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Model Sensitivity to Measured and Estimated Hydraulic Properties of a Zimmerman Fine Sand

Dep. of Soil and Environmental Sciences, Univ. of California at Riverside, CA 92521

R. R. Allmaras

USDA-ARS and Dep. of Soil, Water, and Climate, Univ. of Minn., St. Paul, MN 55108

J. B. Lamb

Dep. of Soil, Water, and Climate, Univ. of Minn., St. Paul, MN 55108

K. E. Johnsen

USDA-ARS/GPSR, For Collins, CO 80522

*Corresponding author (laowu{at}mail.ucr.edu).

ABSTRACT

Water retention and hydraulic conductivity are necessary for simulating solute transport in process-oriented models because of the fundamental role of water flow. Accuracy and extension of soil hydraulic properties are crucial for using models to give site-specific predictions, yet these evaluations are infrequent and often lack rigor. The objectives of this study were to compare measured and estimated hydraulic properties and their effects on simulation of field soil-water regime. Water contents () and hydraulic conductivities (K) at specific matric potentials () alternatively measured in the laboratory and field or estimated from other soil properties were fitted to water retention and hydraulic conductivity functions to obtain parameters according to the van Genuchten formulation. The estimated parameters were inputs for the Root Zone Water Quality Model to simulate water movement in a Zimmerman fine sand (mixed, frigid Argic Udipsamment). Simulated and in situ measured water contents were compared to assess methods used for measuring or estimating hydraulic properties. Simulation using hydraulic functions generated from field-measured () and K() was closest to measured total water depth in the 1.5-m profile as well as water contents at the 0.2-m depth. Arbitrary variations of and n in the van Genuchten equations had larger effects on () and K() functions compared with variations caused by methods to measure and estimate hydraulic functions. This test demonstrated the crucial role and importance of soil hydraulic properties in simulating water storage and movement even in a sandy soil.

Joint contribution of the Minnesota Agric. Exp. Stn. and USDA-ARS. Minnesota Paper no. 21695.

Received for publication January 18, 1995.