The use of consumer digital cameras or webcams to characterize and monitor different features has become prevalent in various domains, especially in environmental applications. Despite some promising results, such digital camera systems generally suffer from signal aberrations due to the on-board image processing systems and thus offer limited quantitative data acquisition capability. The objective of this study was to test a series of radiometric corrections having the potential to reduce radiometric distortions linked to camera optics and environmental conditions, and to quantify the effects of these corrections on our ability to monitor crop variables. In 2007, we conducted a five-month experiment on sugarcane trial plots using original RGB and modified RGB (Red-Edge and NIR) cameras fitted onto a light aircraft. The camera settings were kept unchanged throughout the acquisition period and the images were recorded in JPEG and RAW formats. These images were corrected to eliminate the vignetting effect, and normalized between acquisition dates. Our results suggest that 1) the use of unprocessed image data did not improve the results of image analyses; 2) vignetting had a significant effect, especially for the modified camera, and 3) normalized vegetation indices calculated with vignetting-corrected images were sufficient to correct for scene illumination conditions. These results are discussed in the light of the experimental protocol and recommendations are made for the use of these versatile systems for quantitative remote sensing of terrestrial surfaces.

No abstract

No abstract

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...