Pacific Southwest Research Station

Monitoring past, present, and future water quality using remote sensing (RS)

Our current view of water quality in Lake Tahoe depends heavily on data records from two points within the lake and the points where some of the streams enter the lake. These point data do not provide the temporal and spatial detail needed to understand the changes taking place at different parts of the lake (such as the nearshore zone), and the linkage between the lake observations and the input sources. The intent of this project was to demonstrate the use of remote sensing for measuring water quality parameters at Lake Tahoe. One of the major benefits of this approach would be that a whole-lake view of water quality changes would be possible, even extending into the nearshore where discrete sources of pollutants could be identified. Linked to this was the possibility that through using archived satellite data, long-term trends in other parts of the lake (beyond the two sites currently monitored by UC Davis) would be feasible. The system capitalized on the local infrastructure developed by NASA and UC Davis, the long-term dataset that was collected by UC Davis, and the numerous freely available satellite datasets.

Lead Researchers: S. Geoffrey Schladow, University of California, Davis; Simon J. Hook, Jet Propulsion Laboratory (NASA/JPL)

Figure: Preliminary near-shore clarity map derived from satellite imagery (ASTER) data

Final Report [27MB pdf]
Summary of Findings[pdf]

Project Summary
A system was developed to semi-automatically acquire, store, and process satellite imagery to quantify water clarity and near-surface chlorophyll a concentration measurements over the entire lake as measures of nearshore and offshore water quality at Lake Tahoe. An automated atmospheric correction procedure and processing code were developed to produce high quality maps and time series of water quality at Lake Tahoe. These lakewide maps show significant variations in Secchi depth and chlorophyll a. The largest variations occur closer to shore, and the lowest Secchi depth and highest chlorophyll a are frequently seen to be associated with stream mouths and to occur at times of spring runoff. An unexpected finding of comparing the 8-year, monthly averaged Secchi depth around the lake periphery is that Secchi depth is consistently lower on the east side of the lake (from Stateline Point to Tahoe Keys) than on the west side of the lake. This appears to hold true at all times of year, and is most pronounced closest to shore (at the nearshore sampling stations). Consistently the lowest clarity region is between Glenbrook and Marla Bay. Chlorophyll a on the other hand, did not show as clear a pattern from east to west. The images in the report showed clearly that the distribution of clarity and chlorophyll a in the nearshore is very often controlled by the transport processes within the lake. In addition to the maps and time series, a web-accessible repository was created to store and distribute these and other satellite data products acquired or developed at Lake Tahoe on a near-real-time basis. The methodology developed for this study can be used to study historical or future changes in nearshore and offshore water clarity for any region of concern around Lake Tahoe and help guide management decisions and monitoring efforts related to water quality.

For more information:

NASA JPL Calibration and Validation

NASA JPL Calibration and Validation Lake Tahoe