Abstract:
This dataset was created by computing the mean bulk density for each layer of each component of each STATSGO map unit, and interpolating the results to a set of 11 standard layers. The results for each of the 48 conterminous states (map units for the District of Columbia are included under Maryland) wre joined to create a single dataset.
Units of measurement: Bulk density values are given in g/cm³.
The Build Density Grid is of 1 km resolution.
Supplemental_Information:
The information below was compiled from the following web page:
http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov
Units of measurement: Bulk density values are given in g/cm³.
The Build Density Grid is of 1 km resolution as a three-dimensional array of 16-bit binary integers, representing the bulk density value multiplied by 100.
Soil may be considered a porous three phase system composed of air, water, and solids. The relative distribution of these three components is important to understanding the hydraulic properties of the soil. Bulk density is the ratio of the mass of soil to its total volume (solids and pores together). Bulk density as recorded in STATSGO is a total, or wet, bulk density which is a measure of the total mass of a moist soil per unit volume. Bulk density is often an input variable in pedotransfer functions used to obtain soil hydraulic properties.
Determining the bulk density for each of a set of standard layers required three main steps:
Computing the mean bulk density for each component layer.
For each component, determining the contribution of each component layer to the 11 standard layers.
For each map unit, combining the contributions of all components to compute the bulk density for each layer.
The Layer table entries for each component use the BDL and BDH variables for each layer to record the low and high ends of the range of bulk density values for the layer. The Layer table also records the depth to the top and bottom of each layer, in variables LAYDEPL and LAYDEPH. The Comp table entries for each component of each map unit use the COMPPCT variable to record the percent of the area of the map unit which is covered by that component. This table also reports the minimum and maximum depth to bedrock for the component in variables ROCKDEPL and ROCKDEPH. Some components consist entirely of water and are represented by the value "WATER" in the COMPNAME variable.
The mean bulk density for each layer was computed as the simple average of BDH and BDL. Some components included one or more layers with BDL = BDH = 0 which were below the mean depth to bedrock (the average of ROCKDEPL and RPCHDEPH); for these components, the mean bulk density was set equal to the particle density, PD.
To determine the contribution of each component of a given map unit to the standard layers, the layers defined in the Layer table for the component were compared with each standard layer. If the standard layer was entirely included within one of the component layers, the BD values for the layer were multiplied by the COMPPCT value to determine the weighted contribution of the component to the standard layer. If the standard layer overlapped two or more component layers, the BD values for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value.
If the bottom of the last layer specified for the component was above the the bottom of the last standard layer (250 cm), the component layer was assumed to have the same BD and porosity as the next higher component layer down to the mean bedrock depth. Below this depth, BD was set equal to PD.
If the component was identified as "WATER", it was excluded from the computation of mean BD. Some components specified BDL = BDH = 0 for all layers. These components were also excluded.
After all components for the map unit were processed, the sum of the weighted contributions to each standard layer by all non-excluded components was divided by the sum of the COMPPCT values for the non-excluded components to determine the mean BD for each standard layer of the map unit.
Two major problems were encountered which affect the validity of the computed values.
Many components specify ROCKDEPL = ROCKDEPH = 60 inches (152 cm) inferring that the soil was not examined below this depth. In most cases, bedrock is not actually present; nonetheless, the values of BD were computed as if bedrock was encountered at this depth.
A number of non-water components, and in some cases entire map units, have BDL = BDH = 0 for all component layers, even though other variables for the layer (e.g., texture) contradict the BD values. These components were ignored in the computation of average values for the map unit. If BDL = BDH = 0 for all non-water components for the map unit, the BD value was set to 0. If the map unit is entirely water, BD was set to zero.