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

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Soil pore volume as well as pore size, shape, type (i.e., biopore vs. crack), continuity, and distribution in soil affects soil water and gas exchange. Vertical drainage and lateral drainage of water by gravitational forces occur through large noncapillary soil pores, but redistribution and upward movement of water occur through capillary soil pores. The purpose of this study was to apply equations for K(θ) and intrinsic permeability for different pressure head (i.e., pore size) ranges of soil based on water retention curves for calcareous sandy loam soil and alluvial clay soils. The equations were then used to compare measured and predicted K(θ) for silt loam soils. Three soils with three depths each were used to examine 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 instantaneous profile method based on evaporation rather than drainage. Predicted unsaturated hydraulic conductivity was point based rather than curve based. The nonswelling sandy loam soil had quite different predicted unsaturated hydraulic conductivity from those of the saline and nonsaline clay soils. The predicted unsaturated hydraulic conductivity was in the vicinity of the measured data from the silt loam columns, but the overall root mean square error 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 caused by sorbed water, as well as enhanced flow through large pores at the wet end.

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