P Inputs and Exports: What’s so Special About 1992?
The watershed team is focused on analyzing P loading to watersheds from human activities, which are a major determinant of how much P is delivered to the Lake by the rivers draining Lake Erie watersheds. This team’s research adapts the nutrient mass balance approach that has been widely used to estimate loadings of nitrogen to coastal waters to apply to phosphorus.
The researchers quantified anthropogenic input of P to each watershed of the Lake Erie Basin from fertilizer, atmospheric deposition, and net import of P in food and feed for six census years from 1978 to 2002. They found that the average annual river export of TP from the selected five watersheds of the Lake Erie Basin (Grand (OH), Raisin, Maumee, Sandusky and Cuyahoga) during 1978-2002 is well correlated with net P inputs (R2=0.81).
In addition, they found the relationships between watershed P input and river P exports are different for before vs. after 1992, which is when the TP load changed dramatically toward DRP. For example, as shown in Figure 1, average fractional delivery of P inputs to rivers for the period from 1992-2002 (16%, Red dots) is about three times higher than that for the period from 1978-1987 (6%, Blue dots). This is likely due to 1) higher contribution of P fertilizer use to total watershed P use after 1992 than that before 1992, 2) higher water discharge during 1992-2002 (area-weighted average: 381 mm) than the period before 1992 (area-weighted average: 342 mm), and 3)shift to no-till agricultural practice, which results in less mixing of P into soil, hence higher flushing in wet years.
Figure 1.Â A change in the P input: export relationships between the period before 1992 (blue dots) and after 1992 (red dots) (See the following two tables for more detailed values).