## 3 Results
### Comparison of the Spatial Variability of the VIUPDD and NDVI Derived VCIs
The correlations of PCI and SPI-6 and SPI-9 are lower than those of NDVI and VIUPD-derived VCIs, MODIS-derived TCI and SMCI in the majority of CDs. SMCI showed stronger correlation with _in situ_ indices than the short-term SPI indices, and showed a weak correlations with PDSI and long-term SPIs for most areas. SMCI is better-suited to monitor the short-term drought conditions of large geographical regions. When compared to other indices, SMCI is particularly sensitive to terrain. As shown in Figure 1, the northeastern areas of Minnesota and Wisconsin are covered by large forests and wetlands, and the r-values of SMCI in these regions were lower than those of cropland or herbaceous regions. The same trend was evident in the eastern areas of deciduous forest, and the northwestern areas of evergreen forests. However, the correlation of SMCI and SPI-1 is higher than that of MODIS-derived TCI in regions of low vegetation density, and with few forests. In order to explore the different performances of VIUPD-derived VCI and other remote sensing-based drought indices in different conditions, we calculated the mean temperature and  
Figure 6: Spatial distribution of climate divisions with correlations (r-value) between VIUPD-derived VCI and other commonly used remote-sensing-based single drought indices with _in situ_ drought indices for the entire growing season (April to October) 2002–2011. mean NDVI of each CD in the growing season from 2002 to 2011 (Figure 7). The differences of r-values between VIUPD-derived VCI and other drought indices in each CD (r-values of VIUPD-derived VCI minus r-values of other remote sensing-based drought indices in each CD, see Figure 8) were also calculated. As shown in Figure 8, when correlated to _in situ_ drought indices, the VIUPD-derived VCI has higher r-values than NDVI-derived VCI in almost all different CDs. When compared to PCI, even the VIUPD-derived VCI performed much better than it did in correlating to long term drought indices such as PDSI, SPI-06 and SPI-09, the VIUPD-derived VCI has much lower correlations with short term SPI than PCI in almost all the CDs. In addition, the VIUPD-derived VCI showed higher r-values than SMCI in the southeastern regions where the mean NDVI is above 0.6 (see Figures 7 and 8). It also should be noticed that the VIUPD-derived VCI performed better than VUA based TCI in all different conditions. However, when compared to MODIS based TCI, even the VIUPD-derived VCI had higher r-values in the hot regions such as CDs of California and Arizona in the southwest regions, it performed worse in cold regions such as Washington. As shown in Figure 6, the performance of MODIS based TCI was worse in the extremely hot regions (CDs in Arizona and New Mexico) and cold regions (northeastern CDs like Michigan and New York to Maine) than other regions, the VIUPD-derived VCI can greatly improve the performance of TCI in hot regions but not in the cold regions. Figure 8: Spatial distribution of climate divisions with correlations (r-value) difference between VIUPD-derived VCI and other commonly used remote-sensing-based drought indices. The difference means the r-value of VIUPD-derived VCI in each CD minus the r-value of other drought indices. Figure 7: The mean temperature and mean NDVI of each CD in the growing season from 2002 to 2011. The sub-figure (**a**) is the mean temperature and sub-figure (**b**) is the mean NDVI of each CD in the growing season from 2002 to 2011. In the sub-figure (**a**), the red color stands for the warmer temperature and the blue color stands for the cooler temperature; in the sub-figure (**b**), red color stands for the lower NDVI and green color stands for the higher NDVI. Even the VIUPD-derived VCI has higher correlations with _in situ_ drought indices in most of CDs of the continental United States than other drought indices compared, it should be noticed that the VIUPD-derived VCI has its own limitations and applicability. Under some circumstances, the VIUPD-derived VCI should be used with caution.