Groundwater models include physical (laboratory) models and mathematical models including process-based numerical models, which are the focus of Applied Groundwater Modeling (Second Edition). Most groundwater models are developed for forecasting (prediction), but models may also reconstruct past conditions in hindcasting simulations and perform engineering calculations. There are also screening models and generic models for hypothesis testing. A model is the primary quantitative tool available in a groundwater investigation. A workflow for groundwater modeling begins with a question that addresses the modeling purpose.

What is the Motivation for Modeling?

Groundwater hydrologists are often asked questions about groundwater flow systems and management of groundwater resources. The following is a representative sampling of these types of questions.

• How will pumping affect groundwater levels in the North China Plain in the next 100 years?
• How will proposed land use change affect groundwater discharge to wetlands and streams in Madison, Wisconsin, USA?
• How will water management decisions related to water diversions affect groundwater levels in the Nubian Sandstone of Egypt and Libya in the next 50 years?
• How will climate change affect groundwater levels and groundwater discharge to surface water bodies in temperate forests in northern Wisconsin, USA?
• How long will it take for water levels in a lake created as a result of open pit mining in Guyana to reach equilibrium after dewatering operations cease?
• What is the capture area of a well field that supplies municipal water to Graz, Austria?
• Where and when should groundwater be sampled to identify potential leakage of a clay liner beneath a landfill in Mexico City?
• How long will it take contaminants leaching into groundwater from an abandoned industrial site in Tokyo to reach the property boundary?

Providing answers to these seemingly straightforward questions requires considerable specific hydrogeologic information and analyses, as well as general hydrogeologic knowledge, insight, and professional judgment. Even relatively simple groundwater problems require values of aquifer parameters and hydrologic stresses such as pumping and recharge rates.

A groundwater model provides a quantitative framework for synthesizing field information and for conceptualizing hydrogeologic processes. The organization imposed by a model helps alert the modeler to errors in assumptions and to processes not previously considered. In other words: “…applying a model is an exercise in thinking about the way a system works” (Anderson, 1983). For this reason, mathematical modeling should be performed at the beginning of every hydrogeological study that addresses nontrivial questions (e.g., see Bredehoeft and Hall, 1995).

Tóth (1963) gave compelling justification for modeling, which is still valid today: “Whereas it is practically impossible to observe separately all phenomena connected with a regime of groundwater flow, a correct theory discloses every feature and draws attention to the most important properties of the flow.” Or put another way, given that the subsurface is hidden from view and analysis is hampered by lack of field observations, a model is the most defensible description of a groundwater system for informed and quantitative analyses as well as forecasts about the consequences of proposed actions.

Therefore, although not all hydrogeological problems require a model, almost every groundwater problem will benefit from some type of model, if only as a way to organize field data and test the conceptual model. A corollary to the question “why model?” is the question “what else if not a model?” The 1st edition of Applied Groundwater Modeling included discussion of the debate over the worth of models then current in the literature. Today, groundwater models are accepted as essential tools for addressing groundwater problems.

So What Is a Model?

A model is a simplified representation of the complex natural world. For example, a road map is a kind of model ( Wang and Anderson, 1982); it depicts a complex network of roads in a simplified manner for purposes of navigation. Similarly, a conceptual model of a groundwater system simplifies and summarizes what is known about the hydrogeology in the form of written text, flow charts, cross sections, block diagrams, and tables. A conceptual model is an expression of the past and current state of the system based on field information from the site, and knowledge available from similar sites. A more powerful groundwater model is one that quantitatively represents heads in space and time in a simplified representation of the complex hydrogeologic conditions in the subsurface. Broadly speaking, groundwater models can be divided into physical (laboratory) models and mathematical models. Learn more about these models in the introductory chapter to Modelling Fundamentals.

This excerpt was taken from  introductory chapter to Modelling Fundamentals from the book Applied Groundwater Modeling (Second Edition) by Mary P. Anderson, William W. Woessner and Randall J. Hunt. This second edition is extensively revised throughout with expanded discussion of modeling fundamentals and coverage of advances in model calibration and uncertainty analysis that are revolutionizing the science of groundwater modeling. The text is intended for undergraduate and graduate level courses in applied groundwater modeling and as a comprehensive reference for environmental consultants and scientists/engineers in industry and governmental agencies.

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