The effect of clumping and stomatal response on evaporation from sparsely vegetated shrublands

Abstract

The development of two-source models of evaporation has improved estimates of evaporation from sparsely vegetated surfaces over those of single-source models. This paper investigates the importance of clumping of vegetation, as defined by the fractional vegetative cover, for evaporation from soil and shrubs. A sensitivity analysis; of a model that includes clumping of vegetation and three sources of water for evaporation, shrubs, soil under shrubs and bare soil outside shrubs, is presented. Resistances to vapour transfer were derived experimentally for a shrubland in southeastern Spain, which was dominated by the leguminous shrub Retama sphaerocarpa. Stomatal conductances of the shrub were found to decline linearly with the in-canopy water vapour saturation deficit (D 0). The slope of this relationship increased linearly with volumetric soil water content. Soil surface resistances were found to increase quadratically with time after wetting of the soil surface. The model that included clumping (the C model) was compared with single- and two-source models. The tested models included feedbacks between plant resistances, in-canopy vapour pressure saturation deficits, and total evaporation from the surface ( λE t ). The single-source and two-source models overestimated evaporation from the plant ( λE p ) by 1% and 8%, respectively, whereas the C model underestimated λE p by 5%. The overestimates of λE p by the conventional two-source model resulted primarily from over-estimation of the radiation being absorbed by the canopy. The underestimation by the C model may have resulted from underestimation of the radiation absorbed by the canopy resulting from high reflected short-wave and emitted long-wave radiation from the soil. The C and two-source models predicted D 0 within 4%, whereas the single-source model underestimated D 0 by 19%, resulting from the omission of energy fluxes from the soil. The good prediction of evaporation from the plant by the single source model resulted from the combination of under-estimating D 0 and overestimating stomatal conductance.