Development of a coupled surface/subsurface hydrologic model and stochastic analysis of a vadose zone and saturated zone numerical model

This work contains three papers dealing with various aspects of hydrologic modeling. The first paper discusses the development and application of a coupled surface/subsurface model. This model simulates overland flow, ponding, evaporation, infiltration and subsequent deep migration of moisture beneath a nuclear subsidence feature at the Nevada Test Site. Both short term (less than 100 days) and long term (over 15 years) simulations were performed. The surface water overland flow component achieved a good agreement between observed and simulated water levels following a precipitation event. The long term simulations underpredicted the volumetric water contents and depth of moisture fronts, but performed acceptably well since it was not calibrated to long term observed values. The temporal mean of the simulated flux for the 5.0 m and 10.0 m depths was 0.75 and 1.4 m yr;The second paper discusses the development and application of new stochastic techniques for vadose zone modeling that include the spatial heterogeneity and covariance among the input parameters. The inclusion of different stochastic structures (dependent, independent or partial dependence) affects simulated moisture front behavior. The difference in simulation results between the stochastic structures becomes more pronounced for longer simulations. The stochastic vadose zone model with a constant surface flux was able to simulate the mean behavior and variability of the volumetric water contents beneath subsidence crater U3fd. The long term simulations suggest that the moisture front beneath subsidence crater U3fd will reach the center of the bomb debris (27. m above the water table) in approximately 921 years.;The third paper investigates the role of using two-dimensional transport models when the hydraulic conductivity is known to be truly three-dimensional. A new method was developed to properly transform the two-dimensional hydraulic conductivity field such that solute arrival times in a saturated zone transport model resemble the three-dimensional flow field. Results indicate that the mean and variance must be increased by a factor that depends upon the variance of the three-dimensional hydraulic conductivity field. Numerical experiments suggest that the transformation decreases the relative error of particle breakthrough curves by as much as 54%.