## 3 Results
### Regional Drought Patterns Derived from NDVI and VIUPD-Derived VCI
Based on the USDM, drought conditions were mainly located in the south, covering the majority of Texas, while New Mexico and Georgia also experienced serious drought conditions. These drought conditions became more severe by October 2011. North regions such as Michigan, Iowa and Illinois and Georgia and Alabama in southeastern regions began to suffer drought from August. For NDVI-derived VCI, severe drought appeared in The Great Plain and northeastern and northwestern regions in April and May. Then from June to August, the severe drought disappeared in north and mainly located in south regions such as Texas. In September and October, the south regions drought severity seems to reduce and north regions of The Great Plain experienced more severe drought conditions. According to VIUPD-derived VCI, the drought area mainly located in south and northeastern regions such as New York experienced severe drought in July. The south drought areas expanded and Michigan, Iowa and Illinois began to suffer drought from September to October. The VIUPD-derived VCI described the spatial extent of the drought over the south of the CONUS reasonably well. From April to October 2011, the VIUPD-derived VCI indicated more severe drought conditions than the NDVI-derived VCI in eastern Texas, most of New Mexico, and the southeastern CONUS. These areas were confirmed to be experiencing severe drought conditions by USDM. In regions in which no drought conditions were detected, the VIUPD-derived VCI indicated less-severe drought conditions than the NDVI-derived VCI. In the west and northwest of the CONUS,  
Figure 2: Drought detected by USDM, VIUPD-derived VCI and NDVI-derived VCI for April to October 2011 over the CONUS. The first column is the USDM, the second column is the NDVI-derived VCI, the third column is the VIUPD-based VCI minus the NDVI-derived VCI, and the last column is the VIUPD-derived VCI. from April to October 2011 normal conditions were experienced according to USDM. The map showing the difference between the VIUPDD and NDVI-derived VCI (column 3) showed that in these regions the VIUPDD-derived VCI had higher values than the NDVI-derived VCI. The NDVI-derived VCI indicated more severe drought conditions than the VIUPDD-derived VCI in the regions classified as experiencing no drought. It is even more obvious that in April and June of 2011, NDVI-derived VCI showed significantly more regions with severe drought conditions than VIUPDD-derived VCI in the north and northwestern regions of the COUNS, while these regions were not detected as drought conditions by the USDM. Similar to the NDVI-derived VCI, the VIUPDD-derived VCI may also produce false drought signals. For example, in the northwestern and northeastern regions, from New York to Maine, the VIUPDD-derived VCI indicated more severe drought conditions than USDM. VIUPDD-derived VCI underestimated drought level as compared to the results of USDM and NDVI-derived VCI in southern States including Alabama and Georgia in September and October. However, on the whole, the VIUPDD based VCI showed more similarity to USDM than the NDVI-based VCI. The mere fact that the overall performance of VIUPDD-derived VCI is better than NDVI-derived VCI is not enough to prove that VIUPDD-derived VCI can reasonably monitor drought condition for a long time. The correlations with _in situ_ drought indices for 10 years (2002-2011) were analyzed in the next section.