Evaporation from sparse crops‐an energy combination theory

SUMMARY A one-dimensional model is adopted to describe the energy partition of sparse crops. Theoretical development of this model yields a combination equation which describes evaporation in terms of controlling resistances associated with the plants, and with the soil or water in which they are growing. The equation provides a simple but physically plausible description of the transition between bare substrate and a closed canopy. Although the aerodynamic transfer resistances for incomplete canopies have, as yet, no experimental justification, typical values, appropriate to a specimen agricultural crop and soil, are shown to have limited sensitivity in the model. Processes which require further study if the equation is to be used to calculate evaporation throughout a crop season are also discussed. Previous steps in the development of a physically based model of the vegetationatmosphere interaction (e.g. Shuttleworth 1976, 1978) explicitly treat the vegetation as a closed, stable canopy of uniform structure. They emphasize the interaction of the vegetation, with fluxes arising at the soil surface introduced as an unspecified, and implicitly small, input to the model (Shuttleworth 1979). In this paper this theoretical work is reinterpreted and developed into the situation of sparse crops, where the use of a one-dimensional model has less obvious justification. In describing such crops the soil and plant components must carry equal status, since they can be of similar size and their relative importance can change significantly with crop cover. The philosophy of this paper is to make minimum concession to the more obvious three-dimensional structure of sparse and row crops. Accordingly a one-dimensional model of the interaction is adopted to derive a combination equation, which can provide a physically plausible transition between the bare substrate and closed canopy limits. The equation is expressed in terms of conceptual resistances now familiar to the micrometeorologist and plant physiologist: canopy resistance and boundary layer resistance etc; it also requires the less familiar concept of a surface resistance for bare soil (Monteith 1981). In the later sections of the paper typical values of these resistances are used to illustrate how energy partition varies between crops of the same height, but with different leaf areas.