This model uses the FAO Penman-Monteith method to calculate reference evapotranspiration (ETo) from meteorological data. This model requires temperature, sunshine duration, and wind speed data as inputs, which are based on specific site conditions. For this model, site conditions measured at the Salt Lake City Airport were used for demonstration purposes.
Reference ET is NOT equal to a Pan Evaporation value. ETo values can be considered equal to evaporation from a large body of water, such as a pond or lake. However, for smaller, shallower bodies of water this relationship does not apply. Many factors affect lake/pond evaporation including surface area, depth, water temperature, and turbidity. To date, not enough research has been done to determine the minimum body of water size at which ETo is still equal.
To get an approximate Pan Evaporation value from the ETo value use this 'rule-of-thumb' conversion: divide the ETo value by a conversion constant. In winter or cooler times of the year, divide by 0.7. During summer or warmer periods, divide by 0.6.
Approx. Winter Pan Evap = ETo / 0.7
Approx. Summer Pan Evap = ETo / 0.6
Date of release: 12/27/2019
Under some conditions (cold temps) it is possible for this function to return a negative value for reference ET. Prevent this by adding a max() function to limit ET0 to a rate that is >= 0 mm/d.
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