Soil permeability is the ease with which air and water move through the soil. A quantitative measurement is made by observing the rate at which a column of water permeates the surrounding soil under saturated conditions. The measured permeability rate is related to the saturated hydraulic conductivity of the soil.

The information below was compiled from the following web page: http://www.essc.psu.edu/soil_info/index.cgi?soil_data&conus&data_cov Determining Mean Permeability for Standard Layers The mean permeability rate was determined for each of 11 standard layers for each map unit of each state using data from 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. Determining the mean permeability rate for the 11 standard layers required three main steps: For each component layer, computing the mean permeability rate, in cm/hr. 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 permeability rate for each standard layer. The results are influenced by the way in which the STATSGO data estimated permeabiity values for mineral soils and treated non-mineral-soil layers. Computing Component Layer Permeability For each layer of each map unit component, the STATSGO Layer table contains two values for the permeability rate, PERMH and PERML, defined as the maximum and minimum, respectively, for the range in permeability rate for the soil layer or horizon, expressed as inches per hour. The mean permeability rate for each component layer was computed as the arithmetic average of PERMH and PERML, and converted to cm/hr. The STATSGO documentation indicates that entering values for the permeability rate is optional when the component layer contains non-mineral-soil material, such as organic matter or rock. Since the STATSGO Layer table entries for permeability rate use a value of 0.0 both to indicate an actual zero value and to indicate that no data were available, it was not possible to distinguish between these cases. In fact, as indicated in the table below , non-zero values were entered for most such components, including essentially all layers designated as stratified or any type of organic material -- even for unweathered bedrock, a small non-zero value was entered for about 40% of the occurrences. Accordingly, the permeability values as entered were used for all component layers with two exceptions: There were 34 component layers (0.01%) for which both PERMH and PERML were zero and the dominant soil texture class for the component layer, given by the Layer table variable TEXTURE1, corresponded to a mineral soil. There were 27 component layers (< 0.01%) for which the value entered for PERMH was less than the value entered for PERML. For both these cases, it was assumed that an error had been made when entering the data, and the component layer was omitted from the computation. Determining Contributions to Standard Layers The contributions of each component layer to the standard layers for a given map unit were determined using the component layer depths specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth to bedrock for each component calculated by averaging Comp table variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map unit covered by each component as specified by COMPPCT. For each component, the layers defined in the Layer table were compared with each standard layer in turn. If the standard layer was entirely included within one of the component layers, the permeability rate value for the layer was 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 permeability rate values for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value. The region from the bottom of the last component layer to the bottom of the last standard layer, if any, was assumed to be the same as the lowest component layer down to the mean bedrock depth. Below this depth, the permeability rate was set to 0. Computing Mean Permeability for Entire Map Unit The weighted contributions of all components to each standard layer were then summed to obtain the mean permeability rate values for the map unit. If none of the component layers contributing to the standard layer were mineral soil or if the entire map unit was specified to be water, the permeability rate was set to zero. NOTE that for many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was not examined below this depth, and bedrock was not actually encountered. In all cases, however, the permeability was computed as if bedrock was encountered at the depth specified by the mean of ROCKDEPL and ROCKDEPH. Accordingly, the permeability rate values for the two lowest standard layers (1.5 to 2.5 m) are, in many cases, misleadingly low. STATSGO Permeability Values for Mineral Soils With very few execptions, the values entered for PERML and PERMH for all component layers containing mineral soil were either 0.00, 0.06, 0.20, 0.60, 2.00, 6.00, or 20.00 inches/hour (0.00, 0.15, 0.5, 1.5, 5.0, 15, or 50 cm/hr); i.e., values were rounded to the nearest factor of 3. This suggests that all values should be regarded as rough estimates. Values for Non Mineral Soils As indicated above , the STATSGO documentation states that permeability rate values could be omitted when compiling data for non-mineral-soil component layers. No flag was provided for indicating this omission; instead, a value of zero was entered, which cannot be distinguished from an actual measured value of zero. To get some idea of how often the permeability rate may have been omitted for non-mineral-soil layers, the number of zero and non-zero values was tabulated for each non-mineral-soil texture class. This gave the following results: Texture PERMH PERML nonzero zero % nz nonzero zero % nz (organic, 3395 total) CBV-MUCK 4 0 100.00 4 0 100.00 FB 19 0 100.00 19 0 100.00 GR-MUCK 1 0 100.00 1 0 100.00 HM 158 0 100.00 158 0 100.00 MK-PEAT 3 0 100.00 3 0 100.00 MPT 467 0 100.00 467 0 100.00 MUCK 2218 1 99.95 2218 1 99.95 PEAT 188 0 100.00 188 0 100.00 SP 287 0 100.00 287 0 100.00 ST-MUCK 2 0 100.00 2 0 100.00 STV-MPT 3 0 100.00 3 0 100.00 STV-MUCK 21 0 100.00 21 0 100.00 STX-MUCK 22 0 100.00 22 0 100.00 STX-PEAT 2 0 100.00 2 0 100.00 (other, 32164 total) CBV-CIND 1 0 100.00 1 0 100.00 CE 29 0 100.00 29 0 100.00 CEM 801 534 60.00 375 960 28.09 CIND 60 0 100.00 59 1 98.33 DE 4 0 100.00 4 0 100.00 FRAG 435 1 99.77 435 1 99.77 G 3 0 100.00 3 0 100.00 GR 1 0 100.00 1 0 100.00 GR-MARL 2 0 100.00 2 0 100.00 GR-VAR 6 0 100.00 6 0 100.00 GRX-FRAG 2 0 100.00 2 0 100.00 GYP 64 60 51.61 36 88 29.03 ICE 0 3 0.00 0 3 0.00 IND 1117 1189 48.44 258 2048 11.19 MARL 47 1 97.92 45 3 93.75 MK-MARL 1 0 100.00 1 0 100.00 SG 284 0 100.00 279 5 98.24 SR- 14391 20 99.86 14176 235 98.37 UWB 8121 11532 41.32 3626 16027 18.45 VAR 413 2140 16.18 298 2255 11.67 WB 6397 4900 56.63 3655 7642 32.35 With the exception of SR- ("stratified"), all the non-mineral-soil texture classes are identified in the STATSGO documentation as "Allowable textural code for which no permeability is given"; their meanings are as follows: CE coprogenous earth MARL marl CEM cemented MPT mucky-peat CIND cinders MUCK muck DE diotomaceous earth PEAT peat FB fibric material SG sand and gravel FRAG fragmental material SP sapric material G gravel UNK unknown GYP gypsiferous material UWB unweathered bedrock HM hemic material VAR variable ICE ice or frozen soil WB weathered bedrock IND indurated

