Radiative transfer technique for retrieving the radiative properties of agricultural soils

Abstract

The knowledge of soil optical properties is very important for agriculture production and decision making in selecting and managing land to cultivate. These properties can be retrieved from the radiative transfer technique if appropriate method is used to solve the direct problem of the radiative transfer equation. The radiative transfer equation describes the propagation of the radiation in the soil. Thus, understanding the interactions of the radiation with agricultural soil is an active research area. In this paper, the reflectance characteristics of typical agricultural soil are retrieved. The soil, considered as a semitransparent plane parallel two-layered media with the refractive index mismatch, is illuminated at the top by a collimated radiation. This refractive index mismatch causes reflection and refraction to occur at the soil boundaries and interfaces. The two layered agricultural soil radiative properties and the scattering phase function are determined from the Mie theory assuming that the soil layers consist of only clay particles. With these radiative properties, the equation of radiative transfer is solved using the discrete spherical harmonics method (DSHM) under Marshak boundary conditions. The computational results are in agreement with existing literature results on two-layered media. The two-layered agricultural soil is then studied with a constant or spatial variation of the refractive index. The effects of soil radiative properties and the scattering phase function on the radiation characteristics are discussed. The anisotropy of the soil reflectance is analyzed and results demonstrate that a predominantly backscattering soil reflects more radiation than the predominantly forward scattering and Rayleigh scattering soil. These findings are applicable in the context of agricultural soil as it provides insight for developing optical equipment for quick soil properties measurement before crop planting.