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

Abstract

Abstract. In many experimental conditions, the evaporative fraction, defined as the ratio between evaporation and available energy, has been found stable during daylight hours. This constancy is investigated over fully covering vegetation by means of a land surface scheme coupled with a mixed-layer model, which accounts for entrainment of overlying air. The evaporation rate follows the Penman-Monteith equation and the surface resistance is given by a Jarvis type parameterization involving solar radiation, saturation deficit and leaf water potential. The diurnal course of the evaporative fraction is examined, together with the influence of environmental factors (soil water availability, solar radiation input, wind velocity, saturation deficit above the well-mixed layer). In conditions of fair weather, the curves representing the diurnal course of the evaporative fraction have a typical concave-up shape. Around midday (solar time) these curves appear as relatively constant, but always lower that the daytime mean value. Evaporative fraction decreases 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 possibility of estimation daytime evaporation from daytime available energy multiplied by the evaporative fraction at a single time of the day is also investigated. 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 about 3 hr before or after noon, are the most appropriate to provide good estimates. The estimation appears also to be much better when soil water availability (or evaporation) is high than when it is low.

Upscaling of evapotranspiration fluxes from instantaneous to daytime scales for thermal remote sensing applications

“…the reliability of this hypothesis, especially when the available energy (the difference between net radiation, R n , and soil heat flux, G 0 ) is assumed as the reference variable (e.g., Brutsaert and Chen, 1996;Lhomme and Elguero, 1999). Brutsaert and Sugita (1992) demonstrated that this ratio, commonly referred to as the evaporative fraction (EF), is relatively constant during the central daytime hours for days with clear skies.…”

Effect of the revisit interval and temporal upscaling methods on the accuracy of remotely sensed evapotranspiration estimates

“…For example, both Gentine et al (2007) and Hoedjes et al (2008) showed that the self-preservation of evaporative fraction is sensitive to soil moisture conditions and fractional vegetation cover. Similarly, Lhomme and Elguero (1999) and later Van Niel et al (2012) showed that the degree of self-preservation can be influenced by cloud cover. As such, the assumption of clear-sky conditions is a significant potential source of error in the ET estimates that must be considered when utilizing or evaluating temporally upscaled moisture flux data (Van Niel et al, 2012;Peng et al, 2013;Cammalleri et al, 2014).…”

Urban outdoor water use and response to drought assessed through mobile energy balance and vegetation greenness measurements

“…Based on the two-endpoints of EC measurements, variation of EF at low-vegetation sites was less than 10% compared to the midday measured EF (figure S1). As Lhomme and Elguero (1999) pointed out, good EF measurements can be obtained 'in the central hours of the day, and preferably about three hours before or after noon' under clear sky conditions. Here, we chose a time window spanning from 10:30 to 16:30 PDT based on vegetation cover at each site to maximize the measurements of EF in a single day.…”