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Determining the Saturated Hydraulic Conductivity of a Soil Containing Rock Fragments¹

ABSTRACT

Many rangeland soils contain various amounts of rock fragments (>2 mm and <75 mm), which influence soil hydraulic and water storage properties. Most published data on these properties are only for the fine earth fraction (<2 mm). Determining properties for soils containing rock fragments requires either in situ measurements or procedures that adjust fine earth fabric properties for the coarse fraction content. The hydraulic conductivity is a major determinant of the infiltration and water movement properties of soils. Additionally, the saturated conductivity is used as a scaling parameter in modeling partially saturated flow in porous media. In this Note, literature is reviewed on calculating the saturated hydraulic conductivity for soils containing rock fragments. The two most widely used methodologies are based on knowing either the volume fraction of the rock fragments or the void ratio of the field soil and the fine earth fabric. Both of these soil properties require somewhat laborious laboratory procedures. We show that a calculation of the saturated conductivity of the bulk soil can be accomplished by using only the rock fragment fraction on a weight basis. For many soils the rock fragment fraction on a weight basis is routinely determined and published. For example, the Soil Conservation Service pedon data files list the rock fraction on a weight basis. We test our calculation of the saturated hydraulic conductivity with published data on the saturated conductivity of a stony soil. We show that our simple formulation using the rock fraction on a weight basis duplicates the observed data. Furthermore, it is as accurate as the more complicated procedure requiring void ratio information.

¹ Contribution from the USDA, Agricultural Research Service; Northwest Watershed Research Center, 270 South Orchard, Boise, ID 83705 and the Hydrology Laboratory, BARC-West, Beltsville, MD 20705.

² Research Hydraulic Engineer, USDA-ARS, Northwest Watershed Research Center, Boise, ID; Hydrologist, USDA-ARS, Hydrology Laboratory, Beltsville Maryland; and Geologist, USDA-ARS, Northwest Watershed Research Center, Boise, ID.

Received for publication September 3, 1985.