Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2007
Publication Date: 7/1/2007
Citation: Hornbuckle, B.K., England, A.W., Anderson, M.C. 2007. The effect of intercepted precipitation on the microwave emission of maize at 1.4 GHz. IEEE Transactions on Geoscience and Remote Sensing. 45:1988-1995. Interpretive Summary: Because the presence of water affects the microwave emissivity of soils, passive microwave remote sensing provides a means for mapping soil moisture distributions over large areas from space. However, water associated with vegetation cover also contributes to the microwave signature, and that contribution must be removed through modeling to get to a true soil moisture value. Most soil moisture models account for water contained within the vegetation components, but the treatment of free water standing on leaf surfaces (intercepted precipitation or dewfall) in these models is currently inadequate. In this study, we examined the effects of intercepted precipitation on the microwave emission from a corn canopy in order to improve soil moisture retrieval. It appears that intercepted precipitation tends to increase the apparent microwave brightness temperature of corn by about 3 K, while dew tends to decrease brightness temperature. Understanding these effects will help us to improve microwave techniques for mapping soil moisture.
Technical Abstract: 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 variability 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 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 intercepted precipitation was only about 1 K for vertically polarized brightness temperature and about 3 K for horizontally polarized 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 horizontally polarized 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.