Abstract
A modeling study on the variations of directional brightness temperature (DBT) for row-structure crops was carried out with the help of the images captured by a large aperture thermal infrared camera over a maize canopy. The model assumes that the DBT is a function of component brightness temperatures and their directional fractions. The canopy has three brightness temperature components: sunlit soil, shaded soil and vegetation, each component has a unique temperature value. Component fractions in the scene of view depend on sun-view geometry and the distributions of gaps within and between plant rows. To describe canopy geometrical features, a system of porous hedgerows with rectangle cross-section has been used; the directional variations of gap fraction are described by Nilson function. The model demonstrates directional variations of DBT as a function of sun-viewing geometry and canopy geometrical structure as well as component brightness temperatures. In the simulation of DBT over a middle dense canopy near the noontime, the results reveal an evident row-direction-oriented hot stripe in the DBT polar map, where appeared the hot spot along the sun direction. The sensitivities of the model to the input parameters have been tested. Further validation analysis has also been conducted which demonstrates modeled DBT agreeing closely with field observations.
Highlights
D IRECTIONAL brightness temperature (DBT) plays an important role in characterizing canopy thermal radiation distributions
As a parameter acquired by remote sensing instruments, DBT has been widely applied in many research areas, such as the estimation of field energy budget, the retrieval of field biophysical and phenological parameters, and the normalization of remote sensing information obtained at different directions or from different platforms, etc. [1]–[10]
This paper provides a simple hybrid geometric optical and radiative transfer model to simulate row crop DBT based on a series of assumptions on canopy geometrical architecture, optical properties, and temperature distributions
Summary
D IRECTIONAL brightness temperature (DBT) plays an important role in characterizing canopy thermal radiation distributions. The model, based on the research of Prévot [5], computed the canopy thermal radiance as a function of soil and leaf temperatures, canopy geometrical structures, and the view angles This kind of model neglected the effect of row structures on the simulation of the canopy DBT. Gastellu-Etchegorry et al [20] presented a thermal infrared module in their discrete anisotropy radiative transfer (DART) model to study the field energy budget and diurnal variation of the DBT While this kind of model needs very complicated information on the canopy architecture and temperature distributions. Snyder et al [18], [19] have modified several semiempirical BRDF models by extending the spectral range from visible and near-infrared spectral domain into the thermal infrared band by developing a lookup table for the Moderate Resolution Imaging Spectrometer land surface temperature algorithm These models demonstrated a potential in row crop DBT studies with necessary modifications. The directional fractions of brightness temperature components over the canopy are modeled
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