Water availability is determined by the volume of water in storage, as surface and ground water, and by controls on water movement through the environment. Existing water use, water quality, and ecological or other in-stream requirements potentially constrain water availability. The U.S. Geological Survey (USGS) has started a pilot study in the Great Lakes Basin for a national initiative on water availability and use assessment. In this study, key indicators for assessing water availability are being identified and refined. Goals of the pilot study include developing methods to quantify indicators and demonstrating application in the Great Lakes Basin. The pilot study also aims to provide scientific information desired by the Great Lakes States in development and refinement of water policies. The main components of the pilot study are: surface-water flows and storage, ground-water flows and storage, and water-use assessment.

Ground-water availability in the Great Lakes Basin is being quantified through regional assessments of recharge and storage, estimates of baseflow, and assessments of ground-water data collection. A ground-water-flow model is being developed for the contributing area to Lake Michigan to demonstrate the use of a large regional model to address water-availability questions. In the Great Lakes Basin; however, many water-availability issues are local, and the regional model may not be able to address these issues directly. Modeling techniques for local refinement to address ground-water/surface-water interaction and local water availability issues will be refined and tested in this project. The ground-water model is an important component of the study because it provides a framework for the system, allows for estimation of indicators that include ground-water flux, and links flow processes to field data.