## 4 Experiment and Analysis
### Long Time-Series Variation of NMRI and Drought Events  
According to data released by the National Drought Mitigation Center (NDMC), two-thirds of the U.S. experienced a severe drought in 2012. This drought was the worst drought since the 1950s, which lasted three years and did not improve until 2015. The NDMC produces Vegetation Drought Index (VegDRI), a product that indicates the effect of drought on vegetation, in collaboration with the U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) and the High Plains Regional Climate Center (HPRCC) ([[https://www.drought.gov](https://www.drought.gov)]([https://www.drought.gov](https://www.drought.gov))). Figure 11 shows the distribution of VegDRI in July for 2010-2016. The area marked by the red box is the research area of this paper. We chose the worst drought year of 2012 to analyze the seasonal changes of VWC in the western part of the CONUS according to the monthly average NMRI and NDVI long time-series variation diagrams of four land cover types (Figure 12). Beginning in March, vegetation begins to grow with the approach of spring. From April to July, the NDVI values grow to their maximum, and then decrease with the arrival of autumn and winter. Compared with NDVI, the NMRI performs differently; it begins to increase in March and reaches the peak value at May, then it experiences a sharp decline owing to the severe drought in summer, since drought is especially severe in the summer because of the hot  
Figure 10: The importance of the predictors in the random forest (RF) model. and dry climate in the western U.S. As a result, the NMRI index, which can reflect the VWC change information, is more sensitive than the NDVI index that only reflects a change in the greenness of the vegetation to the occurrence of a drought event. During the drought period from May to July, NMRI values for cropland, shrubland, and grassland decreased by 50%, while NMRI values for tree cover only reduced by 30%. This indicates that the high vegetation types are less affected by drought. Then, we selected July with the worst drought to analyze the inter-annual variation of VWC in the western part of the CONUS over the decade from 2007 (Figure 13). When the severe drought in 2012 occurred, the water content of all vegetation types experienced a dip with NMRI fallen by 22% to 50%. NDVI has also experienced a reduction, but not as severe as NMRI (only about 4% to 16%). NMRI and NDVI were recovered and gradually became stable after the drought conditions were relieved. Similarly, the tree cover was least affected by drought, with NMRI decreasing by 30% and NDVI decreasing by only 4%. Identical results can be obtained from the previous drought spatial distribution map (Figure 11). In terms of drought spatial distribution, the regions with severe drought are mainly concentrated in the central inland region, where the main vegetation types are shrubland and grassland. Areas with higher vegetation suffer from a weaker drought. Therefore, we will focus on low-vegetation areas that are more sensitive to drought events in the following analysis. Figure 11: Vegetation Drought Index (VegDRI) distribution map for 2010 to 2016. Taken from [[https://www.drought.gov](https://www.drought.gov)]([https://www.drought.gov](https://www.drought.gov)). Figure 14 selects the 500 m NMRI results in July from 2010 to 2016 as the basis for the analysis of the changes in VWC during the summer drought. During the non-drought period from 2010 to 2011, the NMRI was normal and higher in the west, north, and central/eastern regions. However, it is worth noting that there is a marked decrease for NMRI in 2012, especially in the southern part of the western coastal state of California, the southern part of Idaho, Northeastern Colorado, Northeast Utah, and Southwestern Wyoming.