In the GFDL CM2.x experiments, what exactly does the soil moisture variable "mrso" represent?

In the GFDL CM2.x model experiments, the variable "mrso" includes water only within the plant root zone, whose depth varies geographically.

Furthermore, the "mrso" values include only plant-available water content, i.e., water in excess of the water content at the wilting point (which also varies geographically). For example, a value of 200 mm would be consistent with a water content of 0.3, a wilting-point fraction of 0.1, and a rooting depth of 1 m: 200kg/m2=(300 kg/m3 soil water content minus 100 kg/m3 'unavailable' soil water content)*1m of soil.

For additional background information about the model used as the land component of the CM2.x models, one may refer to in the paper... Milly, P. C. D., and A. B. Shmakin, 2002: Global modeling of land water and energy balances. Part I: The land dynamics (LaD) model. Journal of Hydrometeorology, 3(3), 283-299. (available for download, along with other CM2.x related references, at http://nomads.gfdl.noaa.gov/CM2.X/references/") .

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