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The mean available water capacity for each STATSGO map unit was computed for three column lengths, 100, 150, and 250 cm, measured from the surface. These column lengths were chosen for the following reasons:

  1. A number of models use a root-zone depth of 100 cm;
  2. Bedrock information is only reliable to a depth of about 150 cm;
  3. 250 cm is the maximum depth for which data were available for any mapunits.
The STATSGO Layer table specifies a range of AWC values, as volumetric percent, for each layer of each mapunit component. The mean AWC for each layer was computed by taking the simple mean of the low (AWCL) and high (AWCH) ends of the range, and then multiplying by the layer thickness to get total column AWC for the layer. The total AWC in the top 100, 150, and 250 cm of the component was determined by adding the contributions of all component layers that were within these distances of the surface. For layers which were partly above and partly below 100 or 150 cm, the AWC was divided in proportion to the thickness of the layer above and below the specified depth. The values for each component of the mapunit were then weighted by the COMPPCT value in the Component table to determine an average AWC for the mapunit.

Many STATSGO Component table entries for depth-to-bedrock used 60 inches (152 cm) to indicate that bedrock was not encountered within this distance of the surface. In computing AWC, it was assumed that there was no water capacity below the specified depth-to-bedrock unless the STATSGO Layer table explicitly described soil layers extending below this depth. Accordingly, the computed AWC for a 250 cm column will often understate the actual value.


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4/19/99