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Drought Home • Drought Basics • Responding to Drought
A drought is a period of abnormally dry weather that lasts long enough to produce a serious imbalance in the water cycle. A drought is defined and measured in relation to normal climate conditions. In other words, a drought in Virginia could equate to wet conditions in Arizona. The specific factors that define droughts are rainfall amounts, vegetation conditions, agricultural productivity, soil moisture, reservoir levels, changes in river and stream flows and economic impacts. Droughts can last for many years and have devastating effects on agriculture, water supplies and the economy.
Wayne Palmer developed the Palmer Drought Severity Index, the most commonly used drought severity measurement tool, in the 1960s. This index uses temperature and rainfall information in a mathematical formula to determine dryness. The index is most effective over long periods of time, at least several months. Normal conditions are placed at zero, with dry conditions expressed as negative numbers and wet conditions as positive numbers.