1. The amount of water evaporated (both as transpiration and evaporation from the soil) from an area of continuous, uniform vegetation that covers the whole ground and that is well supplied with water. Generally, the amount of moisture that, if available, would be removed from a given land area by evapotranspiration; expressed in units of water depth. It can be measured in a dry basin by determining the amount of irrigation water used, and in wetter regions, by the difference between rainfall and runoff, or by the supply of water required to maintain a constant amount of soil moisture in an isolated block of the soil. See evapotranspirometer. 2. The quantity of water evaporated (both as transpiration and evaporation from the soil) per unit area, per unit time from an extensive stretch of continuous, uniform vegetation that covers the whole ground and that is well supplied with water. ; an empirical index of the above. As given in Thornthwaite's 1948 climatic classification, it is equal to the summation of the 12 successive monthly values of the expression ct^a, where t is the monthly mean temperature in degrees Celsius, and a and c are coefficients that depend upon the annual heat index.