The Hydrologic Studies Unit of MDEQ has developed a method to compute curve numbers using Geographic Information Systems (GIS) land use and soils information.  In brief, the technique assigns a unique number to each soil type and land use.  These numbers are summed and associated with the appropriate runoff curve number.  To use the method, download the instructions and the GIS-CN tables. The hydrogroup files would be needed if your GIS soils data does not already include the hydrogroup classes.  For more information about the runoff curve number method, see Computing Flood Discharges for Small Ungaged Watersheds.

• GIS-CN Instructions (200 KB PDF) 
• GIS-CN tables (3 KB Zip file)
• Hydrogroup files for all Michigan counties (100 KB Zip file) Revised 10/25/2007 to reflect statewide Ssurgo coverage available from Soil Data Mart or State of Michigan's CGI data library.