Groundwater Modeling, Illinois State Water Survey

Groundwater Modeling

The Illinois State Water Survey (ISWS) has a long history in the field of groundwater flow modeling. Starting in the 1960s, ISWS groundwater scientists modeled groundwater flow and aquifer response to pumpage with electric analog models of aquifer systems (Walton and Prickett, 1963; Prickett, 1967). These models used a network of resistors and capacitors, arranged in a grid to approximate aquifer geometry (figure 1), to simulate groundwater flow (resistance) and aquifer storage (capacitance). A dedication for the display of several historical analog models was held on November 14, 2007.

Figure 1. An analog model used grids of capacitors and resistors to simulate aquifer hydraulics.Figure 1. An analog model used grids of capacitors and resistors to simulate aquifer hydraulics.

With the advent of computers, groundwater modeling focused on the digital solution of sets of equations expressing the physics of groundwater flow. Scientists at the ISWS developed one of the first digital aquifer codes, the Prickett Lonnquist Aquifer Simulation Model (PLASM). The setup and use of PLASM to simulate aquifer conditions were described in ISWS Bulletin 55, Selected Digital Computer Techniques for Groundwater Resource Evaluation (Prickett and Lonnquist, 1971). Like the earlier electric analog models, PLASM separated the aquifer into a gridlike structure (figure 2) and used an equation to describe the aquifer flow characteristics at each grid node or vector volume. Each node was assigned an average value for aquifer transmissivity, storage factor, initial head, and net withdrawal rate. The “unknown” in each node equation was the final water level or head in the aquifer. The aquifer boundary was approximated in stepwise fashion, and a zero transmissivity was assigned to nodes outside that boundary.

Figure 2. A typical finite difference grid superimposed over an aquifer system map.Figure 2. A typical finite difference grid superimposed over an aquifer system map.

Early computers used computer cards, and a card deck was prepared containing one card for each node with any system properties different than those defined on a default value card. Program operation necessitated collating the basic aquifer simulation program deck, parameter card, and node deck in the order shown in figure 3.

Computers are ideal for solving sets of equations quickly; thus, digital computer models became the standard for simulating various aquifer conditions. For many years, PLASM was one of the most popular groundwater model codes in the United States—if not the world—and was used for hundreds of aquifer simulations. Digital files and rapidly evolving personal desktop computers superseded the old card decks and the old mainframe computers. Consequently, even more scientists had access to groundwater modeling tools. The essence of aquifer simulation and the governing equations have not changed, however.

Figure 3. Groundwater modeling with PLASM used an aquifer simulation card deck such as this.Figure 3. Groundwater modeling with PLASM used an aquifer simulation card deck such as this.

Outgrowths of PLASM were the Random Walk simulator for solute transport using PLASM as the flow model (Prickett et al., 1981) and GWPATH for examining groundwater flow paths and travel times (Shafer, 1987). While not as widely embraced as the original PLASM model, these codes and publications embody a continuing interest by ISWS scientists in modeling groundwater flow and transport.

In large part, MODFLOW and its associated modules, created by the U.S. Geological Survey, have replaced all of these codes, including PLASM. Yet there is no doubt that PLASM was one of the first computer codes for aquifer simulation, predating MODFLOW by 10 years or more. Computing technology and visual displays have progressed exponentially in the decades since the early analog and digital models, and ISWS scientists will continue to use the latest tools for modeling groundwater flow to assess aquifer yields and groundwater development impacts.

Selected References

Prickett, T.A. 1967. Designing Pumped Well Characteristics into Electric Analog Models. Ground Water 5(4):38–46.

Prickett, T.A., and C.G. Lonnquist. 1971. Selected Digital Computer Techniques for Groundwater Resource Evaluation. Illinois State Water Survey Bulletin 55, Champaign, IL.

Prickett, T.A., T.G. Naymik, and C.G. Lonnquist. 1981. A "Random-Walk" Solute Transport Model for Selected Groundwater Quality Evaluations. Illinois State Water Survey Bulletin 65, Champaign, IL.

Shafer, J.M. 1987. GWPATH: Interactive Ground-Water Flow Path Analysis. Illinois State Water Survey Bulletin 69, Champaign, IL.

Walton, W.C., and T.A. Prickett. 1963. Hydrogeologic Electric Analog Computers. Proceedings, American Society of Civil Engineers, paper 3695, 89(HY6).


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