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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #226586

Title: Prediction of Hydraulic Conductivity as Related to Pore Size Distribution in Unsaturated Soils

item Logsdon, Sally

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/14/2009
Publication Date: 9/4/2009
Citation: Amer, A.M., Logsdon, S.D., Davis, D. 2009. Prediction of Hydraulic Conductivity as Related to Pore Size Distribution in Unsaturated Soils. Soil Science. 174(9):508-515.

Interpretive Summary: Soil water and gas exchange occur in soil pores (holes or voids), and depend on the size, shape, and continuity of the pores. This study calculated how fast water moves through soil depending on the size of soil pores. The non-swelling sandy soils had a higher fraction of large pores and a lower fraction of small pores compared with clay soils. For silty soils (flour-size particles), the calculated rate of water flow matched the measured rate. This information is important for scientists who need to predict water flow rates in soils with different size pores and different size particles.

Technical Abstract: Soil pore volume as well as pore size, shape, type (i.e. biopore versus crack), continuity, and distribution in soil affect soil water and gas exchange. Vertical and lateral drainage of water by gravitational forces occurs through large, non-capillary soil pores, but redistribution and upward movement of water occurs through capillary soil pores. The purpose of this study was to develop equations to describe unsaturated hydraulic conductivity in relation to soil pore size, and to test on another data set. Three soils with three depths each were used to develop the concepts, based on water retention curve, saturated hydraulic conductivity, and bulk density measured from undisturbed cores. Unsaturated hydraulic conductivity was determined from undisturbed columns of silt loam soils using instanteous profile method based on evaporation rather than drainage. Predicted unsaturated hydraulic conductivity was point-based rather than curve-based. The non-swelling sandy loam soil had quite different predicted unsaturated hydraulic conductivity than the saline and non-saline clay soils. The predicted unsaturated hydraulic conductivity was in the vicinity of the measured data from the silt loam columns, but the overall RMSE was 1.042 for log-transformed data. The point-based unsaturated hydraulic conductivity equations were useful for fine-textured soil, and incorporated flow reduction in dry soil due to sorbed water, as well as enhanced flow through large pores at the wet end.