Abstract:
Available water capacity is defined in the NRCS Soil Survey Manual as "The volume of water that should be available to plants if the soil, inclusive of rock fragments, were at field capacity".
This dataset contains the available water capacity (AWC) in the top 100, 150, and 250 cm of soil, derived from the State Soil Geographic (STATSGO) soils data compiled by the Natural Resources Conservation Service (NRCS) of the U.S. Department of Agriculture.
AWC values are given in centimeters (more properly cubic centimeters of water per square centimeter of soil).
Supplemental_Information:
The information below was compiled from the following web page:
http://dbwww.essc.psu.edu/geotree/dbtop/amer_n/us_48/data/soilprop/statsgo_albers/awc/doc.html
AWC values are given in centimeters (more properly cubic centimeters of water per square centimeter of soil).
CAUTION: 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.
OVERVIEW: Available water capacity is defined in the NRCS Soil Survey Manual as "The volume of water that should be available to plants if the soil, inclusive of rock fragments, were at field capacity". Climatologists and hydrologists often use this concept in simple water soil water balance models. 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: A number of models use a root-zone depth of 100 cm; As noted in the "Caution" above, bedrock information is only reliable to a depth of about 150 cm; 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.
To increase compatability with different types of data analysis software, the dataset is available in several different data file formats. These include both Arc/Info polygon format and a gridded version at 1 km resolution; the latter is available in both Arc/Info grid format and as a three-dimensinal array of 8-bit unsigned binary integers. The description of dataset files provides additional details on formats, the data files asociated with each format, and instructions for file retrieval.
For the gridded version of the dataset, any 1-km grid cell which contains portions of two or more mapunits was assigned the AWC value for the mapunit which occupies the largest fraction of the cell. For the Arc/Info grid version, the AWC values may be accessed using the mapunit serial numbers associated with each STATSGO mapunit; the AWC has been incorporated into the Value Attribute Table (VAT) entry for each mapunit.
STATSGO mapunit serial numbers: Each STATSGO mapunit is identified by an alphanumeric mapunit ID (MUID) consisting of the two-letter postal abbreviation for the state followed by a three-digit number; for example, AL001 is the first mapunit for Alabama. For the gridded versions of the data, however, the alphanumeric MUID must be replaced by a numeric value. Accordingly, the 10,500 mapunits in the 48 conterminous states have been assigned mapunit serial numbers between 1 and 10500.
The assignment was made by sorting all the MUIDs for the 48 states alphabetically, and then assigning mapunit serial numbers sequentially to the sorted MUID values. The relation between mapunit serial number and mapunit ID is defined by a translation table.