The value of long-term monitoring in the development of ground-water-flow models

Abstract

As environmental issues have come to the forefront of public concern, so has the aware-ness of the importance of ground water in the overall water cycle and as a source of the Nation’s drinking water. Heightened interest has spawned a host of scientific enterprises (Taylor and Alley, 2001). Some activities are directed toward collection of water-level data and related information to monitor the physical and chemical state of the resource. Other activities are directed at interpretive studies undertaken, for example, to optimize the location of new water-supply wells or to protect rivers and lakes fed by ground water. An important type of interpretive study is the computer ground-water-flow model that inte-grates field data in a mathematical framework. Long-term, systematic collection of hydro-logic data is crucial to the construction and testing of ground-water models so that they can reproduce the evolution of flow systems and forecast future conditions. This Fact Sheet provides an example of the incorporation of historical hydrologic data in a regional ground-water model for southeastern Wisconsin. The example demonstrates how the collection of three categories of long-term records—water levels from observation wells, streamflow measurements at gaging stations, and water-use records from high-capacity wells—allows the model to reproduce past conditions and simulate future trends. It is the availability of these long-term data, dating back to measurements from the earliest water-supply wells in the 19th century, that couple the computer model to real-world conditions, and, thus, give it value as a scientific and water-resources management tool. The model discussed in this Fact Sheet was a collaborative effort among the U.S. Geologi-cal Survey, the Wisconsin Geological and Natural History Survey, and the Southeastern Wisconsin Regional Planning Commission. A complete description of the hydrologic study of southeastern Wisconsin including ground-water flow modeling is described in Feinstein and others, 2004. Additional information and documentation are available from the Wiscon-sin Geological and Natural History and from the website http://wi.water.usgs.gov/glpf/.