The rasters originate from the Gridded SSURGO (gSSURGO) database, a National Cooperative Soil Survey (NCSS) SSURGO product in the format of an Environmental Systems Research Institute, Inc. (ESRI®) file geodatabase. Both SSURGO and gSSURGO are considered products of the NCSS partnership. SSURGO generally has the most detailed level of soil geographic data developed by the NCSS in accordance with NCSS mapping standards (1:15,840 scale) and is stored in a vector format which display soil map unit delineations. The gSSURGO product was generated by creating a 10-m resolution raster of the original SSURGO vector data. The soil property tabular data are stored in the National Soil Information System (NASIS) database. For each soil layer, available water capacity, used in the computation of AWS, is recorded as three separate values in the database. A low value and a high value indicate the range of this attribute for the soil component. A “representative” value indicates the expected value of this attribute for the component. For the derivation of AWS, only the representative value for available water capacity is used. The available water storage for each map unit component is computed as described above and then aggregated to a single value for the map unit by the process described below. A map unit typically consists of one or more “components.” A component is either some type of soil or some nonsoil entity, e.g., rock outcrop. For the attribute being aggregated (e.g., available water storage), the first step of the aggregation process is to derive one attribute value for each of a map unit’s components. From this set of component attributes, the next step of the process is to derive a single value that represents the map unit as a whole. Once a single value for each map unit is derived, a thematic map for the map units can be generated. Aggregation is needed because map units rather than components are delineated on the soil maps. The composition of each component in a map unit is recorded as a percentage. A composition of 60 indicates that the component typically makes up approximately 60 percent of the map unit. For the available water storage, when a weighted average of all component values is computed, percent composition is the weighting factor.
