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

摘要

In many experimental conditions, the evaporative fraction, defined as theratio between evaporation and available energy, has been found stable duringdaylight hours. This constancy is investigated over fully covering vegetation bymeans of a land surface scheme coupled with a mixed-layer model, which accountsfor entrainment of overlying air. The evaporation rate follows the Penman-Monteithequation and the surface resistance is given by a Jarvis type parameterizationinvolving solar radiation, saturation deficit and leaf water potential. Thediurnal course of the evaporative fraction is examined, together with theinfluence of environmental factors (soil water availability, solar radiationinput, wind velocity, saturation deficit above the well-mixed layer). Inconditions of fair weather, the curves representing the diurnal course of theevaporative fraction have a typical concave-up shape. Around midday (solartime) these curves appear as relatively constant, but always lower that thedaytime mean value. Evaporative fraction decreases when soil water decreases orwhen solar energy increases. An increment of saturation deficit above themixed-layer provokes only a slight increase of evaporative fraction, and windvelocity has almost no effect. The possibility of estimation daytimeevaporation from daytime available energy multiplied by theevaporative fraction at a single time of the day is also investigated. Itappears that it is possible to obtain fairly good estimates of daytimeevaporation by choosing adequately the time of the measurement of theevaporative fraction. The central hours of the day, and preferably about 3 hrbefore or after noon, are the most appropriate to provide good estimates. Theestimation appears also to be much better when soil water availability (orevaporation) is high than when it is low.