Number range CHAPTER© 2004 by Leo Poppoff

January 29, 2004

Basin Watch

by

Leo Poppoff

WHAT DO LAKE WATER CLARITY MEASUREMENTS TELL US?

For almost four decades, researchers have been checking on Lake Tahoe's
water clarity. That record of water clarity measurements is the longest
and, arguably, the most valuable data set we have. Yet, what have we
learned from it?

Tahoe Research Group scientists have ventured out on the lake, rain or
shine, every couple of weeks. They lower a white disk, about the size of
a dinner plate, into the water and watch it disappear. Then, they raise
it and note the depth at which it reappears. The average of those two
observations is called the Secchi depth. Professor Secchi, invented the
technique for his studies on the Adriatic Sea.

As we all know, the long-term trend of Secchi depths is that Tahoe's
water clarity has diminished. For that reason, we've invested in
research to figure out why water clarity is decreasing. And, we're
investing heavily in erosion control projects to control the pollution
that washes into the lake from the watershed. How will we know if what
we're doing, and planning to do, is helping the lake? No matter what
ideas we might have to check on progress, the ultimate test is how the
lake responds. So, the Secchi depth data is carefully watched for signs
of improvement.

If we examine the record carefully, we see that clarity doesn't decrease
each year. In fact, there were several periods when clarity had actually
increased for a while. When that occurs, we cheer and pat ourselves on
the back. But then, clarity continues its disappointing long-term loss.
Lately, we've had another period of cheering because it seems that water
clarity has improved.

Folks who deal with natural systems know that many factors influence
short-term changes. And many of those changes have nothing to do with
what you want to study. For example, climatologists warn against using
especially hot summers or unusually cold winters to make conclusions
about global warming. Climatologists use 20 or 30-year records, or even
century-long records, to decide if temperatures are changing.

Can we conclude that recent Secchi depth data show that our erosion
control efforts are successful? Individual projects might well be
effective. The question is: when taken cumulatively, are they having a
significant effect on the lake? UC-Davis scientists, Alan Jassby, John
Reuter, and Charles Goldman have been looking into this question. Their
results were published recently.

The researchers point out that several factors influence year-to-year
and season-to-season changes in lake water clarity. For example, the
amount of precipitation, its form, and when it occurs, can affect runoff
from the watershed. And this affects summer clarity measurements. Winter
storms and cooling events influence upwelling that brings up clearer
water from the depths of the lake. And that affects water clarity during
the winter.

Because summer Secchi measurements are the shallowest, and most affected
by watershed runoff, Jassby, Reuter and Goldman used those data for
their study. Their aim was to see what was driving the observed Secchi
depths.

They noted that year-to-year changes in water clarity were quite large,
averaging three meters and sometimes (during the period, 1981 to 1982)
as large as ten meters. On the other hand, the long-term trend is a loss
of three-tenths of a meter per year. According to the scientists, "the
interannual variability in summer clarity is enormous and driven almost
entirely by precipitation differences." They also note that multiyear
periods of drought or rainfall are common in our climate. Therefore,
they conclude that annual means are not appropriate for judging
compliance with water quality standards. Instead, long term trends
should be used.

Precipitation causes erosion of the watershed, and washes various sizes
of soil particles into the lake. The researchers explain that larger
particles could settle out of the lake in a matter of years. But, small
particles could remain suspended for decades. So, they reason,
year-to-year changes are probably caused by the larger particles,
whereas tiny particles control the long-term trend. This means that if
erosion control projects could stop the flow of sediment to the lake, it
would even out the year-to-year fluctuations of Secchi depths. Yet, the
long-term trend of diminishing clarity would likely continue, according
to Jassby, et al, unless "erosion control projects ensure that even the
smallest particles do not penetrate into the lake."

Jassby, Reuter and Goldman conclude that it's mineral particles that
have the most effect on the lake during summer. Algae are an important
factor in determining winter water clarity. With the model that the
researchers developed for this study, they demonstrated that "the
increasing Secchi depths seen during the three years from 1999 through
2001 were simply climate-driven and do not represent a recovery of the
lake."

The information gleaned from this research reinforces the value of the
current TMDL program to understand watershed and lake processes -- as
well as the need to put quantitative limits on pollutant loading and set
targets for load reduction efforts.

Send any comments to basinwatch@sbcglobal.net