Measures of Drought

Dec 4, 2013

Drought is a universally understood phenomenon — especially here in the arid Southwest. But what does drought really mean? To help define the term, and the concept, scientists use several commonly used drought indices. Each summarizes thousands of data points on rainfall and other information into a single handy number.

A scene of Southwestern drought
Credit University of Arizona Climate Assessment of the Southwest

Meteorological drought is probably the most widely used metric. Arizona uses the Standardized Precipitation Index for drought. For any given location and month, rainfall figures are ranked from wettest to driest for the entire period of record.

The current month is then placed into those rankings. If it's the driest, it's in the bottom 1 percent.  That ranking gets converted to an index from plus 2 for abnormally wet up to minus 2 for extremely dry. 

But rainfall is only part of the picture. So farmers who want to know about the effects of dryness on their crops often refer to another measure: the Palmer Drought Severity Index. It takes into account available soil moisture, evaporation from soil and plants, and groundwater recharge rates. Calculated so that comparisons can be made between locations, the index works best for large areas of uniform topography.

But the Southwestern states have mountainous terrain and complex regional microclimates. So some more refined indices consider additional hydrological factors, like variations in snowpack. Colorado, for example, has developed its own Surface Water Supply Index that evaluates each river basin separately. With that index, water managers can take a detailed local look at local water supply issues.

Earth Notes is produced by KNAU and the Sustainable Communities Program at Northern Arizona University.