This dataset contains the mean porosity for each of 11 standard soil layers for the 48 conterminous states derived from the State Soil Geographic (STATSGO) soils data compiled by the Natural Resources Conservation Service (NRCS) of the U.S. Department of Agriculture. 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. Porosity is a measure of the volume of air and water-filled pores in the soil. The STATSGO database does not contain a direct measure of soil pore space. However, a value for porosity can be calculated from the bulk density (BD) and particle density (PD), the ratio of the mass of all solid particles to the volume of the solids.

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 Porosity values are given as a fraction in the range 0 to 1.00 (with 1.01 flagging mapuints for which all non-water components specified bulk density values of zero). Overview 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 BD is the bulk density and 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. This dataset was created by computing the mean porosity 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. 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-dimensional array of 8-bit binary integers, representing the porosity value multiplied by 100. 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 map units was assigned the mean porosity values of the map unit which occupies the largest fraction of the cell. For the Arc/Info grid version, the mean porosity value for each layer may be accessed using the map unit serial numbers associated with each STATSGO map unit; the porosity data have been incorporated into the Value Attribute Table (VAT) entry for each map unit.

For many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was sampled only to this depth, and no bedrock was encountered. As a result, for many map units an entry of "bedrock" for the two lowest standard layers (1.5 to 2.5 m) may actually indicate "no data". 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.

Miller, D.A. and R.A. White, 1998: A Conterminous United States Multi-Layer Soil Characteristics Data Set for Regional Climate and Hydrology Modeling. Earth Interactions, 2. [Available on-line at http://EarthInteractions.org] http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov&ph

Process_Description:

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: 

Computing the mean bulk density and porosity 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 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. 

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.

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.

The 11 standard layers are :

Layer     Thickness       Depth to Top    Depth to Bottom

1      5 cm (2 in)        0 cm (0 in)     5 cm (2 in)
2      5 cm (2 in)        5 cm (2 in)    10 cm (4 in)
3     10 cm (4 in)       10 cm (4 in)    20 cm (8 in)
4     10 cm (4 in)       20 cm (8 in)    30 cm (12 in)
5     10 cm (4 in)       30 cm (12 in)   40 cm (16 in)
6     20 cm (8 in)       40 cm (16 in)   60 cm (24 in)
7     20 cm (8 in)       60 cm (24 in)   80 cm (31 in)
8     20 cm (8 in)       80 cm (31 in)  100 cm (39 in)
9     50 cm (20 in)     100 cm (39 in)  150 cm (59 in)
10     50 cm (20 in)     150 cm (59 in)  200 cm (79 in)
11     50 cm (20 in)     200 cm (79 in)  250 cm (98 in)

The above selection of the number and depths of these standard layers reflects three main considerations:
The wide variation of numbers, thicknesses, and depths of layers for different components means that there are no "natural" or "obvious" choices for the standard layers.
Many models are particularly sensitive to the properties of the top few centimenters of soil; hence extra priority should be given to preserving all available information for this region.
To minimize data volumes, layer thicknesses should not be much less than the thicknesses of "typical" component layers at similar depths.
To aid in the selection of standard layers, therefore, the frequencies of depths and thicknesses of layers were tabulated for all components. This tabulation indicated that roughly 50% of components have surface layers thicker than 20 cm (8 inches); only about 4% of surface layers have a thickness of 5 cm (2 inches) or less, and about 16%, 10 cm (4 inches) or less. Deeper layers are in general thicker -- roughly 60% of all layers were at least 50 cm (20 inches) thick. The majority of components did not record layers extending below 60 inches (approximately 1.5 m); only about 10% include layers extending beyond 2.0 m (79 inches).

poros_grid

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