Abstract
Interpreting remotely-sensed data requires realistic, but simple, models of radiative transfer that occurs within a vegetation canopy. In this paper, an improved version of the stochastic radiative transfer model (SRTM) is proposed by assuming that all photons that have not been specularly reflected enter the leaf interior. The contribution of leaf specular reflection is considered by modifying leaf scattering phase function using Fresnel reflectance. The canopy bidirectional reflectance factor (BRF) estimated from this model is evaluated through comparisons with field-measured maize BRF. The result shows that accounting for leaf specular reflection can provide better performance than that when leaf specular reflection is neglected over a wide range of view zenith angles. The improved version of the SRTM is further adopted to investigate the influence of leaf specular reflection on the canopy radiative regime, with emphases on vertical profiles of mean radiation flux density, canopy absorptance, BRF, and normalized difference vegetation index (NDVI). It is demonstrated that accounting for leaf specular reflection can increase leaf albedo, which consequently increases canopy mean upward/downward mean radiation flux density and canopy nadir BRF and decreases canopy absorptance and canopy nadir NDVI when leaf angles are spherically distributed. The influence is greater for downward/upward radiation flux densities and canopy nadir BRF than that for canopy absorptance and NDVI. The results provide knowledge of leaf specular reflection and canopy radiative regime, and are helpful for forward reflectance simulations and backward inversions. Moreover, polarization measurements are suggested for studies of leaf specular reflection, as leaf specular reflection is closely related to the canopy polarization.
Highlights
Solar radiation intercepted by vegetation canopies undergoes various physical and biological processes until it is either absorbed or escapes from the canopy boundaries
The bidirectional reflectance factor (BRF) dataset collected from summer maize at Luancheng Agricultural Ecosystems Experimental Station, Chinese Academy of Sciences, Hebei province, China (37◦52 N, 114◦39 E) [17] is used to evaluate the improved version of the stochastic radiative transfer model (SRTM)
We investigate the influence of leaf specular reflection on the canopy radiative regime by comparing the vertical profiles of mean radiation flux densities
Summary
Solar radiation intercepted by vegetation canopies undergoes various physical and biological processes until it is either absorbed or escapes from the canopy boundaries. Numerous radiative transfer models have been developed based on the RT theory They can be broadly categorized as 1D [5,6,7], 3D [8,9,10], and stochastic [11,12] models. It has been shown that the SRTM integrates advantages of 3D and 1D RT models; it is as realistic as the 3D RT model and as simple as the 1D RT model [11] It has been successfully adopted in generating leaf area index (LAI) and its sunlit portion from data collected from the EPIC instrument in the Deep Space Climate Observatory (DSCOVR) mission [13]
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