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Estimating In Situ Unsaturated Hydraulic Properties of Vertically Heterogeneous Soils

USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501

J. B. Sisson

Idaho National Engineering Lab., E., G. and G. Idaho Inc., Geosciences Division, P.O. Box 1625 MS-2107, Idaho Falls, ID 83415-2107

T. R. Ellsworth

Agronomy Dep., Univ. of Illinois, 1102 S. Goodwin Ave., Urbana, IL 61801

*Corresponding author.

ABSTRACT

The majority of procedures for in situ measurement of the unsaturated hydraulic conductivity are variations of the instantaneous profile method. A vertically nonuniform soil requires the unsaturated hydraulic functions to be estimated at each horizon. Scaling systems have evolved in an attempt to reduce the number of hydraulic functions needed to characterize water flow through heterogeneous soils. In this study, we extended the concept of water content () scaling to nonuniform soil profiles, tested the effectiveness of scaling for reducing apparent spatial variability, and estimated the unsaturated hydraulic functions for a naturally occurring loamy sand field site. Two instantaneous profile experiments conducted at Etiwanda, CA, provided soil water content and pressure head (h) data vs. depth (z) and time (t). Water retention, (h), and hydraulic conductivity, K(), functions fitted to data from the 15-cm depth at Plot 1 were arbitrarily chosen as the reference hydraulic properties to which the other depths and plots were scaled. Based on a unit-gradient analysis of the drainage data, the slope of the hydraulic conductivity function, dK/d, was estimated as z/t. Scaling other depths and plots to the reference location was done using an iterative procedure that provided least-squares estimates of the two scaling parameters ( and µ) and a corresponding transformed depth variable (z*). Scaled water content, *, plotted vs. z*/t, using data from all depths and plots, coalesced to a single curve. Scaling successfully coalesced heterogeneous soil hydraulic properties into unique functions for both (h) and K().

Joint contribution of E., G. and G. Idaho Inc. and USDA-ARS, U.S. Salinity Lab.

Received for publication August 27, 1991.

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