A two-stage procedure for determining unsaturated hydraulic characteristics using a syringe pump and outflow observations

Dorthe Wildenschild, Karsten Høgh Jensen, Karl-Josef Hollenbeck, T.H. Illangasekare, D. Znidarcic, Torben Obel Sonnenborg, Michael Brian Butts

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    Abstract

    A fast two-stage methodology for determining unsaturated flow characteristics is presented. The procedure builds on direct measurement of the retention characteristic using a syringe pump technique, combined with inverse estimation of the hydraulic conductivity characteristic based on one-step outflow experiments. The direct measurements are obtained with a commercial syringe pump, which continuously withdraws fluid from a soil sample at a very low and accurate how rate, thus providing the water content in the soil sample. The retention curve is then established by simultaneously monitoring the capillary pressure. The measured retention curves were compared with those obtained by static methods and found to agree closely. Subsequently, a transient outflow experiment is carried out on the same soil sample, in the same apparatus, and the information is obtained on a time-scale of days. The one-step outflow data and the independently measured retention data are included in the objective function of a traditional least-squares minimization routine, providing unique estimates of the unsaturated hydraulic characteristics by means of numerical inversion of Richards equation. As opposed to what is often assumed in practical applications, we found it necessary to allow the exponent (gamma) of van Genuchten's hydraulic conductivity expression to vary to obtain satisfactory estimates. Optimized hydraulic conductivity estimates were ultimately compared with directly measured values, and visual agreement was quite satisfactory. However, we encountered numerical instabilities of the optimization code used (SFIT), a flaw that may constitute a potential problem when using this code for the optimization.
    Original languageEnglish
    JournalSoil Science Society of America Journal
    Volume61
    Issue number2
    Pages (from-to)347-359
    ISSN0361-5995
    Publication statusPublished - 1997

    Cite this

    Wildenschild, D., Jensen, K. H., Hollenbeck, K-J., Illangasekare, T. H., Znidarcic, D., Sonnenborg, T. O., & Butts, M. B. (1997). A two-stage procedure for determining unsaturated hydraulic characteristics using a syringe pump and outflow observations. Soil Science Society of America Journal, 61(2), 347-359.