The parameter Wp_FRACT is the fractional soil moisture (expressed as a fraction of the maximum soil moisture; max. soil moisture = porosity * layer depth) at the wilting point. Wilting Point is set at the water content at a tension of –1500 kPa, and is approximated for the different soil textures.
Soil textures are from the STATSGO database (USDA NRCS), which has been compiled from a state-by-state format into a U.S. database, available through Penn State University at http://dbwww.essc.psu.edu/dbndx/tree/amer_n/us_48/data/soilprop/statsgo_geo.html
This reports 16 different soil texture classes for 11 layers, which cover a depth of 2.5 meters. The majority of data does not extend beyond 1.5 meters, which includes the first 9 of their 11 layers. The Penn State data is available in gridded form at a resolution of 30 arc-seconds.
The soil texture data is then imported into Arc/Info, where it is indexed to values of Wp according to the table SOILTEXT. The data in this table are placed into an INFO file in Arc/Info, and the individual relates are then created to be used in an Arc/Info macro to create the Wp grid. An example Arc/Info macro can be downloaded here.