Evaporation

10.1 General Description

Wyoming's low relative humidity, high percentage of sunshine, and high persistent winds contribute to a climate that is considered semiarid. Annually, evaporation ranges from 30 to about 50 inches (Figure 10.1, Figure 10.2, Figure 10.3, and Figure 10.4). Evaporation varies much less on a yearly basis than either streamflow or precipitation. Extreme variations in annual total evaporation are within 25 percent of the long term annual average.^82a However, at Wyoming's highest elevations, precipitation actually exceeds evaporation. To illustrate this point further, if a desert is simply defined as the net annual deficit of precipitation from evaporation, then the amount of precipitation in and of itself cannot determine if a region is a desert. Case in point: In Barrow, Alaska, located well north of the Arctic Circle, the average annual precipitation total is just over four inches and its average annual temperature is just over 10°F. However, evaporation is well under four inches annually and this results in Barrow's landscape being surrounded by marsh in summer and thick perma-frost during the remainder of the year.

Most of Wyoming has been subjected to erosion for tens of thousands of years and less than 10 percent is covered with a mantle of recent (geologically speaking) water-transported soil. The lack of such soil and adequate moisture limits the natural vegetation to hardy plants such as sagebrush, greasewood, and short grass. Low relative humidity and a high rate of evaporation add to the problem. A number of abandoned homesteads of one time enthusiastic settlers bear silent testimony to the lack of moisture. Even so, dryland farming is carried on successfully in some areas.

Figure 10.1. Pan evaporation during the warm season measured in inches

Figure 10.2. The free water surface evaporation is defined by multiplying the pan evaporation (Figure 10.1) by the appropriate coefficient from Figure 10.4.

Figure 10.3. The average free water surface evaporation for the year

Figure 10.4. Pan to free water surface evaporation coefficients (measured as a percent of one another)

Approximately 42% of the state's total area is privately owned land, the majority of which is used for grazing, although some is timberland. The fact that most of the state is still government owned attests to the semiarid climate which has made the land less attractive to homesteaders. Nearly four percent of the state is cultivated cropland, including both irrigated and non-irrigated. Another 13 percent is covered with forests, while parks and recreational areas take up about four percent.

The majority of the state is used for grazing and has a general appearance of dryness most of the time. However with the more abundant spring moisture, a greener landscape and increase in biodiversity often occurs. As the season merges into summer, grasses and flowers turn brown, but continue to serve as food for livestock. Native grasses are nutritious, although scant. There are some very fine grazing areas with luxuriant grasses, especially in or near the mountains. Grass is generally so scarce that large ranches are required for profitable operation. The average for most cattle grazing is about 35 to 40 acres per cow. The mountain areas provide timber and a storage place for the winter snows which in the spring and summer feed lakes and reservoirs used in the irrigation districts. Most of the irrigated land is located in the valleys of the following river systems and their tributaries: North Platte, Wind River, Bighorn, Tongue, and Green Rivers. Principle crops in the irrigation districts are sugar beets, beans, potatoes, and hay. On the non-irrigated land the principle crops are hay and small grains, such as wheat, barley, and oats.

10.2 Snow Fences⁸³

A remarkable fact about blowing snow is that after being transported along the ground for five miles, it has essentially lost all its moisture through a process called sublimation (Figure 10.4a).

Figure 10.4a. Both theory and experiments show that 80 percent of snow drifting off a five-mile fetch evaporates.

10.2.1 Structural Fences vs. Living Snow Fences⁸⁵, ⁸⁶

The conventional slatted fence(4 feet high) and the Wyoming Design board snow fence (8 to 13.8 feet high) are the most popular structural fence designs.

Their advantages include:

• Erected and in use very quickly.
• Used at sites where vegetation is not practical.

Their disadvantages include:

• High establishment and annual maintenance costs.
• Life span of materials is 20 years.

Advantages of Living Snow Fences include:

• Much less expensive than structural fences (nearly 7 times less).
• Life span is 50 years.
• Designed for wildlife habitat, livestock protection
• Used as visual screens and helps to reduce soil erosion.

Their disadvantages include:

• Long time lag before vegetation starts to trap snow.
• Barrier density may vary as vegetation grows at different rates.
• Vegetation is susceptible to damage from insects, disease, etc..

Since 1989, the Laramie County Conversation District has more than 200 living snow fences in place. For more information, contact your local conversation district manager.

Figure 10.5. Example of a snow fence's effectiveness

Figure 10.6. An equilibrium snowdrift cross section is used to compute how much snow transport the snow fence will trap

10.3 Evapotranspiration (ET)⁸⁷

Of all the climate data requested at the Wyoming climate office, ET is perhaps at the top of the list. This should come as no surprise since the agricultural community has a critical need to know the planting success rate of their crops that depend on available soil moisture. With evaporation exceeding precipitation by at least four times, irrigation water management becomes the best means to maximize productivity. Understanding plant root zone depths, soil types, and volume of irrigated water to apply, the ET or consumptive use of water by plants can then be more efficiently derived.

