THE VALUE OF LONG-TERM MONITORING IN THE DEVELOPMENT OF GROUND-WATER-FLOW MODELS

Groundwater flow models are essential tools for managing groundwater. They integrate field data into a mathematical framework that allows water-supply professionals to test different water-use scenarios and make quantitative predictions. Long-term, systematic collection of hydrologic data is crucial to the construction and testing of groundwater models so that they can reproduce the evolution of flow systems and forecast future conditions.

Here we provide an example of the incorporation of historical hydrologic data in a regional groundwater 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 the long-term data, dating back to measurements from the earliest water supply wells in the 19th century, that couples the computer model to real-world conditions, and, thus, gives it value as a scientific and water-resources management tool.

The computer model focused on a 10-county region in southeastern Wisconsin where intensive groundwater use has affected the groundwater flow system throughout the region. Along with producing a large cone of depression, pumping over time has reduced flow from groundwater to surface water (including Lake Michigan), shifted groundwater divides, increased leakage from shallow to deep parts of the groundwater flow system, and changed direction of groundwater flow.

Long-term monitoring of water levels, stream flow, and water-use was essential for calibration of the model. We used water levels recorded by the Wisconsin Geological and Natural History Survey (WGNHS) and the U.S. Geologic Survey (USGS) before 1915 and from 1940 to the present from about 50 wells in the Wisconsin Ground-Water Observation Well Network, streamflow measurements from the USGS stream-gaging network in southeastern Wisconsin, and water use records compiled by Wisconsin Department of Natural Resources, the WGNHS, and the Illinois Water Survey. These records allowed us to match past conditions and better simulate future groundwater flows and drawdowns.