It is important to emphasize that, in addition to the limitations associated with generalizing from detailed soil maps to representative soil profiles in the STATSGO data, another level of generalization has been added by taking area-weighted averages over all the components in each STATSGO mapunit. Hence, for most mapunits, the average soil profile will not closely match any actual soil profile.

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:

Determining Mean Permeability for Standard Layers

The mean permeability rate was determined for each of 11 standard layers for each map unit of each state using data from 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. 

Determining the mean permeability rate for the 11 standard layers required three main steps: 

For each component layer, computing the mean permeability rate, in cm/hr. 

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 permeability rate for each standard layer. 

The results are influenced by the way in which the STATSGO data estimated permeabiity values for mineral soils and treated non-mineral-soil layers. 

Computing Component Layer Permeability
For each layer of each map unit component, the STATSGO Layer table contains two values for the permeability rate, PERMH and PERML, defined as the maximum and minimum, respectively, for the range in permeability rate for the soil layer or horizon, expressed as inches per hour. The mean permeability rate for each component layer was computed as the arithmetic average of PERMH and PERML, and converted to cm/hr. 

The STATSGO documentation indicates that entering values for the permeability rate is optional when the component layer contains non-mineral-soil material, such as organic matter or rock. Since the STATSGO Layer table entries for permeability rate use a value of 0.0 both to indicate an actual zero value and to indicate that no data were available, it was not possible to distinguish between these cases. In fact, as indicated in the table below , non-zero values were entered for most such components, including essentially all layers designated as stratified or any type of organic material -- even for unweathered bedrock, a small non-zero value was entered for about 40% of the occurrences. Accordingly, the permeability values as entered were used for all component layers with two exceptions: 

There were 34 component layers (0.01%) for which both PERMH and PERML were zero and the dominant soil texture class for the component layer, given by the Layer table variable TEXTURE1, corresponded to a mineral soil. 

There were 27 component layers (< 0.01%) for which the value entered for PERMH was less than the value entered for PERML. 

For both these cases, it was assumed that an error had been made when entering the data, and the component layer was omitted from the computation. 

Determining Contributions to Standard Layers
The contributions of each component layer to the standard layers for a given map unit were determined using the component layer depths specified by Layer table variables LAYDEPL and LAYDEPH, the mean depth to bedrock for each component calculated by averaging Comp table variables ROCKDEPL and ROCKDEPH, and the percent of the area of the map unit covered by each component as specified by COMPPCT. 

For each component, the layers defined in the Layer table were compared with each standard layer in turn. If the standard layer was entirely included within one of the component layers, the permeability rate value for the layer was 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 permeability rate values for each component layer were first weighted in proportion to the amount of overlap before multiplication by the COMPPCT value. The region from the bottom of the last component layer to the bottom of the last standard layer, if any, was assumed to be the same as the lowest component layer down to the mean bedrock depth. Below this depth, the permeability rate was set to 0. 

