Examination of evaporative fraction diurnal behaviour using a soil-vegetation model coupled with a mixed-layer model

In many experimental conditions, the evaporative fraction, defined as the r atio between evaporation and available energy, has been found stable during daylight hours. This constancy is investigated over fully covering vegetat ion by means of a land surface scheme coupled with a mixed-layer model, whi ch accounts for entrainment of overlying air. The evaporation rate follows the Penman-Monteith equation and the surface resistance is given by a Jarvi s type parameterization involving solar radiation, saturation deficit and l eaf water potential. The diurnal course of the evaporative fraction is exam ined, together with the influence of environmental factors (soil water avai lability, solar radiation input, wind velocity, saturation deficit above th e well-mixed layer). In conditions of fair weather, the curves representing the diurnal course of the evaporative fraction have a typical concave-up s hape. Around midday (solar time) these curves appear as relatively constant , but always lower that the daytime mean value. Evaporative fraction decrea ses when soil water decreases or when solar energy increases. An increment of saturation deficit above the mixed-layer provokes only a slight increase of evaporative fraction, and wind velocity has almost no effect. The possi bility of estimating daytime evaporation from daytime available energy mult iplied by the evaporative fraction at a single time of the day is also inve stigated. It appears that it is possible to obtain fairly good estimates of daytime evaporation by choosing adequately the time of the measurement of the evaporative fraction. The central hours of the day, and preferably abou t 3 hr before or after noon, are the most appropriate to provide good estim ates. The estimation appears also to be much better when soil water availab ility (or evaporation) is high than when it is low.