Soil-vegetation-atmosphere transfer (SVAT) models can be used to produce estimates of plant-available water as well as the fluxes of energy and moisture across the land-atmosphere interface. These estimates could be greatly improved by assimilating independent measurements of key state variables to force the model back to the correct state. Microwave radiometry is sensitive to one of these key state variables, the water content of the top few centimeters of the soil. Although several different SVAT models have been developed in the past, they are not suitable for use with current models of microwave emission. We are integrating high-fidelity biophysically-based SVAT and microwave emission models together into comprehensive point-scale Land Surface Process / Radiobrightness (LSP/R) models. LSP/R models will provide the climate modeling community with the physical insight needed to create accurate yet operational land surface parameterizations which will be able to assimilate radiobrightness observations made by current and future microwave remote sensing satellites.

Models for grassland and agricultural stubble on the plot scale (O 10² m²) have already been validated by our research group in several Radiobrightness and Energy Balance EXperiments (REBEXs). We are currently developing a new LSP/R model adapted specifically to field corn. Corn represents the next level of modeling for our group because the size of some canopy components (stalks and cobs) are on the order of microwave wavelengths and hence will scatter, rather than simply absorb, the radiation emitted from the soil. Data gathered during two extensive REBEXs held during the summers of 2000 and 2001, will be used to force and validate the model.

We present measurements made of near-surface soil moisture during REBEX-7 and REBEX-8 . We define near-surface soil moisture as the water content of the first few centimeters of the soil. Near-surface soil moisture is an extremely important variable in the context of this work since it is the quantity of water which a microwave radiometer can measure. Over 100 measurements of near-surface soil moisture were made daily using a hand-held impedance probe (ThetaProbe, Delta T Devices). These measurements allow us to more accurately represent the hydrologic state of the field site as compared to previous studies. The site was found to be surprisingly uniform, both in terms of the mean and variance of soil moisture measurements. Near-surface soil moisture was also recorded continuously with buried time-domain reflectometry (TDR) probes (CS-615, Campbell Scientific). Accurate calibration of these specific TDR instruments to a specific soil has been difficult to accomplish in the past. Our presentation focuses on the method we used to continuously calibrate the buried TDR instruments with the hand-held impedance probe, as well as an analysis of the near-surface soil moisture data collected during the two experiments.