A model for microwave emission from vegetation‐covered fields

Abstract

A radiative transfer model for simulating the measured brightness temperatures over vegetation‐covered fields is studied. The model treats the vegetation as a uniform layer, or canopy, with a constant temperature over a moist soil which emits polarized microwave radiation. The equation of radiative transfer is solved analytically subject to boundary conditions at the soil surface and canopy top. Scattering by the vegetation is primarily in the forward direction and given by a single scattering albedo ω*. The effect of soil surface roughness is introduced by modifying the smooth surface reflectivity with a roughness height parameter h and polarization mixing factor Q. The analytic formula for the microwave emission has four parameters, h, Q, τ0* (effective canopy optical thickness), and ω*. The model provides a good representation of the observed angular variations for both the horizontally and vertically polarized brightness temperatures at 1.4 GHz and 5 GHz frequencies over fields covered with grass, soybean, and corn. The effective canopy optical thickness is found to be directly proportional to the amount of water present in the vegetation canopy, while the effective canopy single scattering albedo depends on the type of vegetation.

Calculations of radar backscattering coefficient of vegetation-covered soils

“…For L-band, it has been shown that c = 0.12 [21]. The T values for C-band are 2 to 5 times larger than those for L-band.…”

“…A previous investigation [21] shows that T is proportional to the vegetation canopy water content W (in kg/m 2 ), and that it can be given by the simple form,…”

Passive L-Band Microwave Remote Sensing of Organic Soil Surface Layers: A Tower-Based Experiment

“…Once the weighting factor η p and the brightness temperatures of the surrounding area TB O,p are known, the brightness temperatures emitted from the observed soil-vegetation scene TB S,p can be derived from the L-band radiometer measurements using Equation (1). The brightness temperatures emitted from the soil-vegetation scene (TB S,p ) were simulated using a simple radiative transfer (RT) model, referred to as the tau-omega (τ-ω) model [15], which describes Remote Sens. 2018, 10, 304 7 of 18 the emission contributions of soil and vegetation.…”

“…During recent years, the effect of soil organic matter on passive L-band microwave signals has also been studied in the context of the ESA SMOSHiLat project [13]. L-band radiometer algorithms for soil moisture retrieval are based on the use of a radiative transfer (RT) model [12,14,15] and a dielectric mixing model [16][17][18] to derive soil moisture information from brightness temperature observations. The listed dielectric mixing models were developed for mineral soils and no information on organic matter content can be accounted for.…”

Calculations of the Microwave Brightness Temperature of Rough Soil Surfaces: Bare Field

“…For example, it can be shown [7] that if a 20% polarization mixing is assumed for the L-band case in Figure 2, the calculated T V value at 70 0 would be 265K, instead of 290K as shown in Figure 2. However, the assumption of polarization mixing is not employed in this study, because other uncertainties may exist.…”