RI0199D
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RI0199D. Field Studies and Numerical Modeling of Unsaturated Flow in the Chihuahuan Desert, Texas, by B. R. Scanlon, F. P. Wang, and B. C. Richter. 56 p., 37 figs., 6 tables, 2 appendices, 1991. doi.org/10.23867/RI00199D. Downloadable PDF.
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ABSTRACT
Field studies and numerical modeling were used to evaluate hydraulic controls on unsaturated flow in the Chihuahuan Desert of Texas. These studies were part of a program to characterize a site for a proposed low-level radioactive waste disposal facility. The study area was instrumented with neutron-probe access tubes to monitor moisture content and with thermocouple psychrometers to monitor water potential. The absence of temporal variations in moisture content monitored in deep (41 m) profiles indicated that water pulses are not moving through the system. Penetration of moisture after rainfall was restricted to the uppermost meter of the unsaturated zone because of the low degree of saturation of the surficial sediments. Water potentials were as low as -15.6 MPa near land surface. Except in the shallow subsurface after precipitation events, water potentials generally decreased upward; this trend indicated an upward driving force for liquid water movement, probably controlled by evapotranspiration.
The computer code TRACRN was used to evaluate various unsaturated-flow processes in this system. One-dimensional simulation of infiltration was calibrated using 36CI data. Sensitivity analyses suggested that applied flux and initial water potential are the most critical factors in controlling the propagation of the wetting front. Analyses of potential leakage from the base of the proposed waste disposal facility indicated that the direction, as well as the net rate, of water movement is modified by lithologic layering in the system. Numerical modeling showed that a saturated zone will develop at the contact between the shallow coarse material and underlying clays when the downward leakage rates are similar to the saturated hydraulic conductivity of the deep clays.
Keywords: Chihuahuan Desert, desert soils, hydraulic parameters, psychrometry, sensitivity analyses, unsaturated flow, unsaturated-flow modeling
CONTENTS
Abstract
Introduction
Purpose of study
Study area
Hydrodynamic approach
Numerical modeling
Methods
Field techniques
Moisture content
Water potential
Hydraulic conductivity
Saturated hydraulic conductivity
Unsaturated hydraulic conductivity
Laboratory techniques
Water potential
Saturated hydraulic conductivity
Soil texture, moisture content, bulk density, and porosity
Moisture-retention curves
Numerical modeling
Results
Field and laboratory studies
Soil texture and moisture content
Water potential
Temperature
Hydraulic conductivity
Numerical modeling
Test problem 1
Test problem 2
Discussion
Moisture content and water potential
Comparison with other arid regions
Limitations of the numerical simulations
Conclusions
Acknowledgments
References
Appendix 1. Psychrometry
Appendix 2. Gravitational, water, total, and osmotic potential of samples from 13 boreholes
Figures
1. Location of study area
2. Conceptual model of potential pathways for radionuclide migration
3. Daily precipitation recorded at one of the four rainfall stations from July 1988 to December 1989
4. Mean monthly snowfall, precipitation, and temperature recorded at Fort Hancock and mean monthly pan evaporation recorded at El Paso
5. Screen-caged, Spanner-type thermocouple psychrometer
6. SC-10 thermocouple psychrometer sample changer
7. Location of sampled boreholes, unsaturated-zone monitoring equipment, and hydraulic-conductivity tests in the study area
8. Schematic cross section detailing vertical distribution of monitoring equipment and sampled boreholes
9. Calibration curve that relates moisture content to neutron count
10. Distribution of psychrometers in borehole 20
11. Calibration curve that relates SC-10 thermocouple psychrometer output to water potential
12. Typical Spanner-type thermocouple response during evaporation for different water potentials
13. Calibration curve of in situ psychrometers that relates thermocouple psychrometer output to water potential
14. Variation in moisture content with depth and time in neutron-probe access tubes 18 and 19
15. Variation in moisture content with depth and time in neutron-probe access tubes 61, 62, 66, and 71
16. Profiles of grain size, moisture content, and water potential for boreholes 15, 21, 23, 41, 41C, 42, 50, and 74
17. Profiles of grain size, moisture content, and water potential for boreholes 30 and 31, and water potential for boreholes 54, 56, and 57
18. Temporal variations in water potential measured daily at 0900 hours in borehole 20
19. Relationship between variations in water potential and null output in field psychrometers installed at 0.3-m depth
20. Vertical distribution of water potentials measured from Julian day 90 to Julian day 350
21. Temporal variations in temperature measured daily at 0900 hours in borehole 20
22. Hourly variations in temperature measured from Julian day 155 to Julian day 158
23. Vertical distribution of temperature measured between Julian days 90 to 200 and Julian days 225 to 350
24. Comparison of water potential and temperature profiles measured on Julian day 175 and Julian day 350
25. Propagation of the wetting front with time during the ponding phase of the instantaneous-profile test and moisture content and matric potentials measured during the drainage phase of the instantaneous-profile test
26. Retention curves and unsaturated hydraulic conductivity versus moisture content and versus water potential based on the instantaneous-profile test
27. Grain-size classifications from borehole 50 used in numerical simulations, water potentials, and corresponding moisture contents
28. Retention curves for different soil textures and calculated relative permeability
29. Water potential and saturation profiles based on one-dimensional simulation of infiltration into a dry system
30. Results of sensitivity analyses evaluating the effect of changing the applied flux, initial water potentials, residual water content, and saturated hydraulic conductivity
31. Two-dimensional grid and boundary conditions used in simulations of infiltration
32. Results of numerical simulations of infiltration and redistribution shown by propagation of the core and fringe of the moisture plume
33. Water-potential isolines at 5 yr and 50 yr, which result from infiltration, and at 100 yr, which result from redistribution
34. Water-potential isolines that result from applying a constant flux of 10 mm yr-1 at a depth of 10 min a heterogeneous soil system and a homogeneous system
35. Propagation of wetting front shown by temporal variations in water potential and saturation
36. Perspective view of spatial variation in saturation
37. Comparison of water-potential profiles measured at the Hueco Bolson, Hanford, and Beatty sites
Tables
1. Summary of boreholes drilled, samples collected, and monitoring equipment installed
2. Osmotic potentials of NaCl solutions and temperatures used in calibration of in situ psychrometers
3. Soil texture and field-saturated hydraulic conductivity results based on Guelph permeameter measurements
4. Saturated hydraulic conductivity results based on laboratory measurements of cores
5. Soil texture and hydrologic parameters for numerical simulations
6. Summary of input data for numerical simulation test problems
Citation
Scanlon, B. R., Wang, F. P., and Richter, B. C., 1991, Field Studies and Numerical Modeling of Unsaturated Flow in the Chihuahuan Desert, Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 199, 56 p.