Defining Drought Conditions
By Dr. Xiusheng Yang, Connecticut State Climatologist
The University of Connecticut, 2002

Drought refers to an extended period of deficient rainfall relative to the statistical mean for a region. Drought can be defined according to meteorological, hydrological, and agricultural criteria. Meteorological drought is qualified by any significant deficit of precipitation. Hydrological drought is manifest in noticeably reduced river and stream flow and critically low groundwater tables. The term agricultural drought indicates an extended dry period that results in crop stress and harvest reduction.
The Palmer Drought Severity Index (PDSI) is a widely used measure of drought in the United States to track moisture conditions. The PDSI is defined as “an interval of time, generally in months or years in duration, during which the actual moisture supply at a given place rather consistently falls short of the climatically expected or climatically appropriate moisture supply”. The range of PDSI is from –4.0 (extremely dry) to +4.0 (excessively wet), with the central half (-2.0 to +2.0) representing the normal or near normal conditions. Biweekly broad-scale PDSI is published at http://drought.unl.edu/monitor/monitor.html.
(by Xiusheng Yang, the Connecticut State Climatologist)