On a Relationship Between Evaporation and Evapotranspiration
Heat balance considerations indicate that the annual amount of evaporation from an extensive water surface is greater by some 33 percent than evapotranspiration from an extensive vegetation-covered land surface having an ample supply of water. The assumption is that the same amount of insolation reaches both surfaces. Turbulence theory is used to show that the above estimate leads to values of the friction velocity (or shearing stress) which are in close agreement with independent results for vegetation-covered land surfaces, indicating the correctness or approximate correctness of the above estimate. The assumption of the demonstration is that the geostrophic wind is the same over both surfaces. It is estimated that the annual amount of sensible heat transferred from the vegetation-covered land surface is some 5 to 10 percent of the insolation reaching that surface.