Invasive Plants in PNW Ecosystems
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Mapping the Tides for Managing Invasive Spartina alterniflora Control in Willapa Bay. Keven Bennett and Teresa Zena Alcock Olympic Natural Resources Center GIS is producing maps showing predicted tidal exposure durations for resource managers and stakeholders to plan herbicide applications to control Spartina alterniflora in Willapa Bay. The efficacy of the two primary herbicides used drops dramatically when treatment area drying time durations drop below a certain level. We have been using a LIDAR based Mean Lower Lowest Water (MLLW) referenced elevation model and NOAA National Ocean Service (NOS) tide predictions to provide these spatially explicit tidal exposure duration maps. Validation and accuracy assessment is underway. One challenge we overcame is that the MLLW datum itself, which is used by all tide stations for which predictions are published by the NOS, varies from point to point in the bay. Another is the fact that features within the bay exert their own dynamic hydrologic effects on all aspects of tide mechanics, which makes correlation with NOS tide predictions, which are typically open-water data, very difficult. Our approach to these challenges has been to obtain GPS points along the water line to detect the intersection between the water surface and the underlying terrain to detect quantifiable errors. Using this error data, we are in the process of refining the MLLW-referenced elevation model that is the basis of our mapping methodology. When properly validated, these tidal exposure duration maps can be far superior to the use of tide tables alone. They provide a useful and elegant decision support tool to economically and effectively incorporate chemical applications into Integrated Pest Management strategies.
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