Determination of Bulk Density And Porosity

The bulk density (BD) and porosity for each of 11 standard layers was determined for each map unit of each state using the STATSGO Comp and Layer tables. The standard layers were introduced because of the wide variation in the number, thickness, and depth to top and bottom of soil layers in the STATSGO data from one soil component to another, even within the same map unit. Variable layers cause problems for many environmental models and GIS operations.

Bulk density is defined as the total mass of soil per unit volume. It is determined from the STATSGO Layer table variables BDH and BDL.

Porosity is defined as the total volume occupied by pores per unit volume of soil and is calculated from bulk density and particle density. The formula is: Porosity = 1 - (BD/PD), where PD is the particle density, normally assumed to be 2.65 g/cm³. It should be noted that porosity is a derived quantity and is NOT a part of the original STATSGO database.

Determining the bulk density and porosity for each of a set of standard layers required three main steps:

  1. Computing the mean bulk density and porosity for each component layer.

  2. For each component, determining the contribution of each component layer to the 11 standard layers.

  3. For each map unit, combining the contributions of all components to compute the mean bulk density and porosity 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 the 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 approximate 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; this is specified 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. The corresponding porosity was computed as 1 - (BD/PD), using a particle density PD = 2.65 g/cm³. 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, and the porosity was set to zero.

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 and porosity 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 and porosity 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 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 the particle density, PD, and the porosity was set to zero.

If the component was identified as "WATER", it was excluded from the computation of mean BD and porosity. As noted below, some components specified BDL = BDH = 0 for all layers; these components also were 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 and porosity for each standard layer of the map unit.

Two major problems were encountered which affect the validity of the computed values.

  1. Many components specify ROCKDEPL = ROCKDEPH = 60 inches (152 cm) to specify that the soil was not examined below this depth. In most cases, bedrock is not actually present; nonetheless, the values of BD and porosity were computed as if bedrock was encountered at this depth.

  2. 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 and the porosity to a flag value, 1.01. If the map unit is entirely water, BD is set to zero and the porosity to 1.00.

Last change: 1999 Oct. 18, R. A. White / raw@essc.psu.edu