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
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Comments and Questions
8/27/2001