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The Vegetation Health Index (VHI) is a composite index and the elementary indicator used to compute the ASI. It combines both the VCI and the Temperature Condition Index (TCI). The TCI is calculated using a similar equation to the VCI, but relates the current temperature to the long-term maximum, as it is assumed that higher temperatures tend to cause a deterioration in vegetation conditions. A decrease in the VHI following, for example, a decline in the VCI (relatively poor green vegetation) and an increasing TCI (warmer temperatures) would signify stressed vegetation conditions, and over a longer period would be indicative of drought. The VHI components (VCI and TCI) are given equal weights when computing the index. The VHI images are computed for the two main seasons and in three modalities: dekadal, monthly and annual.
http://www.fao.org/giews/earthobservation/country/index.jsp?code=TLS
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