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    Introduction to Groundwater Modeling in Arizona

What is a Model?

Models serve as a representation of reality and it processes. A model's representation can be physical, mathematical or logical in form. Models can be found in all fields including financial, climatological, environmental, and engineering.

Models are developed from known conditions, typically in the form of measured data. Each model represents a conceptual representation of the processes being examined. From past and present conceptualizations, models can be used to simulate future conditions.

Physical Groundwater Model

         Example of a Physical Groundwater Model

         Photo Credit: West Virginia Conservation Agency

What Types of Models are Used for Groundwater Evaluation?

The two types of models typically used to evaluate groundwater conditions in Arizona are analytical models (Winflow, Thwells) and numeric models (USGS MODFLOW).

Analytical models present a simplistic evaluation of an aquifer (single geologic unit, simplified aquifer parameters). Typically analytical groundwater models utilize the Theis non-equilibrium radial flow equation. The Theis equation is a mathematical equation that treats the aquifer as a uniform porous media, and can solve for future drawdown at a well based on assumed aquifer parameters, boundary conditions and projected pumping rates.

Numeric models have the ability to capture more of the landscapes complexity. Numeric models solve groundwater flow equations by dividing an aquifer system into individual model cells that have characteristic aquifer parameters and pumpage assigned to each cell. The ADWR Groundwater Modeling Unit uses the USGS 3D numeric groundwater flow model code to evaluate regional aquifer behavior throughout Arizona.


How are the ADWR Regional Models Used?

The ADWR uses regional groundwater flow models to evaluate current conditions and simulate possible future scenarios. Each model cell within the numeric MODFLOW model represents conditions unique to part of the model. Components of the landscape including pumpage, recharge, evapotranspiration, stream flow, and flow across model boundaries are simulated within each regional model developed by the ADWR.

Cross Section Diagram

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