Characterizing partial upwellings and surface circulation at Lake Tahoe, California-Nevada, USA with thermal infrared images

Thermal infrared data from recently launched satellite instruments provide an opportunity to address key scientific questions and develop new applications, which could not be addressed or developed with data from earlier instruments. The satellite instruments that provide these data include ASTER, Landsat ETM+, and MODIS, and this study demonstrates how data from them can be used to map upwelling and circulation associated with the transport of heat, solutes, and particles in lakes. Upwelling in a density-stratified water body such as a lake or reservoir results from a surface wind stress being balanced by a horizontal pressure gradient, causing denser water to rise at the upwind lake boundary. Upwellings are considered partial when intermediate-depth water reaches the surface and total when bottom water surfaces. Upwellings are an important part of ecosystem functioning, since they transport nutrients from deeper in the lake, where they accumulate, to the surface layer, where they facilitate phytoplankton growth. Thermal infrared images acquired by ASTER, ETM+, and MODIS can be used to observe partial upwelling events in lakes and provide insight into their spatial variability and horizontal distribution, information totally lacking from conventional in situ measurements. At Lake Tahoe, partial upwellings were found to occur every few days throughout the spring and summer, transporting water from 10–30 m below the surface to the surface layer. They commonly display a jet-like appearance, traveling from the upwind to the downwind side of the lake, with current speeds of 12–17 cm/s. Partial upwellings were found to generally decrease lake clarity, although deeper upwelling events can increase clarity. Sinking zones, other convergence areas, and divergence areas were also observed. The temperature variability associated with upwelling, which could be clearly mapped in the thermal infrared satellite images, illustrates the advantage of synoptic thermal infrared satellite measurements over in situ point measurements alone for detecting upwelling events, since, depending on location, an in situ instrument might not capture an upwelling event. The spatial information conveyed by the synoptic satellite measurements can be used to help improve monitoring of the clarity and general water quality of Lake Tahoe.