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Soil water retention and maximum capillary drive from saturation to oven dryness
Author(s) -
MorelSeytoux Hubert J.,
Nimmo John R.
Publication year - 1999
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/1999wr900121
Subject(s) - saturation (graph theory) , water retention , suction , capillary action , dryness , water retention curve , soil water , water content , soil science , mathematics , environmental science , hydrology (agriculture) , chemistry , thermodynamics , geotechnical engineering , physics , geology , medicine , surgery , combinatorics
This paper provides an alternative method to describe the water retention curve over a range of water contents from saturation to oven dryness. It makes two modifications to the standard Brooks and Corey [1964] (B‐C) description, one at each end of the suction range. One expression proposed by Rossi and Nimmo [1994] is used in the high‐suction range to a zero residual water content. (This Rossi‐Nimmo modification to the Brooks‐Corey model provides a more realistic description of the retention curve at low water contents.) Near zero suction the second modification eliminates the region where there is a change in suction with no change in water content. Tests on seven soil data sets, using three distinct analytical expressions for the high‐, medium‐, and low‐suction ranges, show that the experimental water retention curves are well fitted by this composite procedure. The high‐suction range of saturation contributes little to the maximum capillary drive, defined with a good approximation for a soil water and air system as H c M = ∫ 0 ∞k r w d h c , where k rw is relative permeability (or conductivity) to water and h c is capillary suction, a positive quantity in unsaturated soils. As a result, the modification suggested to describe the high‐suction range does not significantly affect the equivalence between Brooks‐Corey (B‐C) and van Genuchten [1980] parameters presented earlier. However, the shape of the retention curve near “natural saturation” has a significant impact on the value of the capillary drive. The estimate using the Brooks‐Corey power law, extended to zero suction, will exceed that obtained with the new procedure by 25 to 30%. It is not possible to tell which procedure is appropriate. Tests on another data set, for which relative conductivity data are available, support the view of the authors that measurements of a retention curve coupled with a speculative curve of relative permeability as from a capillary model are not sufficient to accurately determine the (maximum) capillary drive. The capillary drive is a dynamic scalar, whereas the retention curve is of a static character. Only measurements of infiltration rates with time can determine the capillary drive with precision for a given soil.