Passive microwave remote sensing of soil moisture, evapotranspiration, and vegetation properties during a growing season of cotton

For accurate prediction of weather and near-term climate, root-zone soil moisture is one of the most crucial components driving the surface hydrological processes. The microwave brightness at low frequencies is very sensitive to soil moisture in the top few centimeters in most vegetated surfaces. The first Microwave Water and Energy Balance Experiment (MicroWEX-1) was conducted to gain better understanding of the interactions among microwave brightness, moisture, and energy fluxes at the land surface for a growing season of cotton. During the experiment, we observed the microwave brightness temperatures at 6.7 GHz and micrometeorological parameters. In this paper, we describe the polarization dependence of observed brightness temperatures; explore the relation of the brightness temperatures changes in soil moisture, ET, and vegetation properties during the growing season. We found that vertically polarized (V-pol) brightness temperatures were less sensitive to the changes in biomass and soil moisture than horizontally polarized (H-pol) temperatures. During the early glowing season, the brightness temperatures were polarization dependent and the temperatures became polarization independent as the biomass increased above 1 kg/m².