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

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 (D0). 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 (λEt). The single-source and two-source models overestimated evaporation from the plant (λEp) by 1% and 8%, respectively, whereas the C model underestimated λEp by 5%. The overestimates of λEp 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 D0 within 4%, whereas the single-source model underestimated D0 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 D0 and overestimating stomatal conductance.

[1]  John L. Monteith,et al.  A four-layer model for the heat budget of homogeneous land surfaces , 1988 .

[2]  C. B. Tanner,et al.  Evaporation from a Drying Soil , 1967 .

[3]  P. Jarvis,et al.  A heated leaf replica technique for determination of leaf boundary layer conductance in the field , 1995 .

[4]  J. Wallace,et al.  Calculating evaporation: resistance to factors , 1995 .

[5]  Peter M. Lafleur,et al.  Application of an energy combination model for evaporation from sparse canopies. , 1990 .

[6]  William J. Massman,et al.  A surface energy balance method for partitioning evapotranspiration data into plant and soil components for a surface with partial canopy cover , 1992 .

[7]  A. Dolman,et al.  Lagrangian and K-theory approaches in modelling evaporation from sparse canopies , 1991 .

[8]  A. J. Dolman,et al.  A multiple-source land surface energy balance model for use in general circulation models , 1993 .

[9]  J. Wallace,et al.  The estimation of transpiration from sparse dryland millet using stomatal conductance and vegetation area indices , 1990 .

[10]  H. Jones,et al.  Plants and Microclimate. , 1985 .

[11]  J. Grace,et al.  A low‐cost psychrometer for field measurements of atmospheric humidity , 1994 .

[12]  J. Kondo,et al.  A Parameterization of Evaporation from Bare Soil Surfaces. , 1990 .

[13]  Jay M. Ham,et al.  Aerodynamic and surface resistances affecting energy transport in a sparse crop , 1991 .

[14]  Robert J. Gurney,et al.  The theoretical relationship between foliage temperature and canopy resistance in sparse crops , 1990 .

[15]  E. F. Bradley,et al.  Flux-Gradient Relationships in a Forest Canopy , 1985 .

[16]  P. Jarvis,et al.  Do stomata respond to relative humidity , 1991 .

[17]  Michael R. Raupach,et al.  A practical Lagrangian method for relating scalar concentrations to source distributions in vegetation canopies , 1989 .

[18]  B. Hurk,et al.  Implementation of near-field dispersion in a simple two-layer surface resistance model , 1995 .

[19]  William J. Davies,et al.  Root Signals and the Regulation of Growth and Development of Plants in Drying Soil , 1991 .

[20]  W. Rouse,et al.  Impacts of summer warming on the energy and water balance of wetland tundra , 1992 .

[21]  L. Simmonds,et al.  Use of microlysimeters to measure evaporation from sandy soils , 1993 .

[22]  Robert J. Gurney,et al.  A RESISTANCE PARAMETER FOR BARE‐SOIL EVAPORATION MODELS , 1986 .

[23]  M. Gilabert,et al.  A comparison of direct and indirect methods for measuring leaf and surface areas of individual bushes , 1995 .

[24]  B. Hicks,et al.  The Forest-Atmosphere Interaction , 1985 .

[25]  H. Mayeux,et al.  Evaporation from rangeland with and without honey mesquite. , 1991 .

[26]  R. Leuning,et al.  Estimation of transpiration by single trees: comparison of a ventilated chamber, leaf energy budgets and a combination equation , 1990 .

[27]  J. B. Thornes,et al.  Mediterranean desertification and land use. , 1996 .

[28]  N. Turner.  Measurement and influence of environmental and plant factors on stomatal conductance in the field , 1991 .

[29]  W. Nichols,et al.  Energy budgets and resistances to energy transport in sparsely vegetated rangeland , 1992 .

[30]  Energy and water budgets of an area of patterned woodland in the Sahel , 1993 .

[31]  T. Meyers,et al.  Testing of a higher-order closure model for modeling airflow within and above plant canopies , 1986 .

[32]  David I. Stannard,et al.  Comparison of Penman‐Monteith, Shuttleworth‐Wallace, and Modified Priestley‐Taylor Evapotranspiration Models for wildland vegetation in semiarid rangeland , 1993 .

[33]  J. Monteith,et al.  Principles of Environmental Physics , 2014 .

[34]  B. Marshall,et al.  Radiation interception and growth in an intercrop of pearl millet/groundnut , 1983 .