## 3 Results
### Comparison of the Spatial Variability of the VIUPDD and NDVI Derived VCIs
During the period that vegetation develops reproductive organs, drought has the greatest effect on vegetation health, thus the vegetation is more sensitive to drought conditions during the rapid growing months. The results of experiments conducted by [PERSON] _et al._[3] showed that the relationship between vegetation and 3-month SPI in June to September is stronger than in May is agreed with our studies. On the other hand, the fact that VIUPD-derived VCI exhibited significantly stronger correlations with _in situ_ drought indices than the NDVI-derived VCI did in all sub-periods, cropland- and grassland-dominated climate divisions indicated that VIUPD-derived VCI are not only more sensitive to vegetation moisture demand in rapid growing season but also the beginning of the vegetation growth. The VIUPD-derived VCI and NDVI-derived VCI generally displayed a similar trend with regard to their correlations with _in situ_ drought indices in all sub-periods of the growing season. At the start of the growing season (April to May), the correlation between the VIUPD-derived VCI and SPI-1 differed markedly from that of the NDVI-derived VCI. The VIUPD-derived VCI exhibited a considerably greater number of climate divisions with r-values higher than 0.4 compared to the NDVI-derived VCI, and there was also a stronger correlation with SPI-1 over the whole CONUS (Figure 5). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Month**} & **Drought** & \multicolumn{8}{c}{**r (_n = 6880_)**} \\ \cline{3-10}  & **Indices** & **PDS1** & **Z** & **SPI-1** & **SPI-3** & **SPI-6** & **SPI-9** & **SPI-12** & **SPI-24** \\ \hline \multirow{2}{*}{April-May} & VIUPD-VCI & 0.351 & 0.339 & 0.425 & 0.381 & 0.360 & 0.429 & 0.385 & 0.326 \\  & NDVI-VCI & 0.257 & 0.269 & 0.293 & 0.329 & 0.315 & 0.265 & 0.191 & 0.117 \\ \hline \multirow{2}{*}{Jun-July} & VIUPD-VCI & 0.750 & 0.699 & 0.463 & 0.531 & 0.601 & 0.651 & 0.643 & 0.550 \\  & NDVI-VCI & 0.527 & 0.485 & 0.355 & 0.417 & 0.383 & 0.428 & 0.446 & 0.312 \\ \hline \multirow{2}{*}{August-September} & VIUPD-VCI & 0.768 & 0.529 & 0.402 & 0.647 & 0.699 & 0.723 & 0.684 & 0.514 \\  & NDVI-VCI & 0.552 & 0.338 & 0.226 & 0.476 & 0.475 & 0.493 & 0.444 & 0.374 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparisons between VIUPD and NDVI-derived VCIs in different sub-periods of the growing season. Figure 5: Spatial distribution of climate divisions with correlations (r-value) between VIUPD and NDVI-derived VCIs, and _in situ_ drought indices for the three sub-periods from April to September. The sub-figure (**a**) stands for the correlations (r-value) from April to May; the sub-figure (**b**) stands for the correlations (r-value) from June to July, sub-figure (**c**) stands for the correlations (r-value) from August to September. Comparison of the Spatial Variability of VIUPD-Derived VCI and Other Remote-Sensing-Based Single Drought Indices  
The TCI, PCI and SMCI are widely used remote-sensing-based single drought indices [2; 35]. To assess the ability of VIUPD-derived VCI to monitor drought, the spatial patterns of VIUPD-derived VCI and the widely used remote-sensing-based single drought indices were compared for April to October 2002-2011 (Figure 6 and Tables 3-5). The spatial patterns detected by the drought indices differed substantially (Figure 6). Each drought index has its own strengths and limitations. As shown in Figure 6, MODIS-derived TCI, NDVI-derived VCI and VIUPD-derived VCI presented a similar spatial variability. There were higher correlations with PDSI and long-term SPI in warm southern regions for MODIS-derived TCI, but lower correlations in northeastern regions such as Minnesota. PCI exhibited stronger correlations with SPI-1 in almost all parts of the CONUS than the other indices. Additionally, PCI showed a stronger correlation with SPI-3 than NDVI-derived VCI, MODIS-derived TCI and SMCI in the eastern and western CDs. However, the correlations of PCI and longer-term SPI indices are lower in most of the CONUS.