Impacts of Climate Change and Land Use  on the Southwestern United States

Climatic variability

Historic Variations in Moisture Availability

Katherine H. Anderson
INSTAAR, University of Colorado


Robert S. Thompson
U.S. Geological Survey

Historic Variations in Moisture Availability

In the southwestern United States interannual variability of moisture may be extreme. The Palmer Index (Palmer, 1965) quantifies the scope, severity, and frequency of prolonged periods of unusually dry or wet weather. The Palmer Index is based on moisture balance values that include information on precipitation, surface soil moisture (availability and loss), evapotranspiration, surface moisture, and recharge, and assumes no surface run-off or run-on. Newman (1987) points out that the Palmer Index is used as a management tool for government (i.e. urban water systems) , industry (i.e. water shortages that affect the economy), and agriculture (i.e. soil moisture for growing crops).

Cook and others from NOAA have put together a Palmer Drought Severity Index (PDSI) of instrumental drought data (1895-1995) as well as tree-ring data (1700-1978).

Shown here are two extreme cases for the United States where reds are severe drought conditions and blues indicate very wet conditions (on a scale of -6 to 6).


In 1934 a severe drought affected most of the United States and caused extreme agricultural and economic hardship. The southern Great Plains region was particularly hard hit, and resulted in severe dust storms, reactivation of sand dunes, loss of productive land and widespread abandonment of farms.


Conversely, 1983 was extremely wet in the southwestern United States and coincided with a strong El Niño.

Water, which is so essential to life, can be monitored by the PDSI. This can help policy makers, industry and farmers make appropriate decisions for the times.


Newman, J.E. 1987. Palmer index. in The Encyclopedia of Climatology. Oliver, J.E. and R.W. Fairbridge (eds.) Van Nostrand Reinhold Company, New York.

Palmer, W.C., 1965. Meteorological drought. Weather Bureau Research Paper No. 45.

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