Adequate characterization of runoff water quality from agricultural fields under a changing climate will require years of monitoring. This study compared runoff volumes and phosphorus loads from two fields in central Iowa. Both fields were in corn-soybean rotation but with only one received swine manure, which was applied following soybean harvest to meet the following corn crop’s N requirement. Runoff flumes (H-type) were installed along grassed waterways prior to the 2000 growing season; runoff samples were collected on a flow-interval basis using automated samplers, retrieved within 24 h, and analyzed for total P. Monitoring continued through 2010, providing eleven years of data. Results are only for rainfall-runoff events because sites were remote and consistent snowmelt monitoring was not feasible. Although average annual runoff was greater from the non-manured site (54 mm yr^-1) than the manured site (37 mm yr^-1), P loads were greater from the manured site 1.4 vs 1.1 kg P ha^-1yr^-1. The difference is small, especially given soils in the non-manured field approached P-deficiency by 2010 after years of maintenance P-fertilizer rates. Interestingly, 40-56% of the runoff volume and 54-62% of the total P load during 2000-2010 resulted from runoff generated by 30-60 mm of precipitation. Only 20-30% of runoff and 12% of the total P load (both fields) were generated from ten events that exceeded 60 mm rainfall. This result suggests conservation systems designed to control runoff from 60 mm rainfall events could provide soil and water conservation against a trend of increasing rainfall in the Midwest.