See examples of PET given for many crops at different growth stages and how this concept is used in irrigation scheduling. The other critical factor in this discussion is the amount of water actually lost by the vegetated surface, referred to as actual evapotranspiration or ET. Actual rates of evapotranspiration only reach potential rates when water is not limiting i. However, when the amount of plant-available water in the soil decreases, the actual rate of ET quickly falls to a fraction of PET.
After long stretches of dry weather, the ratio of actual to potential evapotranspiration rates commonly falls to values below 0. As can be seen in the figure, rates of PET have been abnormally high ever since the middle of May the first half of that month was wetter and cloudier than normal , with a difference of 2.
The difference during this period is due to several meteorological differences, including greater than normal solar radiation levels 8. Not only have soils generally not been able to supply sufficient water to meet crop needs due to the extended dryness, but rates of PET based on atmospheric conditions have been significantly greater than normal this year, which has exacerbated the effects of the drought.
This article was published by Michigan State University Extension. What is evapotranspiration and why it matters. Figure 1. Observed potential evapotranspiration PET from May 1 to July 24, red line , versus the long-term average , black line at East Lansing, Mich. PET data is used to calculate the water requirements of crops and landscape plants, and adjust irrigation schedules in a way that encourages efficient water use, reduces waste, and saves money.
Reduction in over-watering also reduces pollution from fertilizer and chemical run-off into waterways. With projected increases in population and water demand, water conservation is rapidly becoming an issue in all parts of Texas, not just regions of low rainfall. Conservation methods must be implemented now, as one method of addressing future demands.
Bureau of the Census, , p. 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. Mean daily solar radiation in the United States and Puerto Rico.
Source: Data from the U. Department of Commerce, Figure 3. Mean annual lake evaporation in the conterminous United States, Data not available for Alaska, Hawaii, and Puerto Rico. Source: Data from U. 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 mile-per-hour wind will increase still-air evapotranspiration by 50 percent Chow, , p. 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 Eagleman, , p. 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 Kozlowski, , p. Most plants that have a shallow root system, however, will experience moisture stress, which results in decreased transpiration during prolonged droughts.
The reflective characteristics of the land surface 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, , p.
The seasonal trend of evapotranspiration within a given climatic region follows the seasonal trend of solar radiation and air temperature. Minimum evapotranspiration rates generally occur during the coldest months of the year; maximum rates, which generally coincide with the summer season, when water may be in short supply, also depend on the availability of soil moisture and plant maturity.
However, the seasonal maximum evapotranspiration actually may precede or follow the seasonal maximum solar radiation and air temperature by several weeks. In the conterminous 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; 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 million acres; those of the western United States cover million acres and include about 24 million acres in Alaska U. Department of Agriculture, , p. 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, Some of the greatest users of water are phreatophytes, which are plants characterized by a deep root system that extends to or near the water table.
Saltcedar, which is a particularly aggressive phreatophyte, is estimated to cover 16 million acres in the flood plains of the 17 Western States; it thrives in the arid regions south of the 37th parallel and below an altitude of 5, feet in the Southwestern States Robinson, , p.
Mean annual evapotranspiration by this phreatophyte was estimated to be about 56 inches for areas of dense cover along the flood plain of the Gila River in south-central Arizona Culler and others, In contrast to the two major forest areas, the central Great Plains are characterized by large regions of rangeland and cropland irrigated and nonirrigated.
The total rangeland in the conterminous United States and Alaska is about million acres, and the total cropland is about million acres U. Department of Agriculture, Within these areas, irrigated grass or croplands occupy about 60 million acres Irrigation Journal, 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. Most estimates of evapotranspiration are derived from studies of small areas a few acres or less where climate, available moisture, and plant cover are relatively uniform; thus, regional estimates are uncommon.
However, the magnitude and distribution of mean annual evapotranspiration for regions of the United States have been estimated from hydrologic budgets given for each State in the National Water Summary U. Geological Survey, , as shown in figure 4. The estimated mean annual evapotranspiration for each State was determined from the mean annual statewide values of four principal components of the hydrologic budget-precipitation, surface-water inflow, surface-water outflow, and consumptive use.
All four components were measured or estimated, and evapotranspiration was computed as a residual of these components. Figure 4. Estimated mean annual evapotranspiration in the United States and Puerto Rico. Source:Data compiled from U.
For the United States and Puerto Rico, the estimated mean annual evapotranspiration ranges from a maximum of 45 inches per year in Puerto Rico to a minimum of 7. Within the conterminous United States, the estimated mean annual evapotranspiration is greatest in the Southeast about 35 inches per year or about 70 percent of the precipitation , which is an area of abundant precipitation, permeable soils, and substantial solar radiation; it is least in the semiarid region of the Southwest where precipitation is limited.
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