [44]. However, less precipitation is generally accompanied by a temperature increase, which further enhances evapotranspiration and excessive water consumption in the vadose zone. Under other constant conditions, when the meteorological droughts intensify and spread further, the precipitation and runoff may decrease, while the water in the vadose zone will continue to be consumed and not be supplemented, and thus, the soil water condition will further deteriorate [45,46]. Our findings indicated a strong positive correlation between the soil moisture in Guangxi and the ocean temperature in the surrounding sea area, which was in agreement with our assumption that the ocean surface temperature anomaly creates the atmospheric circulation anomaly or monsoon circulation anomaly, and later affects the rainfall and soil moisture anomaly in Guangxi.

This study makes up for the deficiency of previous studies on drought in Guangxi from the perspective of soil moisture, but the analysis results still have some uncertainties and deficiencies. First, this study only selects TerraClimate soil moisture products with high spatial resolution, but the applicability of this data in karst areas has not been fully evaluated. However, comparing the soil moisture products through model and reanalysis, TerraClimate soil moisture, which is more reliable and corrected by remote sensing and models, has been applied to the study of Guangxi for the first time. In addition, there are many factors affecting the dynamics of soil moisture in the driving force analysis. This study uses the most fundamental driving factor—ocean surface temperature, which may not fully explain the long-term evolution characteristics of soil drought in Guangxi.

#### **5. Conclusions**

In this study, the SSMI model was constructed using the TerraClimate soil moisture data; additionally, the applicability of SSMI in soil drought monitoring in Guangxi was evaluated. The following conclusions were drawn: (1) The annual autumn and winter soil droughts in Guangxi were moderate from 1990 to 2018, and the probability of moderate and higher-grade drought after 2005 is much lower than that before 2005. (2) The level of soil drought in Guangxi is mainly light drought and moderate drought, and the possibility of severe drought and extreme drought is relatively low. (3) Two severe soil droughts that occurred in 1998 and 2003 exhibited a large disaster-affected area and persisted for a long duration. (4) The principal component variables of ocean surface temperature and soil moisture showed a strong positive correlation, implying that the ocean surface temperature anomaly may be the root driving force of soil moisture variation in Guangxi. These findings provide scientific guidance for the early warning, prevention, and mitigation of social, ecological, and economic losses associated with soil droughts in Guangxi. Moreover, the results serve as a valuable reference for understanding the impacts of large-scale climate anomalies on soil moisture.

**Author Contributions:** Conceptualization, R.L. and W.W.; methodology, W.W.; software, R.L.; validation, R.L., W.W. and J.S.; formal analysis, J.S.; investigation, R.L.; resources, W.W.; data curation, J.S.; writing—original draft preparation, R.L.; writing—review and editing, R.L.; visualization, J.S.; supervision, R.L.; project administration, R.L.; funding acquisition, W.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This work was funded partially by the National Natural Science Foundation of China (no. 52079033), and Guangxi Key R&D Program (Guike AB19259015) assisted with data presentation.

**Conflicts of Interest:** All individuals included in this section have consented to the acknowledgement.