From field tests, about 40 percent of the total moisture intake by plants is extracted by the plant's roots from zero- to six-inch depths; 30 percent from six- to 12-inches; 20 percent from 12 to 18 inches; and 10 percent from 18 to 24 inches. Each soil type has an inherent "available water" holding capacity which can vary from 1.0 inch per acre foot for a loamy sand to 2.5 inches for a silty clay loam. Consequently, a two-foot root zone will typically have an irrigation water requirement that can vary between 1.5 inches (40,500 gallons per acre) for a course textured soil to 2.5 inches (67,500 gallons per acre) for a fine-textured soil. Most flood irrigation systems are between 45 to 70 percent efficient, therefore 3.0 inches (81,000 gallons per acre) is a recommended application.⁸⁸

As a rule of thumb, irrigation should be applied when 30 to 50 percent of the available water is depleted in the zero to 12-inch root zone and when about 15 to 30 percent is depleted in the 12 to 24-inch root zone. It takes about 48 hours for the surface moisture to recharge the soil to a 24-inch depth for most soils. As a general guideline, a fully mature tree at peak water consumption can remove 0.2 to 0.3 inches of moisture per acre per day. This translates to irrigating between eight and 10 days if no precipitation occurs during this interval. Of course solar radiation, wind, humidity, temperature, precipitation, crop variety, soil drainage, and water quality are important factors for successful irrigation.

10.4 Evaporation Rates for Select Wyoming Stations ^88a

Standardized daily evaporation is measured using the four-foot diameter Class A evaporation pan (Figure 10.7). The pan's water level reading is adjusted when precipitation is measured to obtain the actual evaporation. Most Class A pans are installed above ground allowing for effects such as radiation on the side walls and heat exchanges with the pan material to occur. These effects tend to increase the evaporation totals. The amounts can then be adjusted by multiplying the totals by 0.7 or 0.8 to more closely estimate the evaporation from naturally existing surfaces such as a shallow lake, wet soil, or other moist natural surfaces. Many stations do not measure pan evaporation during winter months. The "---" indicates no monthly measurements were taken (Table 10.A.). At Afton, daily ET has been available since late 1987 (Figure 10.8). During the hottest days of summer, ET rates can exceed 0.3 inch per day. The frequency distribution of Afton ET is show in Figure 10.9.

Figure 10.7. Class A evaporation pan

Figure 10.8. Afton daily ET (1988-2002)

Figure 10.9. Afton daily ET frequency by month (1988-2002)

Monthly and annual means, standard deviations, and highest and lowest evaporation values for the years of record were calculated for each location (Table 10.B.). High, low, and mean values for pan coefficients other than 0.7 can easily be obtained from the data of Table 10.B. by dividing the values by 0.7 and multiplying by the desired coefficient. However, the standard deviations will change somewhat for different pan coefficients. The range of annual values average approximately 15 percent of the mean annual values. The greatest variation is at Rock Springs with the highest and lowest annual values 19 percent greater and 21 percent less than the mean annual value, respectively. The least variation is at Sheridan with the highest and lowest annual values about 13 percent above and 7 percent below the mean annual value, respectively.

Monthly and annual means, standard deviations, and highest and lowest net evaporation values for the years of record were calculated for each of the seven locations (Table 10.C.). Again, a pan coefficient of 0.7 was used. The greater variability of net evaporation as compared to evaporation is shown by the values of Table 10.B. and Table 10.C. The range of annual net evaporation values average 34 percent above and 42 percent below the mean annual values (Table 10.C.). These are over twice the magnitude of the percentages for evaporation (Table 10.B.). The standard deviations of the annual values are also nearly twice the magnitude for net evaporation than for evaporation.

The variations of estimated evaporation and net evaporation are indicated by the values of Table 10.B. and Table 10.C.. Mean annual values of estimated evaporation range from a low of 39.1 inches per year at Sheridan to a high of 47.9 inches per year at Whalen. That is, the annual mean at Whalen is about 22.5 percent higher than the annual mean evaporation at Sheridan. Mean annual net evaporation ranges from a low of 23.7 inches per year at Sheridan to a high of 37.7 inches per year at Rock Springs. As shown, the variation of net evaporation between areas can be quite large.

83#. http://www.wrds.uwyo.edu/wrds/rmfres/refwork.html ,   http://www.wrds.uwyo.edu/wrds/rmfres/rmfres.html

84#. Tabler, Ronald D. 1973a. New snow fence design controls drifts, improves visibility, reduces road ice. In: 46th Annual Transportation Engineering Conference [University of Colorado; Denver; Feb. 22-23, 1973] Proceedings: 16-27.

85#. http://www.lccdnet.org/trees/living_snow_fence.html

86#. http://www.na.fs.fed.us/spfo/hot_topics/winter_97/snowfnc.htm

87#. Comprehensive ET study in Wyoming, see: http://library.wrds.uwyo.edu/wrp/85-21/85-21.html

88#. personal correspondence with Rudy Garcia, USDA/NRCS Field Agronomist

88a#. Available on page 20 of the NCDC Annual Climatological Data publication. In recent years, Heart Mountain and Sheridan Field Stn total evaporation data has been available. Free to .edu and .gov organizations.

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Last Modified: Fri, 17 Oct 2008