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Drought Indicators

In order to get a complete picture of drought conditions, an analyst should examine several drought indicators and indices. These include simple indices like the percent of normal precipitation and number of days with no precipitation, specific indices created to assess drought (such as the Palmer Drought Index and Standardized Precipitation Index [SPI]), complex models (such as the National Land Data Assimilation System [NLDAS]) which calculate soil moisture and other hydrologic variables, indices used for water supply forecasting (such as the Surface Water Supply Index [SWSI]), and indices which reflect impacts on vegetation (such as the Vegetation Health Index [VHI] and Vegetation Drought Response Index [VegDRI]) and water availability (such as groundwater well levels and streamflow). The analyst should also examine indices at many different time scales to assess short-term to long-term drought conditions. The U.S. Drought Monitor does this by depicting drought integrated across all time scales and differentiates between agricultural and hydrological impacts.