Drought: A Paleo Perspective

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Palmer Drought Severity Index (PDSI)


PDSI Animation, 1930s and 1950s (6 year time frame) The Palmer Drought Severity Index (PDSI) has been the most commonly used drought index in the United States and was developed to measure intensity, duration, and spatial extent of drought. PDSI values are derived from measurements of precipitation, air temperature, and local soil moisture, along with prior values of these measures. Values range from -6.0 (extreme drought) to +6.0 (extreme wet conditions), and have been standardized to facilitate comparisons from region to region.

This drought index has been used to evaluate drought impact on agriculture. Because of the time scale built into this index, it is not suitable for the determination of longer-term hydrologic drought such as those that impact stream flow, reservoirs, and aquifers. The PDSI treats all precipitation as rain, so the index does not perform as well in higher elevations in the western U.S. in winter where much of the precipitation falls as snow.

The animation (shown to the right) demonstrates the distribution of drought from instrumental data for the 1930s Dust Bowl Drought and the 1950s Drought that affected much of the Southwest and Southern Great Plains of the U.S. Red indicates areas of extreme drought, while blue indicates very wet conditions. Notice how extensive an area is under severe drought as the 1930s decade progresses. Texas is a key area for the 1950s drought. These PDSI maps clearly indicate the severity and widespread nature of these droughts.

To view images from the Palmer Drought Severity Index, please click here.

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