Field Studies and Numerical Modeling of Unsaturated Flow in the Chihuahuan Desert, Texas. Digital Download

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.