|Van Genuchten, Martinus|
Submitted to: Federal Interagency Hydrologic Modeling Conference
Publication Type: Proceedings
Publication Acceptance Date: 12/12/2005
Publication Date: 4/2/2006
Citation: Pachepsky, Y.A., Guber, A.K., Van Genuchten, M.T., Nicholson, T.J., Cady, R.E., Simunek, J., Jacques, D., Gish, T.J., Daughtry, C.S. 2006. Model abstraction in hydrologic modeling. Proceedings of Federal Interagency Hydrologic Modeling Conference. Paper No. 8D. 2006 CDROM. Interpretive Summary: This work describes and demonstrates the methodology of model abstraction (MA) in contaminant hydrology. MA is defined as a 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. Thus, MA reduces the complexity of the system to be simulated to its essential components and processes through a series of conceptualizations, selection of significant processes and appropriate scales, and identification of the associated parameters. MA has been successfully used in many research and engineering fields that actively employ modeling. Objectives of this work were (a) to develop a systematic overview of MA techniques applicable in subsurface contaminant hydrologic modeling, (b) to suggest a systematic and comprehensive procedure of the MA implementation, and (c) to develop a case study of MA application. We found a large number of MA techniques that have been or can be applied in subsurface contaminant hydrologic and developed a taxonomy of those techniques. The original, or base, model may need to be simplified if (a) it is difficult to obtain its reliable calibration, (b) the base model tends to magnify errors in its input values, and the predictions are very uncertain, (c) simulation results from the base model are counterintuitive and inexplicable, (d) there are not enough resources are available to run the base model, prepare inputs and process outputs, (f) the modeling process has to be more transparent and tractable to be acceptable by decision-makers or public. A step-by-step MA implementation is suggested that takes advantage of the available MA techniques. The MA methodology is demonstrated with the experimental study and simulations of water flow in field soil. A well-accepted model has given inexplicable results when applied to simulate observed soil water fluxes. The models abstraction resulted in the model that gave a good description of field data and also explained behavior of the base model. The potential benefits of MA applications in contaminant hydrology are very substantial.
Technical Abstract: Model abstraction (MA) is a 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 MA explicitly deals with uncertainties in model structure and in model parameter sources. It has been researched in various knowledge fields that actively use modeling. We present (a) the taxonomy of model abstraction techniques being applied in subsurface hydrologic modeling, (b) the systematic and comprehensive procedure of the MA implementation including (1) defining the context of the modeling problem, (2) defining the need for the model abstraction, (3) selecting applicable MA techniques, (4) identifying MA directions that may give substantial gain, and (5) simplifying the base model in each direction. The need in MA may stem from (a) difficulties to obtain a reliable calibration of the base model, (b) the error propagation making the key outputs uncertain, (c) inexplicable results from the base model, (d) excessive resource requirements of the base model, (e) the intent to include the base model in a larger multimedia environmental model, (f) the request to make the modeling process more transparent and tractable, and (g) the need to justify the use a simple model when a complex model is available. The example illustrates the MA application in field-scale simulations of water flow in variably saturated soils and sediments. The MA (a) can result in the improved reliability of modeling results, (c) make the data use more efficient, (c) enable risk assessments to be run and analyzed with much quicker turnaround, with the potential for allowing further analyses of problem sensitivity and uncertainty, and (d) enhance communication as simplifications that result from appropriate model abstractions may make the description of the problem more easily relayed to and understandable by others, including decision-makers and the public.