Approaches for Estimating Saturated Hydraulic Conductivity Using Soil Texture. Data Methodologies have been proposed in the literature for estimating hydraulic conductivity and infiltration rates on the basis of soil texture information. These methodologies range from relatively qualitative estimates based on soil type (e.g. Table 2.2, p. 29, Freeze and Cherry, 1979) to relatively quantitative estimates based on data from soil gradation analyses. Estimates of hydraulic conductivity from soil gradation analyses include the Xxxxx formula, which is based on an effective grain size (Freeze and Cherry, 1979); the Xxxxxxxx and Xxxx equation, which is based on the mean and the standard deviation of the grain size (Xxxxx and DeWeist, 1966); and the Fair-Hatch equation, which is based on the complete gradation curve (Freeze and Cherry, 1979). These approaches are generally applicable to relatively uniform sands. One approach that has been proposed for estimating infiltration rates is to use regression equations based on percentage of sand, percentage of clay, and porosity. The general idea is to measure infiltration rates from a large set of samples and to correlate these rates to measurements of the percentage of sand, percentage of clay, and porosity. The resulting regression equations are then assumed to be valid for other similar soils. This approach was used by Xxxxx and Xxxxxxxxxx (1985). Regression equations were developed on the basis of measurements taken from more than 5,000 soil horizons from 1,323 soil types in 32 states. The data used to develop these regression equations were collected from soils with clay content ranging from 5 to 60 percent and with a sand content of from 5 to 70 percent. (Clay content was defined as particle sizes smaller than 0.002 mm. Sand was defined as particle sizes between 0.05 and 2 mm.) The data that were used to develop the regressions are described in Xxxxx et al., 1982. Although the regresssions developed by Xxxxx and Xxxxxxxxxx were developed with soils with clay contents of between 5 to 60 percent and with sand contents of from 5 to 70 percent, they have been used to describe soils with higher sand contents (Xxxxxx and Xxxxxxx, 1988; Xxxxx et al., 1997). The accuracy of these regressions for soils with higher sand content is not known. Soils considered for infiltration facilities generally have a relatively low clay content. This may limit the use of these equations for estimating saturated hydraulic conductivity for soils in western Washington. Note that for layered systems, the soil texture inf...
