Investigation of the utility of the vegetation condition index (VCI) as an indicator of drought

The relationship between the satellite-based Vegetation Condition Index (VCI) and frequently used agricultural drought indices like Palmer Drought Severity Index, Palmer?s Z-index, Standard Precipitation Index, percent normal and deciles was evaluated using a comparative correlation analysis. These indices were compared at the county level for all 254 Texas counties for the growing-season months (March to August) using monthly data from 1982-1999. The evaluation revealed that the VCI was most strongly correlated with the 6-month SPI and the PDSI. This suggests that the VCI is most similar to drought indices that account for antecedent moisture conditions. There was also significant spatial variability in the magnitude of the correlations between the VCI and the drought indices. The reasons for this variability were explored by utilizing additional data such as irrigation, prevalent landuse/landcover, water table depth, soil moisture levels and soil hydrologic/hydraulic properties. The results demonstrated that mean annual precipitation, soil moisture, landuse/landcover, and depth of the water table accounted for a significant amount of the spatial variability (explaining more than 75% of the variance) in the relationship between the VCI and traditional drought indices.