EFFECT OF SURFACE SOIL-MOISTURE GRADIENTS ON MODELING RADAR BACKSCATTERING FROM BARE FIELDS

In agriculture, soil moisture gradients near the surface are important . The objective of this study was to determine their impact on the est imation of the radar backscatter when using surface diffusion models. Two surface radar backscatter models were evaluated in a bare soil wit h soil moisture stratification induced by irrigation and evaporation. The models were the semi-empirical model (OM) and the Integral Equatio n Model (IEM). These models were coupled with a penetration depth mode l. Three approaches were used to calculate the reflectivity at the air -soil interface required by the models. The first one was based on the incoherent reflectivity contribution of each stratum. The second appr oach calculated the Fresnel reflectivity based on the mean complex die lectric constant over a fixed depth, and the third one computed the Fr esnel reflectivity using the mean complex dielectric over the penetrat ion depth of the incident signal. The estimations from the OM model we re highly correlated with the observations for all combinations of inc idence angles, frequencies and polarizations. However, the backscatter was systematically underestimated and bias increased with wavelength. The IEM provided the most accurate estimate but was sensitive to rada r configuration and roughness. The two Fresnel reflectivity based appr oach tended to overestimate the backscatter in the drying sequence aft er intensive irrigation following held work. The incoherent reflectivi ty on the other end underestimated the backscatter in freshly dry dist urbed soil. The sampling depth had to be chosen carefully when estimat ing the backscatter from the second approach, especially in K-u and L- bands.