Spatial and temporal distributions of watershed and lake loading sources

The following provides a quick snapshot of key findings.  A more in-depth discussion and references to the scientific literature can be found here.

We built phosphorus mass balances for 18 Lake Erie watersheds to explore how the sources and sinks of phosphorus vary across the region.  As can be seen below, TP inputs to the Lake St. Clair, Clinton, Detroit, Huron, Cuyahoga, and Ashtabula watersheds (#2-4, 13, 14) are dominated by human sources; inputs to the St. Clair, Ottawa-Stony, Raisin, Maumee, Cedar-Portage, Sandusky, Huron-Vermilion, and Cedar Creek watersheds (#1, 6-11, 24) are dominated by fertilizer; and inputs to the Grand (Ont) and Thames watersheds (#19, 20) are dominated by manure.

Figure 13

 

As a result of differences among watershed phosphorus balances and variations in precipitation patterns, loads to Lake Erie are also not distributed equally across the basin.  As illustrated below, the Western Basin received approximately 60% of the 2003-2011 average total phosphorus (TP) loads; whereas the Central and Eastern Basins received about 30% and 10%, respectively.  The Western Basin received 68% of the 2005, 2007-2011 average dissolved reactive phosphorus (DRP) loads; whereas the Central and Eastern Basins received 24% and 8%, respectively. The loads from individual tributaries within each basin also vary considerably for both TP and DRP, with largest contributions coming from the Maumee, Detroit, Sandusky, and Cuyahoga rivers.  Thus, it is clear that loads to the WB are a very important determinant of the WB and CB eutrophication response.

TP riversDRP rivers

 

Just as tributary loads are not evenly distributed among major watersheds, non-point sources within those watersheds vary considerably. For example, 36% of DRP and 41% of TP come from ~25% of the agriculturally dominated Maumee River sub-watersheds.  Similar disproportionate contributions of DRP and TP were found for the Sandusky River watershed (33% and 38%, respectively) and Cuyahoga watershed (44% and 39%, respectively).

These collective results suggest that spatial targeting of management actions would be an effective P reduction strategy.  However, it is important to note that these loads represent flux to the stream channels at the exit of each subwatershed, not P delivered to the lake.  Thus, the maps of important contributing sources of TP and DRP to the lake could be different if flux to the lake were considered.

SWAT output