Decades of Change: Monthly and Seasonal Perspective of Sediment and Phosphorus Loads in Agricultural Watersheds

ABSTRACT Agricultural land use significantly contributes to the deterioration of water quality in the Great Lakes basin. The Pollution from Land Use Activities Reference Group (PLUARG) analysed diverse agricultural watersheds for nonpoint source (NPS) pollution during 1975–1977. The Multi‐Watershed Nutrient Study (MWNS), conducted from 2015 to 2020, revisited some of these watersheds to assess long‐term changes in NPS pollution. For direct comparison with the PLUARG study, a subset of MWNS data from 2017 to 2019 was selected to match the two‐year duration of the PLUARG dataset. A comparative analysis of the two studies reveals notable temporal and spatial variations in runoff, sediment, and total phosphorus (TP) loads. Season I (January–April) exhibited the highest sediment and TP loads. During the PLUARG period, sediment and TP loads were primarily concentrated in February and March. In contrast, MWNS data show an extended loading period spanning February, March, April, and May indicating a shift in seasonal and monthly load distribution patterns. For five of the six watersheds, annual average sediment and TP load magnitudes showed only minor changes between the two study periods. However, North Creek showed a marked increase in both sediment and TP loads, suggesting a shift in load generation behaviour (Sediment loads increased from 0.82 to 1.95 kg/ha/day and TP loads increased from 3.84 × 10 −3 to 6.03 × 10 −3 kg/ha/day). The transportation of sediment and TP loads is highly event‐oriented, with 80%–90% of annual loads occurring during the top 5%–10% of high‐flow events in all watersheds, emphasising the role of hydrological extremes in nutrient mobilisation. These findings highlight critical periods for sediment and phosphorus transport and emphasise the importance of targeted watershed management practices to mitigate NPS pollution effectively.