Lake Merced Update, January 2006
I just received the year-end report from the Lake Merced Pumping Station: the water level in Lake Merced is now just a tad less than 23 feet. This is the highest year-end lake level since 1986, the year before the drought that brought us all, or at least quite a few of us, together. That's about 4 feet higher than the year-end level that we saw in 2001, just four years ago.
There are a number of reasons that might explain this increase:
* We've had a lot of rain. Since March of last year its been raining at a rate of about 29 inches of annual rainfall, about 8 inches above the long-term average. However, for the 4-year period rainfall has been just about 22.6 inches per year, not much above average. This doesn't seem to be enough to explain the increase.
* We had a relatively foggy and cool summer. Perhaps as a result the evaporation rate wasn't so great, and that may be part of the reason that the decline in lake level this summer was about 2 inches a month, just half the rate observed for several prior years.
* The PUC added about 2 feet of water to the lake, but that was back in 2003, more than two years ago. One finding of the CH2M-Hill study with which most of us agree, the effect of prior additions of water to the lake was short-lived, and disappeared after a couple of years. Something now seems to be holding up the lake after water has been added.
* Since then there have been small additions of water, from Hetch-Hetchy and from a test project diverting water from the Vista Grande Canal into Lake Merced. However, these are not believed to have added to more than a few inches to the lake level.
* Finally, Daly City started providing recycled water to the golf courses in August 2004, and the conjunctive use program, by which Daly City reduces pumping from the aquifer and receives increments of Hetch-Hetchy water, has been underway for a couple of years. This may have had some impact on the water table.
In fact, a number of these observations might be the result of a higher water table in the Westside Basin aquifer. However, we were told, by the PUC's consulting hydrologist Joe Scalmanini, that "it will take decades" for aquifer replenishment to have an impact on lake level. Since the three lakes, North, South and Impound, are now directly connected, we don't have a lake-level gradient to see if the north-to-south flow of water through the lake is having an effect that might also be eliminated with a higher water table. In short, there's a lot we don't know.
However, one thing is clear, we need better science:
* More data is needed, and that which exists should be publicly available. We don't, for example, know the condition of the aquifer as reflected by data taken from monitoring wells; that data has simply not been provided.
* The confidential "mediated negotiations" initiated by the CalTrout action should be terminated. They have produced nothing at all for more than three years, and instead have provided a means for the PUC and the City Attorney's office to withhold needed information under the guise of "confidentiality." What is it that they need to hide?
* Better analytic skills are needed to evaluate this data. The poor quality of the recently released analysis of the Vista Grande stormwater diversion study underscores this need. The old saw of "liars, damn liars, and statisticians" doesn't hold up here. We need competent statistical analysis.
* The model of the Westside Basin aquifer being developed by Gus Yates and John Fio should be calibrated and used for assessment of issues such as those raised here. Can we expect the observed increases in lake level to be maintained, or even to continue? Is building a barrier in the shallow aquifer between the lake and the wells to the south still the best use of water to support lake level?
Again, there are many questions for which we do not have answers. Frankly, I don't understand what is getting in the way of these developments.
Best wishes for the New Year!
January 6, 2006