| Many impacts of climate change will be expressed in the hydrologic cycle. Microwave radiometry is sensitive to the quantity and distribution of water in soil and vegetation. Recent advances in technology will allow global measurements at useful spatial resolutions. Critical to this vision is the development of reliable models of microwave brightness. In this dissertation, measurements of 1.4 GHz brightness, micrometeorology, and soil moisture were collected over the course of the growing season in a field of corn. It was determined that the brightness of a field-corn canopy at both polarizations is isotropic in azimuth during much of the season. At senescence, brightness is a function of row direction. This phenomenon is caused by water loss from the leaves, which when dry become essentially invisible. The question is raised whether other biophysical processes associated with critical periods of drought or extreme wetness could cause similar changes in the effective constitutive properties of the canopy. A current model of microwave brightness, appropriate for weakly-scattering canopies, was unable to predict change in brightness with incidence angle. Significant scatter darkening was observed. A new model was formulated with an anisotropic canopy. The new model was compared to continuous measurements of brightness collected during the highest canopy biomass of the season. With the aid of coincident measurements of micrometeorology and soil moisture, the radiometric sensitivities to vegetation canopy temperature, soil moisture, and canopy water, either in the form of intercepted precipitation or dew, were determined and compared to sensitivities in a hypothetical nonscattering canopy of equivalent density, such as thick grass. Sensitivity to canopy temperature is similar in both types of canopies. Soil moisture sensitivity is higher in the corn canopy where moisture is concentrated in stems and fruit. An increase in canopy water has the net effect of decreasing the brightness equally at both polarizations in corn, while an increase in brightness occurs in nonscattering canopies. Dew can decrease the brightness more than a soaking rain. With an appropriate emission model, there will be year round sensitivity to soil moisture in most, and perhaps all, agricultural crops. |
