This time-enabled layer shows areas of moderate to extreme drought for the lower 48 states for all months for the years 1895 through 2012.
Drought status is represented by the Palmer Drought Severity Index (PDSI) for each climate division in the United States. Only instances where drought exceeded near-normal values (PDSI < -2) were included in this dataset.
The PDSI is an index of the relative dryness or wetness effecting water sensitive economies, indicating prolonged and abnormal moisture deficiency or excess. Inputs to its calculation include weekly precipitation total, average temperature, division constants (water capacity of the soil, etc.), and previous history of the index.
The PDSI is an important climatological tool for evaluating the scope, severity, and frequency of prolonged periods of abnormally dry or wet weather. It can be used to help delineate disaster areas and indicate the availability of irrigation water supplies, reservoir levels, range conditions, amount of stock water, and potential intensity of forest fires.
