Palmer Drought Severity Index (PDSI) The PDSI was the first comprehensive drought index developed in the United States. It is a meteorological drought index based on a balance between moisture supply and demand and responds to weather conditions that have been abnormally dry or abnormally wet. The index is calculated based on precipitation and temperature data, as well as the local Available Water Content (AWC) of the soil, i.e., soil moisture. From the index’s inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer. The index, however, is less well suited for mountainous regions or areas of frequent climatic extremes. Human impacts on the water balance, such as irrigation or drainage, are not considered. The index roughly ranges from extremes of -6 to + 6, with negative values denoting dry spells and positive values indicating wet periods. There are also a few values in the magnitude of -7 or +7. Ideally, the PDSI is designed so that a -4.0 in South Carolina has the same meaning in terms of the moisture departure from a climatological normal as a -4.0 in Idaho. The following shows the more typical PDSI classifications: 4.00 or more – Extremely wet 3.00 to 3.99 – Very wet 2.00 to 2.99 – Moderately wet 1.00 to 1.99 – Slightly wet 0.50 to 0.99 – Incipient wet spell 0.49 to -0.49 – Near normal -0.50 to -0.99 – Incipient dry spell -1.00 to -1.99 – Mild drought -2.00 to -2.99 – Moderate drought -3.00 to -3.99 – Severe drought -4.00 or less – Extreme drought Also see Drought Indexes (Indices).
Source: http://www.bvsde.paho.org/bvsacg/i/fulltext/dicciona/dicciona.pdf
Web site to visit: http://www.state.nv.us/cnr/ndwp/home.htm
Author of the Water Words Dictionary source of text: Gary A. Horton
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