Crossing-scale hydrological impacts of urbanization and climate variability in the Greater Chicago Area

This paper uses past hydrological records in northeastern Illinois to understand the e ects of urban development and climatic variability at di erent spatial scales in the Greater Chicago Area. A step increase in annual precipitation occurred in northeastern Illinois during 1965− 1972 according to climate records. Urbanization has occurred as a gradual process over the entire Greater Chicago Area, both before and after the abrupt annual precipitation increase. The analysis of stream ow trends at each gaging station is supplemented by the comparison of the evolution of stream ow indicators in a group of urban and agricultural watersheds, thanks to an original use of the MannWhitney test. Results suggest that urban expansion in the Greater Chicago Area has led to widespread increases in a wide variety of stream ow metrics, with the exceptions being spring ows and some of the peak ow indicators. These increases are more homogeneous in large (> 200 km) urban watersheds than in small (< 100 km) ones, where local impacts play a bigger role. While the impacts of land-use change are identi ed across a wide range of ow indicators and spatial scales, there are indications that some of these e ects are mitigated or made negligible by other factors. For example, while impervious surfaces are found to increase ooding, stormwater management facilities, an adaptation to increased ooding, mitigate their impacts at a wide range of scales. While impervious surfaces are known to reduce in ltration and base ow, a low ow increase was triggered by water withdrawals from Lake Michigan, as a response to a rising water demand which made on-site groundwater extraction unsustainable. Our analysis thus highlights the impacts of adaptive planning and management of water resources on urban hydrology.