lack or insufficiency of rain for an extended period that causes a considerable hydrologic (water) imbalance and, consequently, water shortages, crop damage, streamflow reduction, and depletion of groundwater and soil moisture. It occurs when evaporation and transpiration (the movement of water in the soil through plants into the air) exceed precipitation for a considerable period. Drought is the most serious physical hazard to agriculture in nearly every part of the world. Efforts have been made to control it by seeding clouds to induce rainfall, but these experiments have had only limited success.

There are four basic kinds of drought:

1. Permanent drought characterizes the driest climates; the sparse vegetation is adapted to aridity, and agriculture is impossible without continuous irrigation.

2. Seasonal drought occurs in climates that have well-defined annual rainy and dry seasons; for successful agriculture, planting must be adjusted so that the crops develop during the rainy season.

3. Unpredictable drought involves an abnormal rainfall failure; it may occur almost anywhere but is most characteristic of humid and subhumid climates. Usually brief and irregular, it often affects only a relatively small area.

4. Invisible drought can also be recognized: in summer, when high temperatures induce high rates of evaporation and transpiration, even frequent showers may not supply enough water to restore the amount lost; the result is a borderline water deficiency that diminishes crop yields.