LAGRANGIAN AND K-THEORY APPROACHES IN MODELING EVAPORATION FROM SPARSE CANOPIES

An evaporation model based on Lagrangian turbulent diffusion principles is developed and compared with simpler single- and dual-source K-theory models of evaporation. The performance of the Lagrangian model is assessed against observations of total evaporation by an eddy-correlation instrument and found to be satisfactory for a millet crop in west Africa. Three versions of existing simpler models containing K-theory descriptions of within-canopy turbulence are described and their results also compared. The two K-theory models that explicitly take into account the soil source perform better than the single-source Penman-Monteith model. A Lagrangian analysis does not seem to be necessary for this kind of crop because the low source-density profile of the crop, associated with a low leaf-area index, caused the near-field effect to be very small. The overall difference between the evaporation estimates of the dual-source and Lagrangian models is therefore small. It is concluded that, for practical purposes, K-theory remains an adequate approximation of turbulent transport in sparse-crop evaporation models.