Abstract
Free water in a maize canopy has the net effect of decreasing the brightness temperature at 1.4 GHz (wavelength of 21 cm). It appears that only one form of free water, dew, causes this decrease in brightness temperature. It is not clear how the other form of free water, intercepted precipitation, effects the brightness temperature. This effect occurs at both polarizations, but vertically polarized brightness is affected more than horizontally polarized brightness. We observed a decrease in the horizontally polarized and vertically polarized brightness temperature of a maize canopy of 2 and 4 K, respectively, when intercepted precipitation and dew were present. Since free water in the canopy has been observed to increase the brightness temperature of wheat and grass at 1.4 GHz, we hypothesize that the effect of free water on terrestrial microwave emission depends on the physical dimensions of vegetation canopy components (such as stems, leaves, and fruit) relative to the wavelength of observation. Free water on vegetation will increase terrestrial microwave emission when vegetation canopy components are electrically small, and decrease terrestrial microwave emission when the sizes of some vegetation canopy components are comparable to the observing wavelength and hence scattering in the canopy is significant, as in the case of maize. The electrical size of vegetation components therefore determines the relative enhancement of emission and scattering by free water in the canopy. The most widely used model of microwave emission does not account for the effect of free water on vegetation. Bias introduced by the presence of free water could be a significant source of error in retrieved soil moisture from future 1.4 GHz satellite radiometers.
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