UC Davis today released the first in a new series of annual reports designed to give the non-scientific community an unprecedented compendium of information that documents changing water quality and weather conditions in the Lake Tahoe Basin.

This "Tahoe: State of the Lake Report 2007" presents, in easy-to-read charts, a summary of tens of thousands of scientific observations of lake weather, water conditions, and aquatic life made since the late 1960s by UC Davis scientists. In addition, some data on weather conditions go as far back as 1911. The report also includes the widely watched annual UC Davis Lake Tahoe clarity report (also called the Secchi depth measurement).

Overall, the most striking data in the inaugural report are those showing that the Tahoe climate is warming up. This trend could have profound implications for the natural features that make Tahoe a popular international vacation destination: snowfall in winter and the beautiful cobalt-blue lake in summer.

The report includes these troubling signs of a warming climate:

• Nights are warmer: Night low temperatures have risen more than 4 degrees F. since 1911.

• Cold days are fewer: The number of days with average air temperatures below freezing has dropped from 79 days to 52 days since 1911.

• Less precipitation falls as snow: The percentage of snow in the total precipitation has decreased from 52 percent to 34 percent since 1911.

• Lake waters are warmer: The average July surface water temperature has increased almost five degrees, from 62.9 degrees F. to 67.8 degrees F., since 1999. The lake's surface waters were the warmest on record on July 26, 2006: 78 degrees F.

"The persistent increase in water temperature that we have observed since 1978 is beginning to have noticeable impacts on the entire Lake Tahoe ecosystem," said Geoff Schladow, an expert on lake health and director of the UC Davis Tahoe Environmental Research Center.

"The types of algae we see in the lake are changing, and they are starting to be present earlier in the year. The lake is becoming more hospitable to invasive plants and fish, with warm-water species like bass and carp increasingly common."

The entire report is free and available online at http://terc.ucdavis.edu.

Schladow said he instituted the new State of the Lake Report to give the public a better understanding of the changes occurring in the basin on a year-to-year basis. Data are presented on meteorology, lake physical properties, nutrients, biology and clarity.

In the past, the single annual measurement commonly discussed in Tahoe Basin policy circles has been the clarity report – one number representing how far a white, 10-inch "Secchi disk" is visible below the lake surface. Schladow said he hopes that having dozens of other indicators to complement the clarity report will give the Tahoe community both a better understanding of the many factors that influence lake health, and a better sense of where these key indicators are headed.

As for the clarity measurement: In 2006, clarity declined. The waters of Lake Tahoe were clear to an average depth of 67.7 feet. That is a 4.6-foot loss of clarity compared with 2005, but still an improvement on the all-time low value of 64 feet in 1997.

When measurements began in 1968, the lake was clear to an average depth of 102.4 feet.

Annual variations in Lake Tahoe clarity are controlled largely by the amount of precipitation (the water content of rain and snow) that falls in the lake watershed. "Higher precipitation results in more runoff and affects the amount of soil particles and pollutants that are washed into the lake," said John Reuter, also an expert on lake health, and the associate director of the UC Davis Tahoe Environmental Research Center.

The precipitation total in 2006 was 48.4 inches. That is 84 percent higher than the annual average measured in 2001 through 2005, which was 26.3 inches.

Reuter and Schladow cautioned, as they do every year, that the relationship between precipitation and clarity makes it difficult to use data from any single year — or even a small number of years — to draw conclusions about whether the lake is getting clearer or more murky. "Only with the commitment to long-term monitoring can we truly evaluate environmental changes over time," said Reuter.

UC Davis experts and others believe clarity is falling because of fine particles and nutrients that enter the lake through erosion, runoff and atmospheric deposition. The fine particles scatter light; the nutrients fuel the growth of algae, which absorb light.

UC Davis and many other research institutions and public agencies are working together to restore and preserve the Tahoe Basin ecosystem. Led by the Tahoe Regional Planning Agency (TRPA), the organizations are currently engaged in a comprehensive effort to develop and implement an unprecedented set of environmental management plans for the basin, under the banner of a process called Pathway.

Among many other things, Pathway includes the Lake Tahoe Environmental Improvement Program, the Tahoe Basin's 20-year Regional Plan and Forest Plan Revision, and a water-quality restoration plan termed the total maximum daily load, or TMDL – an effort under the joint direction of the states of California and Nevada.

"During wet years of extraordinary runoff, we expect to see dips in clarity," said Julie Regan, TRPA communications and legislative affairs chief. "The 2006 reading is what we would expect based on runoff conditions.

"Importantly, we remain committed to turning the tide on the long-term trend of declining clarity at Lake Tahoe and we must sustain our investment in environmental improvements if we want to achieve this important goal for future generations," Regan said.

UC Davis scientists, in cooperation with California and Nevada water quality protection agencies, continue to refine a new mathematical model for use in guiding Tahoe Basin restoration efforts. Called the Lake Clarity Model, it can simulate the lake's response to various combinations of pollution types and amounts. This tool is currently being used to evaluate management strategies as part of the TMDL program.

The annual average Secchi measurements for the past several years were:

• 2000: 67.3 feet (20.5 meters)
• 2001: 73.6 feet (22.4 meters)
• 2002: 78 feet (23.8 meters)
• 2003: 71 feet (21.6 meters)
• 2004: 73.6 feet (22.4 meters)
• 2005: 72.4 feet (22.1 meters)
• 2006: 67.7 feet (20.6 meters)