INTEGRATING PASSIVE MICROWAVE MEASUREMENTS WITH A SOIL-MOISTURE HEAT-FLOW MODEL
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Passive microwave remote sensing offers a potential for large area estimates of soil moisture, a crucial element for evaluating the impact of drought on agricultural production. However, the microwave measurements reflect only the soil moisture in the upper few centimeters of the soil profile. For these measurements to be useful for soil moisture monitoring, the depth sensitivity must be verified, and a method must be found for using the near-surface measurements to estimate root-zone soil moisture. Field measurements and simulations of microwave brightness temperature using a soil moisture/heat flow model and radiation theory were used to evaluate depth sensitivity and to assess some of the difficulties in estimating root-zone moisture from near-surface measurements. Results showed that even the longest wavelength (21 cm) was sensitive to only the conditions in the top 2 cm of the profile. Diurnal variations of both soil moisture and soil temperature affected the brightness temperature. Converting brightness temperature to effective emissivity minimized the impact of the soil temperature variation, but the influence of the diurnal moisture variation remained. Thus, it may be necessary to obtain microwave measurements at the same time of day for each measurement date to adequately interpret the measurements. 1983.
