## 3 Results  
### Drought Characteristics of the Loess Plateau  
#### 3.1.1 Spatial and Temporal Variability of Drought  
Spatial and temporal variations of SPEI and SSMI from 2001 to 2020 were analyzed to determine the interannual wet and dry conditions on the Loess Plateau. The monthly, seasonal, semi-annual, and annual trends of SPEI are presented in Figure 3. SPEI fluctuates greatly on short time scales (monthly and seasonal scales) and then slows down with increasing time scales. SPEI_01 has the most frequent wet and dry variations, with a total of 62 drought months occurring from 2001 to 2020, of which there were 39 months of mild droughts, 19 months of moderate droughts, and 4 months of severe droughts, with no extreme droughts. The longest drought period was from May to July 2001. Compared with the monthly scale, the seasonal scale of SPEI_03 shows relatively stable wet and dry changes, with a total of 52 drought months, 41 of which were mild droughts, 8 were moderate droughts, 3 were severe droughts, and there were no extreme droughts. The longest drought period was from May to August 2001. SPEI_06 was significantly slower than the previous two short time scales, with a total of 29 drought months, 26 of which were mild droughts, 3 were moderate droughts, and no severe or extreme droughts. SPEI_12 was the most stable, with only 5 mild drought months. SM data were used and converted to SSMI to reflect the wet and dry status of the soil. During the period 2001-2020, 133 drought months were identified, accounting for more than half of the entire study period, and were dominated by mild drought, with 128 months of mild drought, 5 months of moderate drought, and no severe or extreme drought.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Anomaly** & **Class** \\ \hline \(>\)2 & Strong positive anomaly \\ \(1-\)2 & Moderate positive anomaly \\ \(-\)1\(-\)1 & No anomaly \\ \(-\)1\(--\)2 & Moderate negative anomaly \\ \(<\)\(-\)2 & Strong negative anomaly \\ \hline \hline \end{tabular}
\end{table}
Table 2: Classes of anomalies.  
The spatial trends of SPEI and SSMI were analyzed using Sen trend analysis and the M-K test (Figure 4a,b). SPEI showed an increasing trend in all land cover types. However, the percentage of areas with significant increase was small, with sparse vegetation, grassland, cropland, forest-shrub-grass vegetation mosaic belt, deciduous forest, and evergreen forest with significant increase accounting for 2.01%, 7.19%, 6.87%, 3.01%, 3.74%, and 3.56% of their respective areas, respectively. The SSMI was in an upward trend in sparse vegetation, grassland, cropland, forest-shrub-grass vegetation mosaic belt, and in a downward trend in deciduous and evergreen forests. Among them, grassland and cropland had the most significant increase, accounting for 54.97% and 43.11% of their respective regions, while cropland, deciduous forest, and evergreen forest had significant decreases, accounting for 11.37%, 58.33%, and 20.34% of their respective regions, and in particular, the area with highly significant decreases in deciduous forest reached 34.24%. From the viewpoint of the entire Loess Plateau, the trend of drought is decreasing, and drought has weakened.