Model Abstraction Techniques for Soil-Water Flow and Transport (NUREG/CR-6884)

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Publication Information

Manuscript Completed: September 2005
Date Published: December 2006

Prepared by:
Y.A. Pachepsky
Environmental Microbial Safety Laboratory
Agricultural Research Service
U.S. Department of Agriculture
Beltsville, Maryland 20705

M.T. Van Genuchten
George E. Brown Jr. Salinity Laboratory
Agricultural Research Service
U.S. Department of Agriculture
Riverside, California 92705

A.K. Guber, J. Simunek, M.G. Schaap
Department of Environmental Sciences
University of California Riverside
Riverside, California 92521

T.J. Nicholson, R.E Cady
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

T.J. Nicholson, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6724

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Abstract

This report describes the methodology of model abstraction in subsurface hydrology. Model abstraction is defined as the methodology for reducing the complexity of a simulation model while maintaining the validity of the simulation results with respect to the question that the simulation is being used to address. The need in model abstraction may stem from the need to improve the reliability and reduce uncertainty of simulations, to make the modeling and its results more explicable and transparent to the end users, and to enable more efficient use of available resources in data collection and computations. A comprehensive review of model simplification techniques developed in subsurface hydrology is included. Abstractions of both model structure and model parameter determination are described. A systematic and objective approach to model abstraction is outlined. A case study is presented that is designed to illustrate how model abstraction can affect performance assessment of contaminant migration at a relatively humid site. Although the model abstraction methodology is generic, it is designed to be of practical use to NRC licensing staff in their review of the performance assessment of decommissioning sites and waste disposal facilities. The model abstraction process provides a systematic approach to understand the adequacy of model simplification, and facilitates communication and transparency of the model to regulators, stakeholders, and the general public.