Threshold events responsible for sediment and phosphorus loads in agricultural watersheds of the Great Lakes basin

Nonpoint-source sediment and phosphorus pollution from agricultural lands is a major driver of eutrophication in the Great Lakes basin, yet the storm events responsible for most pollutant transport remain poorly quantified. This study quantified the contribution of high-magnitude runoff events to sediment and total phosphorus (TP) transport, identified seasonal threshold events, and assessed watershed controls on event responses. Event-scale runoff, sediment, and TP data from 10 agricultural watersheds in Southern Ontario were analysed using Lorenz curves, threshold detection, and Bayesian logistic regression. The largest 10% of events contributed 40–50% of annual sediment and TP loads in winter, spring, and fall, increasing to 70% in summer. Threshold behaviour was evident in winter and spring, with sediment and TP thresholds corresponding to return periods of 10 and 8 years in winter and 4 and 3 years in spring, respectively. These findings validate the threshold storm concept using observed watershed data.