Computing Mean Permeability for Entire Map Unit
The weighted contributions of all components to each standard layer were then summed to obtain the mean permeability rate values for the map unit. If none of the component layers contributing to the standard layer were mineral soil or if the entire map unit was specified to be water, the permeability rate was set to zero. 

NOTE that for many STATSGO components, a depth-to-bedrock value of 60 inches (152 cm) was used to indicate that the soil was not examined below this depth, and bedrock was not actually encountered. In all cases, however, the permeability was computed as if bedrock was encountered at the depth specified by the mean of ROCKDEPL and ROCKDEPH. Accordingly, the permeability rate values for the two lowest standard layers (1.5 to 2.5 m) are, in many cases, misleadingly low. 

STATSGO Permeability Values for Mineral Soils
With very few execptions, the values entered for PERML and PERMH for all component layers containing mineral soil were either 0.00, 0.06, 0.20, 0.60, 2.00, 6.00, or 20.00 inches/hour (0.00, 0.15, 0.5, 1.5, 5.0, 15, or 50 cm/hr); i.e., values were rounded to the nearest factor of 3. This suggests that all values should be regarded as rough estimates. 

Values for Non Mineral Soils
As indicated above , the STATSGO documentation states that permeability rate values could be omitted when compiling data for non-mineral-soil component layers. No flag was provided for indicating this omission; instead, a value of zero was entered, which cannot be distinguished from an actual measured value of zero. 

To get some idea of how often the permeability rate may have been omitted for non-mineral-soil layers, the number of zero and non-zero values was tabulated for each non-mineral-soil texture class. This gave the following results: 

     Texture		   PERMH			PERML
                   nonzero  zero     % nz      nonzero   zero    % nz
                                          		      
  (organic, 3395 total)
    CBV-MUCK           4       0    100.00          4       0    100.00
    FB                19       0    100.00         19       0    100.00
    GR-MUCK            1       0    100.00          1       0    100.00
    HM               158       0    100.00        158       0    100.00
    MK-PEAT            3       0    100.00          3       0    100.00
    MPT              467       0    100.00        467       0    100.00
    MUCK            2218       1     99.95       2218       1     99.95
    PEAT             188       0    100.00        188       0    100.00
    SP               287       0    100.00        287       0    100.00
    ST-MUCK            2       0    100.00          2       0    100.00
    STV-MPT            3       0    100.00          3       0    100.00
    STV-MUCK          21       0    100.00         21       0    100.00
    STX-MUCK          22       0    100.00         22       0    100.00
    STX-PEAT           2       0    100.00          2       0    100.00
							       
  (other, 32164 total)					       
    CBV-CIND           1       0    100.00          1       0    100.00
    CE                29       0    100.00         29       0    100.00
    CEM              801     534     60.00        375     960     28.09
    CIND              60       0    100.00         59       1     98.33
    DE                 4       0    100.00          4       0    100.00
    FRAG             435       1     99.77        435       1     99.77
    G                  3       0    100.00          3       0    100.00
    GR                 1       0    100.00          1       0    100.00
    GR-MARL            2       0    100.00          2       0    100.00
    GR-VAR             6       0    100.00          6       0    100.00
    GRX-FRAG           2       0    100.00          2       0    100.00
    GYP               64      60     51.61         36      88     29.03
    ICE                0       3      0.00          0       3      0.00
    IND             1117    1189     48.44        258    2048     11.19
    MARL              47       1     97.92         45       3     93.75
    MK-MARL            1       0    100.00          1       0    100.00
    SG               284       0    100.00        279       5     98.24
    SR-            14391      20     99.86      14176     235     98.37
    UWB             8121   11532     41.32       3626   16027     18.45
    VAR              413    2140     16.18        298    2255     11.67
    WB              6397    4900     56.63       3655    7642     32.35

With the exception of SR- ("stratified"), all the non-mineral-soil texture classes are identified in the STATSGO documentation as "Allowable textural code for which no permeability is given"; their meanings are as follows: 

    CE      coprogenous earth		MARL    marl               
    CEM     cemented			MPT     mucky-peat         
    CIND    cinders			MUCK    muck               
    DE      diotomaceous earth		PEAT    peat               
    FB      fibric material		SG      sand and gravel    
    FRAG    fragmental material		SP      sapric material    
    G       gravel			UNK     unknown            
    GYP     gypsiferous material	UWB     unweathered bedrock
    HM      hemic material		VAR     variable           
    ICE     ice or frozen soil		WB      weathered bedrock  
    IND     indurated


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).

perm_grid

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