From near-surface to root-zone soil moisture using year-round data

Passive microwave remote sensing may provide quantitative information about the water content of a shallow near-surface soil layer. However, the variable of interest for applications such as short- and medium-term meteorological modeling and hydrological studies over vegetated areas is the root-zone soil moisture, which controls plant transpiration. Because near-surface soil moisture is related to root-zone soil moisture through diffusion processes, assimilation algorithms may enable one to retrieve the total soil moisture content from observed surface soil moisture. A variational assimilation method is applied to a new dataset obtained over a fallowland in southwestern France in 1997 and 1998. The new database includes continuous automatic measurements of water content within the top 6-cm soil layer during a 17-month period from January 1997 to May 1998. Once calibrated, the Interactions between Soil, Biosphere, and Atmosphere (ISBA) surface scheme is able to simulate properly the measured surface variables and the root-zone soil moisture. It is shown that, knowing the atmospheric forcing (including the precipitation), four observations of the surface soil moisture spread out over a 9-day period are enough to retrieve the total soil water content by inverting ISBA and that only two observations are required during rainless periods.