Scientific Investigations Report 2007–5008

Scientific Investigations Report 2007–5008

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Model Sensitivity

The model’s wind sheltering coefficient, suspended sediment settling rates, suspended sediment light extinction factor, and bottom sediment temperature were all individually increased and decreased by 20 percent in separate model runs to determine the effect of changing that parameter on the model output. The sensitivity of modeled water temperature, suspended sand and silt concentrations, and suspended clay concentrations to these parameters were examined in the lake and in the lake outflow for 2003. In addition, a qualitative sense of model sensitivity was gained in the model calibration process.

Varying these model parameters by 20 percent did not have large effects on in-lake or outflow average water temperatures (table 4). Of the parameter group, only changes in the wind sheltering coefficient (which modifies wind speed) and the temperature of bottom sediments had any effect at all, and these effects were localized in the water column. Wind did affect the shape of the water temperature profile near the lake surface; higher winds led to more mixing, a deeper mixed surface layer, and less vertical variability in near-surface water temperatures. A 20 percent change in bottom sediment temperature had only a small effect on lake water temperatures, and this effect was greatest for water near the bottom of the lake.

Varying the suspended sand and silt and suspended clay settling rates by 20 percent had a large effect on suspended sediment concentrations in the lake (table 4). Decreasing the suspended sediment settling rates (making sediment settle more slowly), as expected, led to increased average annual concentrations of suspended sediment in the lake and in the outflow. In contrast, increasing the suspended sediment settling rates (sediment settles faster) decreased suspended sediment concentrations both in the lake and the reservoir outflow. These effects were greatest for suspended sand and silt. Smaller effects on suspended sediment concentrations were caused by varying the wind sheltering coefficient and the bottom sediment temperature by 20 percent. Increasing the wind sheltering coefficient increases the wind shear at the lake’s surface and leads to more wind mixing, keeping the sediment suspended longer and leading to a small increase in the simulated suspended sediment concentration. Varying the light extinction factor for suspended sediment had no effect on suspended sediment concentrations in the lake. This light extinction factor is only relevant to the surface photic zone, a relatively small portion of the lake volume. TDS concentrations had no change in any of these sensitivity runs.

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