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Hood Canal - Thermal infrared mapping of groundwater, small streams, and ephemeral creeks in Lynch Cove

  
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Background:
The input of nutrients into Lynch Cove and, lower Hood Canal, by groundwater seeps, small streams, and ephemeral creeks may play a significant role in the nutrient loading of this low dissolved oxygen system. In situ measurements of ground water seepage rates are very limited and may not adequately represent the large spatial variability of outflow, and cannot provide detailed information of horizontal mixing and transport patterns. As a result, the groundwater processes important to understanding hypoxia are poorly known.

To determine the spatial variability and relative intensity of these sources, the USGS Washington Water Science Center teamed with the University of Washington Applied Physics Laboratory to obtain thermal infrared images of the Lynch Cove and lower Hood Canal shoreline at or near low tide. The cool water emerging from seeps and in streams, flowing across the exposed, sun warmed beach is readily evident in the images.

The imaging system consisted of an ICI 320x240 pixel microbolometer with a 19° field-of-view that measures long-wave infrared radiation at 8-12 microns. The ICI thermal camera is sensitive to changes in temperature of 0.1 °C, which is adequate to observe the fine surface-temperature structure in seawater, where variations of 0.5° to several degrees Celsius are typical. An un-calibrated, visible band RGB (red, green, blue) camera (Lumenera 1392x1040 pixels with 30° field-of-view) was also mounted in the system to record color images for later reference, and could be used to qualitatively assess the relative abundance of near surface phytoplankton blooms and turbidity. The location and orientation of each image was measured by a GPS, digital compass, and accelerometers, logged once per second on a laptop and combined later for geo-registration.

These surveys yielded 1500 scenes of the shore line and the majority displayed some evidence of cool, or fresh, water flowing into Lynch Cove during each mission. Each image was referenced to a uniform temperature scene from the middle of Lynch Cove, to provide a common reference, and each TIR image was colorized to accentuate the cool signatures. The coldest temperature differences, < -0.5 °C are shown by the bright cyan color, this shifts to black as the temperature differences approach +0.5 °C. Temperature differences > 0.5 °C are mapped to a grey scale where white corresponds to +5.0 °C.

Images from 2008 Survey

Images from 2009 Survey

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