Evapotranspiration

The Evapotranspiration Process

Assuming that moisture is available, evapotranspiration is dependent primarily on the solar energy, available to vaporize the water. Because of the importance of solar energy, evapotranspiration also varies with latitude, season of year, time of day, and cloud cover. The distribution of mean daily solar radiation for the United States (Figure 2, below) shows a regional variation similar to that of mean annual lake evaporation (Figure 3, below) and mean annual temperature.

According to the 1980 Bureau of Census data (U.S. Bureau of the Census, 1987, p. 181), the area of open-water bodies in the 48 conterminous States totals 38.4 million acres. Mean annual lake evaporation ranges from about 20 inches in parts of Maine, Oregon, and Washington to about 80 inches in parts of Arizona, California, and Nevada.

Figure 2:

Figure 3:

• Which area in the contiguous U.S. receives the maximum solar radiation and the greatest lake evaporation?
• Which areas in the contiguous U.S. receive the minimum solar radiation and have the least lake evaporation?