The Dry Ecoregions

Abstract

The essential feature of a dry climate is that annual losses of water through evaporation at the Earth’s surface exceed annual water gains from precipitation. Due to the resulting water deficiency, no permanent streams originate in dry climate zones. Because evaporation, which depends chiefly on temperature, varies greatly from one part of the Earth to another, no specific value for precipitation can be used as the boundary for all dry climates. For example, 610 mm of annual precipitation produces a humid climate and forest cover in cool northwestern Europe, but the same amount in the hot tropics produces semiarid conditions.