DOI: 10.7910/dvn/vsrhqu
title: Field measurements of hydraulic conductivity and soil shear strength at Gabilan Mesa, CA

description: This data was collected to determine if aspect-dependent differences in field-saturated hydraulic conductivity and soil shear strength exist at Gabilan Mesa, CA. Infiltration and soil shear strength measurements were made at two field locations at Gabilan Mesa on north-facing and south-facing slopes. At each field location, multiple infiltration and soil shear strength measurements were made on north-facing and south-facing slopes. The infiltration measurements were used to estimate field-saturated hydraulic conductivity which is reported here. We also report soil shear strength values made for bare soil and vegetated soil.
description: Data prepared by Paul Richardson, Massachusetts Institute of Technology. Contact info: paul.richardson@usda.gov <br>
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General information: <br>
Infiltration measurements and soil shear strength measurements were made at Gabilan Mesa, CA at two field locations ("weak asymmetry" site and "strong asymmetry" site).
The weak asymmetry site exhibits less topographic asymmetry relative to the strong asymmetry site. Infiltration measurements and soil shear strength measurements of bare and vegetated soil were typically made at the same sites. On a few occasions, only one measurement type was made (for example, infiltration rates were measured while soil shear strength was not). Field saturated hydraulic conductivity values for two field locations (weak asymmetry sites and strong asymmetry sites) at Gabilan Mesa, CA are reported on the "Hydraulic_Conductivity" worksheet. Soil strength measurements for the weak asymmetry and strong asymmetry sites are reported on the "Soil_Strength" worksheet. At each field location, multiple measurements were made on north-facing slopes and south-facing slopes. We also report hillslope gradient at the measurement sites for the hydraulic conductivity data. The gradients were calculated from 10 m USGS National Elevation Dataset (NED) topography. Infiltration and soil strength measurements were from December 30, 2012 to January 9, 2013. <br>
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Coordinates near weak asymmetry sites: 35.918° N, 120.827° W. <br>
Coordinates near strong asymmetry sites: 35.915° N, 120.767° W. <br>
Coordinate are in GCS_WGS_1984. <br>
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Field-saturated hydraulic conductivity details: <br>
We measured the infiltration rates with a single-ring infiltrometer at two ponding depths (5 cm and 10 cm). The inside radius of the infiltrometer was 7.46 cm. Once we completed the 5 cm ponding depth measurements, we added water to the infiltrometer to achieve a ponding depth of 10 cm and made additional measurements at the 10 cm ponding depth. At each ponding depth, we measured the length of time that was required for 200 ml of water to infiltrate into the soil repeatedly until infiltration rates were approximately the same for multiple, contiguous measurements. For each site, the best-fit line to the cumulative infiltration rate for the quasi-steady infiltration measurements against time was chosen as the field-saturated infiltration rate for each ponding depth. We used the approach described in Reynolds & Elrick (1990) to calculate field saturated hydraulic conductivity from the infiltration measurements and report those values here.
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Equations from Reynolds & Elrick (1990) <br>
Kfs = (G/a)*(Q2-Q1)/(H2-H1) [equation 1] <br>
G = 0.316*(d/a)+0.184 [equation 2] <br>
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Kfs = field-saturated hydraulic conductivity <br>
G = empirical, dimensionless parameter dependent on d <br>
d = depth of insertion of the infiltrometer <br>
Q1 and Q2 = field-saturated infiltration rates for the two ponding heights H1 and H2 <br>
H1 and H2 = depth of ponding for the two infiltration measurements corresponding to Q1 and Q2 <br>
a = infiltrometer radius <br>
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We used Gaussian error propagation to calculate the standard deviation for Kfs from known or assumed standard deviations, equation (1), and equation (2). For Q1 and Q2, we treated the standard error of the regressions used to estimate Q1 and Q2 as the standard deviations of Q1 and Q2. We assumed that the standard deviation in insertion depth d was 0.5 cm and that the uncertainty in H1 and H2 was 0.25 cm. The uncertainty in the infiltrometer radius a was assumed to be zero. <br> 
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Attribute information <br>
Site: Name of field site where measurements were made. At each "Field location," a set of north-facing and south-facing "Site" measurements were made. <br>
Latitude: Latitude of "Site." <br>
Longitude: Longitude of "Site." <br>
Field location: Field location is either "weak asymmetry" site or "strong asymmetry" site.<br>
Slope direction: Direction that the slope faces. Either "north-facing" or "south-facing." <br>
Saturated hydraulic conductivity (cm/hr): Field-saturated hydraulic conductivity estimated using the approach described in Reynolds & Elrick (1990) from 5-cm and 10-cm depth infiltration measurements. Uncertainty calculated from the approach described in the above "Field-saturated hydraulic conductivity details." <br>
Gradient: Gradient measured at "Site" from a 10-m NED DEM. <br>
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Soil shear strength measurement details: <br>
Soil shear strength was measured with a shear vane (Humboldt H-4212MH). At each site where we measured the soil shear strength, we made 10 measurements and used the mean as the representative value for the site. We report the standard deviation of soil shear strength made at each site where the 10 measurements were made. Measurements were made on bare soil and vegetated soil. For the vegetated soil measurements, we clipped the vegetation down to the surface before making the measurement to limit the entanglement of the shear vane blades with above-surface vegetation. At two of the north-facing sites, bare soil measurements were not made (values marked as "x" on the "Soil_Strength" worksheet). <br>
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Attribute information <br>
Site: Name of field site where measurements were made. At each "Field location," a set of north-facing and south-facing "Site" measurements were made. <br>
Latitude: Latitude of field site. <br>
Longitude: Longitude of field site. <br>
Field location: Field location is either "weak asymmetry" site or "strong asymmetry" site. <br>
Slope direction: Direction that the slope faces. Either "north-facing" or "south-facing." <br>
Bare soil shear strength (kg/cm2): Mean of 10 soil shear strength measurements made with a shear vane for bare soils at "Site." The standard deviation was calculated from the 10 measurements made at each "Site." <br>
Vegetated soil shear strength (kg/cm2): Mean of 10 soil shear strength measurements made with a shear vane for vegetated soils at "Site." The standard deviation was calculated from the 10 measurements made at each "Site."