Using midday surface temperature to estimate daily evaporation from satellite thermal IR data

The practical experience obtained by our participation in the European project TELLUS (a part of the HCMM programme) led us to state that the thermal inertia concept and sophisticated models are useful for understanding basic processes and for performing informative simulations, but cannot be used for a real estimation of evaporation On the other hand, a simplified procedure like that proposed by Jackson et al. (1977) appears feasible, but its theoretical basis and its field of application are subject to discussion. Two main questions arise, namely, the conditions underlying the use of one instantaneous measurement to estimate daily integrated values, and the significance of the adjustment parameter B in the simplified relationship ET d = Rn d − B(Ts − Ta) i which relates the daily evaporation to daily net radiation by means of one measurement of the surface and air temperature, at a given time of day These questions are discussed with reference to results of recent experiments. A theoretical analysis of the B parameter was undertaken to explain the difference between the value (0.25) derived from an experiment at Crau (south of France) and the value (0.64) proposed by Jackson et al. (1977) for conditions in Phoenix. The influences of wind velocity, thermal stratification and surface roughness are discussed. Consequences of the practical use of the simplified procedure are presented.