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What is ET
NOTES ON EVAPOTRANSPIRATION DATA CALCULATED ON THE PAWS SYSTEM
prepared by Thomas W. Ley, P.E., WSU Extension Irrigation Engineer
1. Alfalfa ETr is the daily reference crop evapotranspiration of a well-watered (i.e., soil water is not limiting ET), actively growing, erect standing alfalfa crop with at least 8-12" of top growth and occupying an extensive area. Appropriate crop coefficients must be applied to estimate actual crop ET for other crops. This includes alfalfa crop ET for growth stages other than described for the reference crop, i.e., after cuttings.
2. Grass ETo is the daily reference crop evapotranspiration from an extensive surface of 3-6" tall, green grass cover of uniform height, which is actively growing, completely shading the ground and not short of water. Appropriate crop coefficients must be applied to estimate actual crop ET for other crops.
3. Crop coefficients (Kc) relate the actual rate of crop water use (ETcrop) to the reference crop ET (ETr or ETo above).
ETcrop = Kc x reference crop ET (ETr or ETo)
Crop coefficients are determined experimentally and reflect the physiology of the crop, the degree of crop cover, the location where data were collected and the method used to determine the reference ET. Thus, there are different crop coefficients for the same crop, but for use either with the alfalfa ETr or the grass ETo reference ET values. These two sets of crop coefficients are not interchangeable.
4. Evaporation pan crop coefficients are a third set of coefficients. Where the evaporation pan is set up and maintained under a set of standard conditions, such as the WSU Research Units (surrounded by green, well-watered, clipped grass in a large irrigated environment and without influence of buildings, trees or large paved areas nearby), then grass reference crop ETo is estimated as:
ETo = 0.80 x Epan
Remember that we use 80% of pan evaporation as an estimate of turfgrass water use. This is simply this relationship. Other crops use water at different rates depending on stage of growth, etc.
Monthly average evaporation pan crop coefficients were published in the WA State Irrigation Guide (see Appendix B of the guide which lists the crop irrigation requirements for several locations and crops). The basis of these crop coefficients are grass reference crop coefficients. Dividing the evaporation pan crop coefficients given in the Irrigation Guide by 0.80 gives crop coefficients which can be used with the grass reference crop ETo available on PAWS. Further information on these crop coefficients and how they were
developed is given in WSU EB 1513 (formerly WSU XB0925): Irrigation Requirements for Washington--Estimates and Methodology.
5. On a similar note, research at Kimberly, ID has shown that weekly average alfalfa reference ETr is about 90% of the weekly average pan evaporation for the carefully controlled evaporation pan conditions given above. The evaporation pan factors given in the Irrigation Guide can be divided by 0.90 to obtain crop coefficients which can be used with the alfalfa reference crop ETr available on PAWS. Error introduced by not making this adjustment to the Irrigation Guide crop coefficients (when they are used with alfalfa ETr) results in estimated crop water use which is about 10% low.
Wind Rose
A wind rose gives a very succinct but information-laden view of how wind speed and direction are typically distributed at a particular location. Presented in a circular format, the wind rose shows the frequency of winds blowing FROM particular directions. The length of each "spoke" around the circle is related to the frequency of time that the wind blows from a particular direction. Each concentric circle represents a different frequency, emanating from zero at the center to increasing frequencies at the outer circles.
Evapotranspiration
The combined processes of evaporation and transpiration.
EVAPORATION - Evaporation is the physical process by which a liquid is transformed to a gaseous state.
TRANSPIRATION - The process by which water in plants is transferred to the atmosphere as water vapor.
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