Why does the root zone soil moisture in the SMAP Level-4 soil moisture products vary in such close unison with the surface soil moisture?

The surface and root zone soil moisture estimates in the SMAP Level-4 soil moisture products are the outputs of a land surface model into which SMAP observations of brightness temperature have been assimilated. The coupling between the surface layer and the root zone layer is known to be very strong in this model (the Catchment model of Koster et al. [2000]), perhaps overly so, and this will indeed lead to similarity in the surface and root zone soil moisture time series. Nevertheless, the two variables are not identical; the surface soil moisture generally shows higher temporal variability as expected.

There are three additional points to consider. First, one's intuition regarding how "uncoupled" the two variables should be may be based on the behavior of soil at a point, as represented by a set of in situ measurements. The SMAP product represents soil moisture averaged over a large area, for which (in nature) the correspondence between the surface and root zone values may indeed be greater. Second, some idealized analyses suggest that, in the absence of information on the true level of coupling in nature at large spatial scales, a model with a stronger coupling between the variables is probably more appropriate for soil moisture data assimilation activities [Kumar et al. 2009]. Third, typical values for surface and root zone soil moisture are strongly determined by soil properties and climate. At large scales, the spatial variability imposed by soils and climate can overwhelm the differences between surface and root zone values.

Koster, R. D., M. J. Suarez, A. Ducharne, M. Stieglitz, and P. Kumar, 2000: A catchment-based approach to modeling land surface processes in a general circulation model, 1, Model structure.  J. Geophys. Res., 105, 24809-24822.

Kumar, S. V., R. H. Reichle, R. D. Koster, W. T. Crow, and C. D. Peters-Lidard, 2009: Role of subsurface physics in the assimilation of surface soil moisture observations.  J. Hydromet., 10, 1534-1547.

* Level-4 soil moisture products: