Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/30/2006
Publication Date: 8/29/2006
Citation: Hornbuckle, B.K., England, A.W., Anderson, M.C., Viner, B.J. 2006. The effect of dew on the microwave emission of maize at L-Band. Agricultural and Forest Meteorology. 138:180-191. 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 dew on the microwave emission from a corn canopy in order to improve soil moisture retrieval. It appears that the effects of dew depend on the nominal size of the canopy component (e.g., leaf size) relative to the wavelength of observation. Because of the large leaf size, dew on corn tends to decrease the observed brightness temperature, while it has the opposite effect on canopies with smaller leaves.
Technical Abstract: Dew has the net effect of decreasing the brightness temperature of maize at L-band. Scattering is enhanced more than emission. This effect occurs at both polarizations, but vertically-polarized brightness is affected more than horizontally-polarized brightness. As more water condenses on the canopy, the brightness temperature continues to decrease. A light dew of approximately 0.1 kg m-2 decreased horizontally-polarized and vertically-polarized brightness temperature by 2 and 4 K, respectively. A very light dew of approximately 0.01 kg m-2 decreased vertically-polarized brightness temperature by 1 K, but did not affect horizontally-polarized brightness. Since dew has been observed to increase the brightness temperature at L-band in other types of vegetation, we hypothesize that the effect of dew on terrestrial microwave emission depends on the size of vegetation canopy components relative to the wavelength of observation. Dew will increase terrestrial microwave emission when the size of the vegetation canopy components are electrically small, and decrease terrestrial microwave emission when the vegetation canopy components are significant fractions of the wavelength. For a moderate to heavy dew, it appears possible that bias introduced by the presence of dew could be a significant source of error in retrieved soil moisture from future L-band satellite radiometers. The most widely-used model of microwave emission can not correctly account for the effect of dew in its present form.