The Critical Trends Assessment Program (CTAP) was a process established by the Illinois Department of Energy and Natural Resources in the early 1990s to monitor changes in Illinois ecological conditions. The initial technical report was published in 1994 and included a volume on Water Resources. Find more information about CTAP and additional documents here.
The work continued with comprehensive evaluations of major Illinois watersheds and ecological regions, called Area Assessments, published between 1996 and 2002. Information on surface water, ground water, water use and water availability are covered in the "Water Resources" volumes of the Area Assessments. Typically, the Area Assessments covered geology, water resources, living resources, socio-economic profile, environmental quality, archaeological resources, early accounts of ecology (from historical literature).
In addition to the Area Assessments, summary resource inventories written for a lay audience were produced for most watersheds/regions.