A New Solution to Richards' Equation for Application to P-Band Radar Remote Sensing of Root Zone Soil Moisture.

The Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) mission is aimed at estimation of North America Net Ecosystem Exchange (NEE) via estimation of root-zone soil moisture (RZSM) from low frequency P-band radar remote sensing data. Due to the substantial penetration depth of the P-band frequency, AirMOSS is able to retrieve RZSM profile down to about 0.5 m. A mathematical function is used for continuous presentation of the vertical moisture distribution from 3 radar observations (i.e. one frequency and three polarizations of HH, VV and HV). Since no more than 3 free parameters is allowed, an empirical second order polynomial is being used currently. In this presentation, a physically-based soil moisture profile (SMP) model with 3 free parameters is introduced based on a new solution to Richards’ equation for unsaturated flow in soils. Evaluation of the new SMP model based on both numerical simulations and measured data revealed that it exhibits broad flexibility for fitting measured and simulated SMPs. The new solution improves upon the second order polynomial SMP model employed in the AirMOSS retrieval algorithm, by adding physically-based parameters and modeled SMP.