Extreme daily loads: role in annual phosphorus input to a north temperate lake
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Changes in fertilizer use, manure management or precipitation may alter the frequency of episodes of high nutrient runoff and thereby affect annual nutrient loads and total nutrient concentrations of lakes. We developed an empirical, stochastic model for daily P loads and used the model to project annual P loads and summer total P concentrations in Lake Mendota, Wisconsin, USA. Daily P loads (8,250 daily observations) were fit closely by a three-part gamma distribution composed of days with low, intermediate, and high P loads. High P load days happen when heavy rains or snowmelt occur on soil with abundant P, often as a result of manure or inorganic fertilizer application. In Lake Mendota, on average 29 days per year accounted for 74 % of the annual load. Simulations showed that median annual P loads increased linearly with the frequency of high P load days. However, the upper quantiles of the annual P load distribution increased more steeply than the median. Increases in the number of high P load days per year also increased summer concentrations of P in the lake. Thus increases in the frequency of high P load days due to larger precipitation events or increased application of fertilizers and manure may worsen widespread problems caused by P pollution of lakes in this agricultural watershed.
KeywordsDaily load extremes Lake Phosphorus load Water quality
This research was supported by NSF through the Water Sustainability and Climate program (Grant #DEB-1038759) and the North Temperate Lakes LTER site, and by the Wisconsin Department of Natural Resources.
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