Estimating soil hydraulic properties using passive and active microwave data

Soil hydraulic properties are of main interest for estimating water and energy fluxes at the land surface. In general, estimation of the hydraulic properties relies on the measurement of the soil water status within the soil profile usually performed with in-situ sensors such as time-domain reflectometry (TDR) or capacitance probes. Unfortunately, these sensors do not account for the high spatial and temporal variability in the soil, especially over large areas. In the opposite, proximal sensors such as ground-based microwave radiometer and radar can be used to obtained information at a larger scale. However, these sensors solely provide shallow subsurface information on the moisture status, and the retrievals might be biased by confounding factors such as soil surface roughness. L-band radiometer and off-ground ground-penetrating radar (GPR) measurements were performed above a rectangular wooden box (2.00 x 2.00 m2 area and 1.00 m height) homogeneously filled with sand. The radiometer was fixed on an arc of 5 m radius with an observation angle of 35° while the GPR was fixed above the sand box at nadir position. In order to avoid the brightness temperatures measured being affected by radiance originating from areas outside the sand box, the surrounding of the sand box was covered with a reflector made up of a mesh grid with a mash-size much smaller than the wavelength of the radiometer. Ground-truth soil moisture data were obtained by using TDR and capacitance sensors installed at 7 depths ranging from 5 to 80 cm. Soil temperature and soil conductivity data were also collected at each depth. The objective of this study is to compare estimated soil hydraulic properties of the sand box from radiometer, radar, and in-situ measurements and analyze the uncertainty related to these estimations.