20 YEARS OF ANALYTIC ELEMENT MODELING IN WISCONSIN

As originally implemented by Otto Strack and his students, two-dimensional, steady-state groundwater flow modeling using Analytic Element Methods (AEM) provide an efficient tool for regional groundwater simulation and is well suited for answering regional water-balance questions. The gridless nature of AEMs is also ideal for simulating interaction at multiple scales simultaneously, allowing models developed for one purpose to be easily refined and extended for later objectives.

These two capabilities formed the driver for early AEM work in Wisconsin, where relatively thin, glacial aquifers are an important and widespread resource. These aquifers are important for human consumption and ecological function through their interconnection with lakes and streams. This interconnection forms ideal internal boundary conditions for AEM’s infinite aquifer. Work in the early 1990s also included the non-glaciated region of the state where regional AEMs were used as boundary conditions for wellhead protection and planning. In the years since, the US Geological Survey Wisconsin Water Science Center, along with other professionals in the state, have developed an extensive list of analytic element models in Wisconsin. All of this work has been conducted with software developed by Otto Strack and his students.

In addition to widespread applications of AEMs, the tools themselves have been extended by important innovations resulting from collaborative projects in Wisconsin. Examples include: formalizing the ability to extract numerical (MODFLOW) models from regional analytic element models; replacing the surface layer of numerical models with coupled analytic element models, analytic lake elements where lake stage is solved for rather than specified, and extending AEM GUI tools to include universal parameter estimation methods that allowed modelers to calibrate to types of data other than heads.

Over the past two decades, tens of analytic element models have been developed by the USGS in Wisconsin. In some cases an original analytic element model resulted in numerous stepwise updates as well as efficient extension of modeling insight into adjacent areas. The strengths of analytic element modeling will continue to make it an important technique for effective groundwater resource planning into the future.