Upstream phosphorus reductions and the downstream effects in a midwestern agricultural watershed
Phosphorus (P) enrichment in surface waters is a major concern in the Yahara Watershed of southern Wisconsin, home to a thriving dairy industry, the city of Madison, and a chain of five highly valued lakes that are eutrophic. As in many agricultural watersheds in North America, a legacy of agricultural land use has led to an accumulation of soil P that may take centuries to draw down, even under the best land use practices. Terrestrial P sources within the Yahara are spatially non-uniform, with the largest sources concentrated upstream of the lakes in areas of heavy dairy production. Management efforts to mitigate nutrient runoff have so far focused on widespread adoption of best management practices, yet it is unclear what benefit could be attained by targeting such efforts to only those areas of high P export.
To investigate the timeline of P accumulation and drawdown within the watershed, and to quantify the potential benefit of targeted P reductions, we use a suite of numerical models to gage the effects of alternative P application scenarios on downstream lake water quality. The model suite includes process-based representation of terrestrial ecosystems (Agro-IBIS), hydrologic routing of water and nutrients across the watershed (THMB), and assessment of lake water quality (YWQM). Phosphorus application scenarios include complete cessation of P application, spatially uniform reductions in P application, and spatially targeted reductions in P application. Model results will examine the magnitude and temporal response of P loading and lake water quality to each of the scenarios.