Thermally-Driven Exchange Flows

Emergent vegetation provides shade to the water column, so that during solar heating the temperature in the open water rises more quickly than in the canopy. The resulting horizontal gradient in water temperature (and thus density), drives a horizontal exchange flow that carries fluxes between the chemically distinct regions of the canopy and open water. This flow can play an important role in lake-scale chemistry and ecology by promoting exchange between stagnant, littoral regions and open water offshore. We are using field and laboratory experiments to observe the formation of thermally-driven exchange flows between open water and water with vegetation of different morphology and with different levels of light penetration. For example, the figure shows two levels of light penetration. When the surface energy flux, Io, is rapidly absorbed by turbid water (blue I(z) curve), the surface heated layer is thin, producing a thin intrusion into the canopy and a wide outflow. In contrast, when Io is distributed over the full water depth (red I(z)), the intrusion and outflow are symmetric. Our studies will ultimately guide the development of models that predict the frequency, duration and spatial footprint of these flows based on meteorological data. The models will enable researchers and lake managers to evaluate how changes in the littoral zone may impact the lake-scale nutrient budget and ecology.

Funded by the National Science Foundation