Comparison of aerodynamic and energy budget techniques in estimating evapotranspiration from a cornfield

On the validity of flux-gradient relationships above very rough surfaces

“…However, these relationships are not valid in the lower part of the surface layer: Tennekes (1973) pointed out that they are only applicable for measurements made at heights z which are very large compared with the roughness length of the underlying surface z,,, i.e., z/z,, > 50-100. This theoretical limitation, already raised by Mukammal et al (1966) has been contirmed by others (Thorn et al, 1975;Garratt, 1978;Raupach, 1979;Denmead and Bradley, 1985) who emphasized the non-validity of these relationships at the top of very rough surfaces such as Savannah and coniferous forest in a layer called the 'transition layer'.…”

A Nighttime Energy and Moisture Budget in Death Valley, California, in Mid-August

“…The Bowen ratio approach has been successfully tested against observations on many occasions (see, for instance, Pruitt and Lourence 1965;Fritschen and Van Bavel 1963;Mukammal, King, and Cork 1966;Mcllroy and Angus 1964;Tanner 1966). dence, (Q + q)n, shows that even in the early hours of the day, values as high as 1.43 ly min-1 could be obtained even on this relatively hazy day.…”

Evaporation and Transpiration

“…However, in a layer below the inertial sublayer, described as the roughness sublayer, it has become evident that the commonly accepted flux-gradient relationships may underestimate fluxes by up to a factor of two or more. This is evident from studies over forests by Thorn et al (1975), Raupach (1979), and Denmead and Bradley (1985); over bushland by Garratt (1978b); and even over tall agricultural crops, e.g., corn (Zea mays) by Mukammal et al (1966) and Cellier and Brunet (1992). The underestimation of fluxes is caused by more efficient momentum, heat and vapor transfer within the roughness sublayer than is predicted by logarithmic similarity (fc-theory) with general stability corrections.…”