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This article in SSSAJ

  1. Vol. 74 No. 4, p. 1113-1118
     
    Received: Sept 18, 2009


    * Corresponding author(s): andy.gregory@bbsrc.ac.uk
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doi:10.2136/sssaj2009.0356

Estimating Relative Hydraulic Conductivity from the Water Release Characteristic of a Shrinking Clay Soil

  1. Andrew S. Gregory *a,
  2. Nigel R. A. Birda,
  3. W. Richard Whalleya and
  4. G. Peter Matthewsb
  1. a Cross-Institute Programme for Sustainable Soil Function, Dep. of Soil Science, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK
    b Environmental and Fluid Modelling Group, Univ. of Plymouth, Drake Circus, Plymouth PL4 8AA, UK

Abstract

To understand the hydraulic properties of soils, accurate prediction of the unsaturated hydraulic conductivity is needed, which is commonly derived from the soil water release characteristic. In fine-textured soils, the modified Mualem–van Genuchten (MMVG) function has been used extensively to predict the change in relative conductivity with changes in matric potential, but this assumes that the soil is rigid with a constant porosity (CP), which is unrealistic for shrinking soils. We used a triaxial cell apparatus to accurately monitor soil volume changes during water release in a clay soil. From this we derived relative hydraulic conductivity functions based on either MMVG (CP), or based on geometrically similar shrinkage (GSS) of all pore sizes by a constant factor as calculated from the measured shrinking porosity (SP). The MMVG (CP) function predicted a decline in relative conductivity of up to three orders of magnitude from 0 to −400 kPa matric potential. This was similar to published data on the response of conductivity in the same soil to consolidation of larger pores. Based on SP, however, the soil remained effectively tension saturated (>0.97) down to a matric potential of −85 kPa due to normal shrinkage. The corresponding GSS (SP) function predicted only a modest decrease in relative conductivity from 1 to 0.85 across this range. In this case the larger pores, although reduced in size, are assumed to be still water filled and conducting. The GSS function was probably the most realistic portrayal of hydraulic functioning in a tension-saturated shrinking clay soil.

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