This story first appeared in the SERC quarterly Newsletter Summer 2007
New Tools in the Watershed
According to ecologist Dr. Kathy Boomer, it's time for a change to the status quo. Boomer has been examining the tools scientists and managers use to predict how much sediment runs into the Chesapeake Bay, and by her account, they're way off the mark.
Sediment running off the land reduces light, suffocates underwater organisms and is a significant source of phosphorous, a nutrient that essentially fertilizes the water causing algal blooms and many other problems in the Bay.
"Cities and Counties are under increasing pressure to meet total maximum daily loads set by State and Federal agencies, and to try to attribute various sources of sediment," she says. "So we looked at validating the tools most widely in use now to predict sediment delivery."
Her research comparing actual observations of sediment delivery rates at several locations and predictions from the most up-to-date models suggests the models aren't very accurate.
The problem, she says, is that the most widely used models all begin with the same tool, the Universal Sediment Loss Equation (USLE). Boomer emphasizes that the USLE was developed to help farmers limit topsoil loss on their fields rather than predict sediment delivery to streams. It gives an average annual erosion rate over a broad area. But not all of the eroded soil makes it into the water, so the estimates don't translate directly into sediment delivery rates. To account for the discrepancy, different models incorporate a wide variety of adjustments. According to Boomer, the adjustments still don't result in the right analysis because erosion rate is not the best information to start with. "Sediment delivery is largely associated with specific rain events and stream bank erosion," she says. "So, USLE-based models that begin with estimated long-term annual average erosion provide limited information to land managers."
Her work has led to a new tactic. "We're moving away from focusing solely on watershed-level processes and looking more at what happens near streams during specific events that effect erosion and delivery of sediment to the Bay."
For more information, or to reach Dr. Boomer, please contact SERC science writer Kristen Minogue.