Abstract
doi: http://dx.doi.org/10.15447/sfews.2015v13iss1art2 The three-dimensional UnTRIM San Francisco Bay–Delta model was applied to simulate tidal hydrodynamics and salinity in the San Francisco Estuary (estuary) using an unstructured grid. We compared model predictions to observations of water level, tidal flow, current speed, and salinity collected at 137 locations throughout the estuary. A quantitative approach based on multiple model assessment metrics was used to evaluate the model's accuracy for each comparison. These comparisons demonstrate that the model accurately predicted water level, tidal flow, and salinity during a 3-year simulation period that spanned a large range of flow and salinity conditions. The model is therefore suitable for detailed investigation of circulation patterns and salinity distributions in the estuary. The model was used to investigate the location, and spatial and temporal extent of the low-salinity zone (LSZ), defined by salinity between 0.5 and 6 psu. We calculated X2, the distance up the axis of the estuary to the daily-averaged 2-psu near-bed salinity, and the spatial extent of the LSZ for each day during the 3-year simulation. The location, area, volume, and average depth of the low-salinity zone varied with X2; however this variation was not monotonic and was largely controlled by the geometry of the estuary. We used predicted daily X2 values and the corresponding daily Delta outflow for each day during the 3-year simulation to develop a new equation to relate X2 to Delta outflow. This equation provides a conceptual improvement over previous equations by allowing the time constant for daily changes in X2 to vary with flow conditions. This improvement resulted in a smaller average error in X2 prediction than previous equations. These analyses demonstrate that a well-calibrated three-dimensional (3-D) hydrodynamic model is a valuable tool for investigating the salinity distributions in the estuary, and their influence on the distribution and abundance of physical habitat.
Highlights
The ecology of the San Francisco Estuary is strongly influenced by tidal hydrodynamics and the distribution of salinity on hourly to interannual time-scales
The comparison of tidally averaged water level for the full analysis period shows that the model accurately predicts spring– neap effects as well as non-tidal forcing such as freshwater flow and storm surge (Figure 5B)
Given that the salinity predictions presented in this paper indicate that the UnTRIM Bay–Delta model predicts observed synoptic and continuous observed salinity accurately and, in particular, more accurately than the TRIM3D model documented by Gross et al (2009), we assert that the data set of X2 calculated using UnTRIM is superior to previous X2 data sets, even including those derived by interpolation of data from continuous surface salinity monitoring stations
Summary
The ecology of the San Francisco Estuary (estuary) is strongly influenced by tidal hydrodynamics and the distribution of salinity on hourly to interannual time-scales. Many hypotheses have been proposed that relate freshwater flow to abundance of organisms in the estuary (Kimmerer 2002) One such hypothesis is that abundance of estuarine species varies with the area or volume of low-salinity habitat, which is related to X2 (Feyrer et al 2007; Nobriga et al 2008; Kimmerer et al 2009, 2013). The exploration of this hypothesis required prediction of the salinity field for a range of Delta outflows in order to calculate X2 and the area, volume, and depth of water within specific salinity ranges for multiple species (Kimmerer et al 2009, 2013)
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