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United States Department of Agriculture

Agricultural Research Service

Title: Estimating Hysteresis in the Soil Water Retention Function from Cone Permeameter Experiments

Authors
item Simunek, J. - U.C. RIVERSIDE
item Kodesova, R. - UNIV. OF S. CAROLINA
item Gribb, M. - UNIV. OF S. CAROLINA
item Van Genuchten, Martinus

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 16, 1999
Publication Date: N/A

Interpretive Summary: Definition of the hydraulic properties of unsaturated soils is increasingly necessary for geotechnical applications. Knowledge of the soil moisture characteristic and hydraulic conductivity curves is particularly important for accurate numerical modeling of variably saturated flow and contaminant transport processes. While these soil properties can be determined in the laboratory, in situ methods are often preferred. Here we present a modified cone penetrometer, called a cone permeameter, which has been designed to inject water into the soil through a screen and measure the progress of the wetting front with two tensiometer rings positioned above the screen. The obtained measured results during the infiltration and redistribution process are analyzed with parameter estimation technique to obtain the hysteretic soil hydraulic properties

Technical Abstract: Data obtained from modified cone penetrometer experiments were used to estimate the hysteretic soil hydraulic properties with a parameter estimation technique which combined a numerical solution of the Richards equation with Marquardt-Levenberg optimization. The modified cone penetrometer was designed to inject water into a soil through a cylindrical screen, measure the infiltration rate with time, and track the movement of the wetting front using two tensiometer rings positioned above the screen. After reaching relatively stable tensiometer readings during the experiments, the source of water was cut off and pressure head readings measured while water in the soil profile redistributed. Cumulative inflow and pressure head readings for two experiments with different supply pressures were analyzed to obtain estimates of the soil hydraulic parameters. Analysis of flow responses obtained during the infiltration period, and those obtained during the combined infiltration and redistribution phases, demonstrated the importance of hysteresis of the soil hydraulic functions. We found that the redistribution phase could not be described accurately when hysteresis was neglected. Hysteresis in the soil hydraulic functions was modeled using a relatively simple empirical model in which wetting scanning curves are scaled from the main wetting curve and drying scanning curves from the main drying curve. This model was deemed adequate for our examples. Optimization results for various combinations of unknown soil hydraulic parameters were compared to results of standard laboratory and in situ methods.

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