Will,

Sorry to have taken so long to respond. It turns out that you've uncovered an error in the MAPS/RUC GRIB table. The wrong variable parameter is currently being used. #86 (units kg/m**2) is not correct. Instead, the variable used in RUC/MAPS and cycled is the volumetric soil moisture content (#144, units = fraction from 0-1). This has been changed for the MAPS grib.table and will be at NCEP when we are allowed to make the change there.

The numbers are fine, and the correct interpretation, regardless of what GRIB parameter is used, is the amount of the total volume in that soil layer occupied by water.

Thanks for helping to uncover this error.

By the way, the corrected grib table is posted at http://maps.fsl.noaa.gov/MAPS40.grib.table.

Stan