Continued Operation of a 25-raingage Network for Collection, Reduction, and Analysis of Precipitation Data for Lake Michigan Diversion Accounting: Water Year 2017

The volume of water diverted from Lake Michigan into the state of Illinois is monitored to ensure that the diversion does not exceed a long-term average of 3,200 cubic feet per second (cfs) as imposed by a 1967 U.S. Supreme Court Order, last updated in 1980. This diversion has a long history, dating back to the mid-1800s with the completion of the Illinois and Michigan Canals. Over the years, the diversion has been affected by such events as the flow reversal of the Chicago River and completion of theChicago Sanitary and Ship Canal in 1900, and has weathered various legal proceedings that attempted to ensure that the diversion could be monitored and did not exceed certain limits. One of the key components of the monitoring procedure, administered by the U.S. Army Corps of Engineers (USACE), Chicago District, is the accurate representation of the precipitation that falls over portions of Cook County, Illinois.This report describes and presents the precipitation component of the diversion monitoring including the maintenance and operation of the Cook County precipitation network, the data reduction and analysis techniques employed, monthly gage precipitation totals, and a brief data analysis for WY 2017, the 28th year of network operation.