Techniques

Modeling

Numerical Models

The Bureau has used numerical models to evaluate flow and transport processes in unsaturated media, suitability of evapotranspirative covers for waste containment, and impact of climate variability and paleoclimate changes on groundwater recharge. Codes that have been used in different studies include

HELP: http://www.wes.army.mil/el/elmodels (Schroeder et al., 1994)

HYDRUS1D: http://www.ussl.ars.usda.gov/MODELS/HYDR1D1.HTM (Simunek et al., 1998)

SHAW: http://www.nwrc.ars.usda.gov/models/shaw/index.html (Flerchinger et al., 1996)

SoilCover: http://www.geo2000.com (Wilson et al., 1994)

SPLaSHWaTr: Simulation Program for Land-Surface Heat and Water Transport

SWIM: http://www.clw.csiro.au/products/swim (Verburg et al., 1996)

TRACR3D: http://ees-www.lanl.gov/EES5/models/lanl_models.html

UNSAT-H: http://hydrology.pnl.gov/resources.asp (Fayer, 2000)

VS2DTI: http://water.usgs.gov/software/vs2di.html (Healy, 1990)

Intercode Comparison Study
A detailed comparison of seven different codes, HELP, HYDRUS-1D, SHAW, SoilCover, SWIM, UNSAT-H, and VS2DTI, was conducted for an EPA study using 1–3 yr water-balance monitoring data from nonvegetated engineered covers (3 m deep) in warm (Texas) and cold (Idaho) desert regions. Results from this study provided valuable information on various approaches to simulating different processes in the various codes and the importance of boundary conditions and hydraulic parameters for simulating subsurface flow. A unique aspect of the code comparison study was the ability to explain the outliers by incorporating the simulation approaches (boundary conditions or hydraulic parameters) used in the outlying codes in a single code and comparing the results of the modified and unmodified code. This approach overcomes the criticism that valid code comparisons are infeasible because of large numbers of differences among codes.

References:

Scanlon, B. R., Christman, M., Reedy, R. C., and Gross, Beth, 2002, Intercode comparisons for simulating water balance in an engineered cover, in 2001 International Containment and Remediation Technology Conference, Orlando, Florida, Institute for International Cooperative Environmental Research, Florida State University, Paper ID. No. 148, http://www.iicer.fsu.edu, 3 p. [PDF]

CodeSimulation of Liquid and Vapor Transport in Deserts

The SPLaSHWaTr code was used to evaluate the relative importance of liquid and vapor flow in semiarid regions by simulating subsurface flow in response to meteorologic forcing and comparing simulated fluxes with those estimated from environmental tracers including tritium and chlorine-36.

References:

Scanlon, B. R., 1992, Evaluation of liquid and vapor flow in desert soils based on chlorine-36 and tritium tracers and nonisothermal flow simulations: Water Resources Research, v. 28, no. 1, pp. 285–297. [PDF]

Scanlon, B. R., and Milly, P. C. D., 1994, Water and heat fluxes in desert soils 2. Numerical simulations: Water Resources Research, v. 30, no. 3, pp. 721–733. [PDF]

Evaluation of System Response to Paleoclimate Variability in the Chihuahuan Desert

The HYDRUS-1D code was used to simulate system response to paleoclimatic forcing using modeling of nonisothermal liquid and vapor flow and Cl transport at semiarid (High Plains, Texas) and arid (Chihuahuan Desert, Texas; Amargosa Desert, Nevada) sites. Nonisothermal liquid and vapor flow simulations indicate that upward flow for at least 1–2 kyr in the High Plains and for 12–16 kyr at the Chihuahuan and Amargosa desert sites is required to reproduce measured upward water potential gradients and that recharge is negligible (<0.1 mm/yr) in these interdrainage areas.

References:

Scanlon, B. R., K. Keese, R. C. Reedy, J. Simunek, and B. J. Andraski. 2003. Variations in flow and transport in thick desert vadose zones in response to paleoclimatic forcing (0–90 kyr): field measurements, modeling, and uncertainties. Water Resources Research 39:1179; doi:1110.1029/2002WR001604.

Simulation of Groundwater Recharge

Recharge controls (climate, vegetation, and soils) were evaluated by simulating drainage in 5-m-thick profiles using a 1-D unsaturated flow code (UNSAT-H), climate data, and vegetation and soil coverages from online sources. Soil hydraulic properties were estimated from STATSGO/SSURGO soils data using pedotransfer functions. Vegetation parameters were obtained from the literature. Long-term (1961–1990) simulations were conducted for 13 county-scale regions representing arid to humid climates and different vegetation and soil types, using data for Texas . Areally averaged recharge rates are most appropriate for water resources; therefore Geographic Information Systems were used to determine spatial weighting of recharge results from 1-D models for the combination of vegetation and soils in each region. Simulated 30-year mean annual recharge in bare sand is high (51–709 mm/yr) and represents 23–60% (arid–humid) of mean annual precipitation (MAP). Adding vegetation reduced recharge by factors of 2–30 (humid–arid), and soil textural variability reduced recharge by factors of 2–11 relative to recharge in bare sand. Vegetation and soil textural variability both resulted in a large range of recharge rates within each region; however, spatially weighted, long-term recharge rates were much less variable and were positively correlated with MAP (r 2 = 0.85 for vegetated sand; r 2 = 0.62 for variably textured soils). The most realistic simulations included vegetation and variably textured soils, which resulted in recharge rates from 0.2 to 118 mm/yr (0.1–10% of MAP). Mean annual precipitation explains 80% of the variation in recharge and can be used to map recharge.

References:

Keese, K. E., B. R. Scanlon, and R. C. Reedy. 2005. Assessing controls on diffuse groundwater recharge using unsaturated flow modeling. Water Resources Research 41:W06010, doi:06010.01029/02004WR003841.