Supply and Demand: Visualizing Water Usage

One tool to investigate groundwater resources in a region is to compare available supply and demand on a county level. The ISWS calculates water use (demand) primarily using reported data to the  Illinois Water Inventory Program , although  additional analyses were used to calculate agricultural demand . Unless specified, supply numbers are the sum of groundwater and surface water resources. See the bottom of this document for technical details of the methodology for this approach.

This dashboard allows water supply planners and other stakeholders to assess the demand and supply of water resources at a county level. The chart to the left shows total supply and demand for both consumptive and non-consumptive uses. The chart in the lower left shows supply and demand broken down by the source of the water supply (groundwater or surface water). The chart in the lower right shows supply and demand broken down by the type of water use (municipal, industrial, agricultural, and thermoelectric). 

The deep Cambrian-Ordovician Sandstone Aquifer System is an important source of water for much of northern Illinois, and the Rock River Region is no exception. The supply from this groundwater resource is determined by the amount of water that infiltrates vertically into the deep sandstone aquifers. Where leakage values are greater, more water leaks into the sandstone and sustainable yields are higher.

Not all sandstone wells are constructed the same in the region. Some are open to all sandstone layers, while others are open to only deeper layers, such as the Ironton-Galesville. It follows that the deeper aquifers like the Ironton-Galesville receive less water from precipitation events (less leakage). As a result, facilities (communities, industries, etc.) that rely on wells open to the Ironton-Galesville are more likely to have much smaller sustainable yields than facilities with wells open to all layers.

The dashboard below shows the estimated supply and reported demand from the Cambrian-Ordovician Sandstone Aquifer System in northern Illinois. The chart shows, for the selected spatial extent, the five counties with the greatest deficit in available supply compared to demand (referred to as sustainable yield exceedance). If you would like to view supply and demand for a specific county or counties, use the selection tool in the upper left corner of each sustainable yield map.

The methodologies used to determine supply differs by source, as outlined below. All methodologies are "Tier 1", indicating that they are part of a larger statewide effort with minimal regional committee feedback.

• Shallow Groundwater Supply. The primary issue for determining supply for shallow groundwater supply is whether withdrawals will have an adverse impact on surface water. Specifically, supply for shallow aquifers is calculated as 10% of the groundwater flow entering surface waters in a county;  withdrawals exceeding this have been demonstrated to have adverse impacts for some sensitive fish species . Two other factors are not currently considered in this analysis: 1) the possibility for declining water levels, typically from confined aquifers with limited inflow from precipitation events and 2) the potential for contamination.
• Deep Groundwater Supply. The primary issue for determining supply from deep aquifers is whether withdrawals will result in long-term declines in water levels. This generally occurs where aquifers are confined; the overlying material prevents precipitation events from infiltrating to the aquifer. The supply of deep confined aquifers (also referred to as sustainable yield) has been determined as the maximum possible leakage into the deep aquifers of the state.  This assessment was conducted using a model analysis.  
• Surface Water Supply. Supply from rivers is calculated as the maximum demand over the last 7 years from a facility as reported to IWIP, as a proxy for available infrastructure. The ISWS is currently developing an alternative approach to investigate the sustainable supply from rivers. More information on rivers in the Rock River region can be found  here .

The ISWS strongly recommends the Rock River Region planning committee carefully considers the methodology behind these values and provides recommendations for more detailed assessments/improvements.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.