Evapotranspiration

In contrast to the two major forest areas, the central Great Plains are characterized by large regions of rangeland and cropland (irrigated and non-irrigated). The total rangeland in the conterminous United States and Alaska is about 817 million acres, and the total cropland is about 427 million acres (U.S. Department of Agriculture, 1987). Within these areas, irrigated grass or croplands occupy about 60 million acres (Irrigation Journal, 1985). The average annual evapotranspiration for irrigated lands varies greatly and, apart from the climatic controls, is dependent on the grass or crop type, quantity of water applied, and length of the growing season. During a drought, natural vegetation may experience moisture stress and wilting, whereas irrigated grasses and crops continue to grow and transpire at a normal rate (if water supplies are available for irrigation).

The type of vegetative cover is not as important in the evapotranspiration process as is solar radiation if the vegetative cover is dense and sufficient soil moisture is available.

• When the weather is particularly dry, what happens to plants that have a shallow root system?
• How does dry weather affect the rate of transpiration?

The reflective characteristics of the land surface (a measurement known as albedo) also have an effect on the magnitude of evapotranspiration. Coniferous forests and alfalfa fields reflect only about 25 percent of the solar energy, thus retaining substantial thermal energy to promote transpiration; in contrast, deserts reflect as much as 50 percent of the solar energy, depending on the density of vegetation (Rosenberg, 1986, p.13).