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STOMP

Las Cruces Trench

The Las Cruces Trench Site refers to a 26.4-m-long by 4.8-m-wide by 6-m-deep trench that was excavated in undisturbed soil approximately 40 km northeast of Las Cruces, New Mexico. The trench was dug to provide samples for characterization of hydrologic property (e.g., bulk density, porosity, particle- size distribution, saturated hydraulic conductivity and water retention parameters) and horizontal access to irrigated areas through the trench face. Soil properties from collected core samples were reported by Wierenga et al. [1989]. Two experimental plots were developed at the trench site with controlled irrigation systems, capabilities for measuring soil moisture content and soil moisture tension, and solution samplers to monitor flow and transport experiments. Results from experiments conducted at these plots are described in the literature [Wierenga et al. 1990; Hills et al. 1991; Hills and Wierenga 1994].

In this application the Water Operational Mode of the STOMP simulator was used with a two- dimensional hydrologic conceptual model to predict the flow and transport behavior of the second experiment at the Las Cruces Trench Site. The objective of this investigation [Rockhold et al., 1996] "was to evaluate the effectiveness of similar media scaling and geostatistical methods for characterizing the spatial variability of soil hydraulic properties" and "to describe a simple method for conditioning the hydraulic properties used for flow and transport modeling based on the spatial distributions of initial field-measured water contents and a set of scale-mean hydraulic parameters." The crux of this application was the development of the hydrologic conceptual model for the experimental plot site. The conceptual model comprised a two- dimensional domain of 7700 nodes, each with unique hydrologic properties for porosity, saturated hydraulic conductivity, and soil-moisture retention characteristics. Simulation results for water content and tritium concentration at one moment in time are shown in the figures below.

This investigation demonstrated that a simple method for conditioning the hydraulic properties based on the spatial distribution of initial field-measured water contents and a set of scale-mean hydraulic parameters could be used to achieve good matches between experimentally observed and numerically simulated flow and transport behavior without model calibration. This study suggests that if enough data are available to reliably estimate scale-mean hydraulic parameters and a depth- averaged pressure head distribution, combined with spatial distribution of water content in the field, then the problem of simulating unsaturated water flow and transport could change from being stochastic to more deterministic.

Observed (top) and Simulated (bottom) Water Contents on day 70 of Las Cruces Trench Experiment 2B

Observed (top) and Simulated (bottom) Water Contents on day 70 of Las Cruces Trench Experiment 2B

Observed (top) and Simulated (bottom) Water Contents on day 310 of Las Cruces Trench Experiment 2B

Observed (top) and Simulated (bottom) Water Contents on day 310 of Las Cruces Trench Experiment 2B


Observed (top) and Simulated (bottom) Relative Tritium Concentration Distributions on Day 310

Observed (top) and Simulated (bottom) Relative Tritium Concentration Distributions on Day 310


References

Hills, R.G., P.J. Wierenga, D.B. Hudson, and M.R. Kirkland. 1991. "The second Las Cruces Trench experiment: Experimental results and two-dimensional flow predictions." Water Resources Research, 27:2707-2718.

Rockhold, M.L., R.E. Rossi, and R.G. Hills. 1996. "Application of similar media scaling and conditional simulation for modeling water flow and tritium transport at the Las Cruces Trench Site." Water Resources Research, 32(3):595-609.

Wierenga, P.J., A.F. Toorman, D.B. Hudson, J. Vinson, M. Nash, and R.G. Hills. 1989. Soil Physical Properties at the Las Cruces Trench Site, NUREG//CR-5441.

Wierenga, P.J., D.B. Hudson, R.G. Hills, I. Porro, J. Vinson, and M.R. Kirkland. 1990. Flow and Transport Experiments at the Las Cruces Trench Site: Experiments 1 and 2, NUREG/CR-5607.

Wierenga, P.J., R.G. Hills, and D.B. Hudson. 1991. "The Las Cruces Trench Site: Characterization, Experimental Results, and One-Dimensional Flow Predictions." Water Resources Research, 27:2695-2705.

Wierenga, P.J., D.B. Hudson, and M.R. Kirkland. 1991. "The Second Las Cruces Trench Experiment: Experimental Results and Two-Dimensional Flow Predictions." Water Resources Research, 27:2707-2718.

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