Shifts in Precipitation, Hydrology, and Land Use Affect Phosphorus Concentrations and Loads in an Agricultural Watershed Steven I. Apfelbaum2, Sophia Heimerl1, and Donald M. Waller1*

Fertilizers and animal manures applied to increase crop yields are often lost via surface erosion, soil leaching, and runoff, increasing nitrogen and phosphorus nutrient loads in surface and sub-surface waters, degrading local water quality and worsening the 'dead zone' in the Gulf of Mexico. We analyzed spatial and temporal variation in stream total phosphorus (P) concentrations and loads in relation to rain events and agricultural land-use practices in a Midwestern watershed. Our data derive from water samples collected between 2001 and 2017 at 25 sites along the Sugar River (Wisconsin), recently listed as impaired. Since 2012, three dairy farms expanded to become concentrated animal feeding operations (CAFOs). We analyzed TP concentrations and loads in relation to day of the year, stream position, discharge volume, and proximity to the CAFOs. Mainstem TP concentrations ranged from 0.025 to 0.748 mg/L (standard: 0.075 mg/L) and increased with stream discharge. Total daily phosphorus loads (concentration x discharge) ranged from 12.6 to 4801 kg (median: 94.5 kg). Phosphorus concentrations peak in early Spring to mid-summer. Maximum TP loads coincided with extreme rainfall events mid-May to mid-July. Although TP concentrations and loads declined until 2012, they have increased since then. Below two CAFOs, TP concentrations increase by 39 and 91% to 0.043 and 0.077 mg/L higher than just upstream. Standards should focus on loads rather than concentrations. Monitoring efforts should include peak events that contribute the most to these loads. Because extreme rainfall events are becoming more frequent and agriculture continues to intensify, efforts to limit soil and TP runoff from manure and fertilizer should focus on Spring and early Summer farming practices particularly in proximity to CAFOs.