Study questions erosion model accuracy

WASHINGTON, D.C.—According to a recent study, published in the January/February issue of the Journal of Environmental Quality, the tools scientists and managers use to predict how much sediment runs into the Chesapeake Bay are inaccurate. "Cities and counties are under increasing pressure to meet total maximum daily loads set by state and federal agencies and to understand where sediments come from," said led by Smithsonian ecologist Kathy Boomer, who led the study. "So we tested the tools most widely used now to predict sediment delivery. There was no correlation at all between the model predictions and the measurements."

The new study compared measurement of sediments in more than 100 streams in the Chesapeake watershed with predictions from several of the most up-to-date models. All the models failed completely to identify streams with high sediment levels.

The problem, Boomer said, is that the most widely used models all begin with the same tool, the Universal Sediment Loss Equation (USLE). The USLE estimates erosion from five factors: topography, soil erodibility, annual average rainfall amount and intensity, land cover, and land management practices. Boomer emphasized that the USLE was developed to help farmers limit topsoil loss from individual fields rather than to predict sediment delivery from complex watersheds to streams.

As often applied, the USLE gives an average annual erosion rate for the whole watershed draining into a stream. But not all of the eroded soil makes it into the water, so the estimates do not translate directly into sediment delivery rates. To account for the discrepancy, different models incorporate a wide variety of adjustments. According to Boomer, the adjusted models still do not work, partly because erosion rate is not the best information to start with.

During the study, Boomer and colleagues—Smithsonian Environmental Research Center ecological modeler Donald Weller and ecologist Thomas Jordan—compared erosion rates and sediment yields estimated from regional application of the USLE, the automated Revised-USLE, and five widely used sediment delivery ratio algorithms to measure annual average sediment delivery in 78 catchments of the Chesapeake Bay watershed.

"We did the same comparisons for an independent set of 23 watersheds monitored by the U.S. Geological Society," Boomer said.

Sediment delivery predictions, which were highly correlated with USLE erosion predictions, exceeded observed sediment yields by more than 100 percent. The RUSLE2 erosion estimates also were highly correlated with the USLE predictions, indicating that the method of implementing the USLE model did not greatly change the results.

"Sediment delivery is largely associated with specific rain events and stream bank erosion," she said. "So, USLE-based models that emphasize long-term annual average erosion from uplands provide limited information to land managers."

With a new focus on what is happening in and near the streams themselves, Boomer and her colleagues hope to develop more reliable tools to predict sediment running into Chesapeake Bay—tools that can be used in other lakes and estuaries as well.


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