Evapotranspiration

Another important climatic factor that contributes to evapotranspiration is wind speed. Winds affect evapotranspiration by bringing heat energy into an area and removing the vaporized moisture. A 5-mile-per-hour wind will increase still-air evapotranspiration by 20 percent; a 15-mile-per-hour wind will increase still-air evapotranspiration by 50 percent (Chow, 1964, p. 6-20). Maximum mean annual wind velocities, averaging more than 14 miles per hour, are recorded in the central United States. Minimum mean annual wind velocities, averaging less than 8 miles per hour, are recorded along the West Coast and in the mountainous part of the east-central United States.

• What role does wind speed have in the evapotranspiration process?

The seasonal trend of evapotranspiration within a given climatic region follows the seasonal trend of solar radiation and air temperature.

• During what season would you expect to find the minimum evapotranspiration rates?
• During what season would you generally expect to find the maximum rates?

Regional and Seasonal Variability of Evapotranspiration

The United States is covered by a variety of vegetation due mostly to the variability in climate and soil types across the country. In the contiguous United States, two major forested areas exist: the eastern forests, which include large areas of conifers and hard- woods, extend from the East Coast to the eastern edge of the central Great Plains; and the western forests, which are predominantly conifers that grow in mountainous areas separated by semiarid basins, extend from the West Coast to the western edge of the central Great Plains. The forests of the eastern United States cover 385 million acres; those of the western United States cover 353 million acres and include about 24 million acres in Alaska (U.S. Department of Agriculture, 1987, p. 475). Estimates of evapotranspiration for the eastern forests range from slightly less than 12 inches per year for spruce-fir forests to slightly more than 36 inches per year for pines and river-bottom hardwoods and for the western forests from about 6 inches per year for pinyon and juniper forests to almost 60 inches per year for Pacific Douglas-fir forests (Kittredge, 1948).