Salinity Effects on the Microwave Emission of Soils

Passive microwave remote sensing has been proposed as a method for measuring both surface soil moisture and soil salinity. Since both of these characteristics will vary over an area, it is important to know how information about one of them can be isolated from the remotely sensed data. Water in the soil changes the microwave dielectric constant, which in turn changes the emission. Several dielectric mixing models have been developed and evaluated to describe soil-water-air systems. However, the concentration of salts in water also affects its dielectric properties in a well-known manner. When saline water is mixed with soil, the dielectric properties of this mixture will be different from those of a pure water and soil mixture. Although there are a few theories available to describe this process, very little verification of these theories has been done. In this study, a series of controlled field experiments were conducted using truck-mounted L-and C-band microwave radiometers. Experiments were designed to develop a data set that isolated the effects of salinity and soil moisture. This data set was used to evaluate the importance of each variable and to test proposed dielectric mixing models. The mixing models explained the observed data trends and range of values; however, the observed effects of salinity were not as large as predicted by the models. These differences may be related to model structure or field sampling problems. Variation in profile properties near the surface may also introduce some error.