P-band radar remote sensing applied during the Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) mission has shown great potential for estimation of root zone soil moisture. When retrieving the soil moisture profile (SMP) from P-band radar observations, a mathematical function describing the vertical moisture distribution is required. Because only a limited number of observations are available, the number of free parameters of the mathematical model must not exceed the number of observed data. For this reason, an empirical quadratic function (second order polynomial) is currently applied in the AirMOSS inversion algorithm to retrieve the SMP. The three free parameters of the polynomial are retrieved for each AirMOSS pixel using three backscatter observations (i.e., one frequency at three polarizations of Horizontal-Horizontal, Vertical-Vertical and Horizontal-Vertical). In this paper, a more realistic, physically-based SMP model containing three free parameters is derived, based on a 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 greater flexibility for fitting measured and simulated SMPs than the currently applied polynomial. It is also demonstrated that the new SMP model can be reduced to a second order polynomial at the expense of fitting accuracy.
Bibliographical noteFunding Information:
We acknowledge funding from the National Science Foundation (NSF) grant No. 1521469 awarded to Utah State University and to the University of Arizona. Support of the National Aeronautics and Space Administration (NASA) AirMOSS EVS-1 mission under a contract to the University of Southern California is also gratefully acknowledged. Additional support was provided by the Utah Agricultural Experiment Station, Utah State University, Logan, Utah 84322-4810, approved as UAES journal paper No. 8888.
- Airborne microwave observatory of subcanopy and subsurface (AirMOSS)
- P-band remote sensing
- Radar backscatter
- Richards' equation
- Root zone
- Soil moisture profile