ISU Soil Moisture Network
Iowa State University maintains a network of observation stations located primarily at the Research and Demonstration Farms, but also at cooperating locations. Data from this network dates back to 1986! The stations comprising the network has changed over the years with a complete network replacement started during 2012.
The Soil Moisture Network uses a non-heated tipping bucket rain gage and is unable to measure snowfall. If you are interested in higher quality precipitation data, check out the NWS COOP network of human observations.