An Evaluation of Unsaturated Flow Models in an Arid Climate

Abstract

An Evaluation of Unsaturated FlowModels in an Arid Climate by Jason DixonDr. James Cardle, Examination Committee Chair Professor of Civil and Environmental Engineering University of Nevada, Las VegasThe objective of this study was to evaluate the effectiveness of two unsaturated flow models in arid regions. The area selected for the study was the Area 5 Radioactive Waste Management Site (RWMS)at the Nevada Test Site in Nye County, Nevada. The two models selected for this evaluation were HYDRUS-1D [Simunek et al., 1998] and the SHAW model [Flerchinger and Saxton, 1989].Approximately 5 years of soil-water and atmospheric data collected from an instrumented weighing lysimeter site near the RWMS were used for building the models with actual initial and boundary conditions representative of the site. Physical processes affecting the site and model performance were explored. Model performance was based on a detailed sensitivity analysis and ultimately on storage comparisons. During the process of developing descriptive model input, procedures for converting hydraulic parameters for each model were explored. In addition, the compilation of atmospheric data collected at the site became a useful tool for developing predictive functions for future studies. The final model results were used to evaluate the capacities of the HYDRUS and SHAW models for predicting soilmoisture movement and variable surface phenomena for bare soil conditions in the arid vadose zone. The development of calibrated models along with the atmospheric and soil data collected at the site provide useful information for predicting future site performance at the RWMS.

SABAE‐HW: An Enhanced Water Balance Prediction in the Canadian Land Surface Scheme Compared with Existing Models

“…Figure 4 displays the daily evaporation corresponding to the SCL medium-textured soil-type case computed by all codes, except code HYDRUS-1D that was forced by the half hourly evaporation output from SABAE-HW. A quick overdrying of soil surface wetness by SHAW is noted, which corresponds to the same conclusion by Dixon (1999). Furthermore, while CLASS continues evaporating surface moisture before and after the precipitation day, SABAE-HW curbs the rate, joining the evaporation curves corresponding to SHAW.…”

“…Moreover, the faster moving wetting fronts by HYDRUS-1D can be noticed in plots corresponding to the 222.5 cm depth, with a lag time of about 10 d for the medium SCL soil. Th e study by Dixon (1999) already noted this behavior when comparing HYDRUS-1D and SHAW model predictions, a direct consequence of the greater unsaturated hydraulic conductivity evaluated from the Mualem model used in HYDRUS-1D. Th e CLASS early wetting fronts are associated with numerical error arising from the coarse layer.…”

“…Furthermore, it was noted that SHAW generally overestimated evaporation and underestimated water storage change and drainage. The effectiveness of HYDRUS‐1D and SHAW models was also evaluated in a study by Dixon (1999) for an arid test site. It was noticed that the SHAW model's overprediction of evaporation occurs immediately after precipitation events, which overdries the near‐surface soil and induces a subsequent underprediction.…”

Hierarchical Bayesian models for soil CO2flux using soil texture: a case study in central Hokkaido, Japan

“…Free drainage was selected for the lower boundary. The most important factor for successful application of unsaturated flow theory to actual field problems is obtaining the parameters for the governing transfer equation (Dixon 1999). The hydraulic parameters used in this study for hydraulic conductivity function were optimized by neural network prediction (Van Genuchten et al 1992).…”

Characterization Report for the 92-Acre Area of the Area 5 Radioactive Waste Management Site, Nevada Test Site, Nevada

“…• Hydrogeologic (IT, 1998Blout et al, 1995;Laczniak et al, 1996;Winograd and Thordarson, 1975) • Unsaturated Flow (Dixon, 1999;BN, 1998a) • Groundwater Recharge (Levitt and Yucel 2002a, b;Hokett and French, 1998) • Subsidence (DOE, 1998;Obi et al, 1996) • Source Term • Transport and Exposure (Cochran, Crowe, et al, 2001a; • Inadvertent Human Intrusion (BN, 2001;Black, 2001;Shott et al, 1998) • Biological (Hansen and Ostler, 2003;Winkel et al, 1995;Wirth et al, 1999) • General Performance Cochran, Beyeler, et al, 2001) Most scenarios for radionuclide release and transport ultimately involve some aspect of the hydrologic system. Additionally, the hydrologic environment affects monitoring, PA, and closure cover design decisions.…”