The Effect of Intercepted Precipitation on the Microwave Emission of Maize at 1.4 GHz

Terrestrial microwave emission is sensitive to soil moisture. Soil moisture is an important yet unobserved reservoir of the hydrologic cycle linked to precipitation variability. Remote sensing satellites that observe terrestrial microwave emission have the potential to map the spatial and temporal variabilities of soil moisture on a global basis. Unfortunately, terrestrial microwave emission is also sensitive to water within the vegetation canopy, and the effect of free water residing on vegetation, either as intercepted precipitation or dew, is not clear. Current microwave emission models neglect the effect of free water. We found that the precipitation intercepted by a maize (corn) canopy increased its brightness temperature at 1.4 GHz. This effect is opposite that of dew: dew decreases the brightness temperature of maize at 1.4 GHz. The increase in brightness temperature due to the intercepted precipitation was only about 1 K for vertically polarized brightness temperature and about 3 K for horizontally polarized (H-pol) brightness temperature. It may be acceptable to neglect the effect of free water in microwave emission models. A more serious concern, however, is the underestimation, by current microwave emission models, of the sensitivity of the H-pol brightness temperature to soil moisture through maize. Understanding the physics associated with the effect of free water in vegetation on the emission, scattering, and attenuation of microwave radiation will lead to improved emission models, and potentially, models that correctly reproduce the sensitivity of the 1.4-GHz brightness temperature to soil moisture at high levels of biomass when vegetation effects are greatest.