Evaporation from heterogeneous and sparse canopies is often represented by multi-source models that take the form of electrical analogues based upon resistance networks. The chosen representation de facto imposes a specific form on the composition of elementary fluxes and resistances. The two-and three-source representations are discussed in relation to previous work where some ambiguities arise. Using the two-layer model (Shuttleworth and Wallace, Q J R Meteorol Soc 111: [839][840][841][842][843][844][845][846][847][848][849][850][851][852][853][854][855] 1985) and the clumped (three-source) model (Brenner and Incoll, Agric For Meteorol 84:187-205, 1997) as a basis, it is shown that the stomatal characteristics of the foliage (amphistomatous or hypostomatous) generate different formulations. New generic and more concise equations, valid in both configurations, are derived. The differences between the patch and layer approaches are outlined and the consequences they have on the composition and formulation of component fluxes are specified. Then, the issue of calculating the effective resistances of the single-layer model from multi-source representations is addressed. Finally, a sensitivity analysis is carried out to illustrate the significance of the new formulations. Keywords Big leaf model · Effective parameters · Evaporation · Heterogeneous and sparse vegetation · Multi-layer models List of SymbolsA Available energy of the whole crop (W m −2 ) A f Available energy of the foliage (W m −2 ) A s Available energy of the substrate (W m −2 ) A vs Available energy of the vegetated soil (W m −2 ) A bs Available energy of the bare soil (W m −2 ) R n Net radiation of the whole crop (W m −2 ) G Soil heat flux (W m −2 ) 123 244 J. P. Lhomme et al. H Sensible heat flux from the complete canopy (W m −2 ) λE Latent heat flux from the complete canopy (W m −2 ) H i Component heat flux (i = f, s, vs, bs) (W m −2 ) λE i Component latent heat flux (i = f, s, vs, bs) (W m −2 ) D a Vapour pressure deficit at reference height (Pa) D m Vapour pressure deficit at canopy source height (Pa) T a Air temperature at reference height ( • C) T m Air temperature at canopy source height ( • C) T i Surface temperature of component i (i = f, s, vs, bs) ( • C) u a Wind speed at reference height (m s −1 ) e a Vapour pressure at reference height (Pa) e m Vapour pressure at canopy source height (Pa) e * (T ) Saturated vapour pressure at temperature T (Pa) c p Specific heat of air at constant pressure (J kg −1 K −1 ) ρ Air density (kg m −3 ) γ Psychrometric constant (Pa K −1 ) Slope of the saturated vapour pressure curve (Pa K −1 ) Canopy Structural Characteristics d Canopy displacement height (m) F Fractional cover of foliage (dimensionless) LAI Leaf area index (m 2 m −2 ) n Parameter with value of 1 for amphistomatous and 2 for hypostomatous foliage z r Reference height (m) z h Mean canopy height (m) z m Mean canopy source height (Canopy aerodynamic roughness length (m) Component Resistances r a Aerodynamic resistance between the source height an...

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Artículo de publicación ISIBackground and Aims: Central Chile represents a large area of viticultural potential for high-quality wine production. Although climate has been commonly accepted as one of the main drivers of Chilean viticultural success, its main features have not been described from a viticultural perspective. Our work focused on analysing the spatial climatic structure in this area with respect to the potential for grapevine production.CYTED of Ibero-Americ

Biosphere-atmosphere water and carbon fluxes depend on ecosystem structure, and their magnitudes and seasonal behavior are driven by environmental and biological factors. We studied the seasonal behavior of net ecosystem CO2 exchange (NEE), Gross Primary Productivity (GPP), Ecosystem Respiration (RE), and actual evapotranspiration (ETa) obtained by eddy covariance measurements during two years in a Mediterranean Acacia savanna ecosystem (Acacia caven) in Central Chile. The annual carbon balance was −53 g C m−2 in 2011 and −111 g C m−2 in 2012, showing that the ecosystem acts as a net sink of CO2, notwithstanding water limitations on photosynthesis observed in this particularly dry period. Total annual ETa was of 128 mm in 2011 and 139 mm in 2012. Both NEE and ETa exhibited strong seasonality with peak values recorded in the winter season (July to September), as a result of ecosystem phenology, soil water content and rainfall occurrence. Consequently, the maximum carbon assimilation rate occurred in wintertime. Results show that soil water content is a major driver of GPP and RE, defining their seasonal patterns and the annual carbon assimilation capacity of the ecosystem, and also modulating the effect that solar radiation and air temperature have on NEE components at shorter time scales.