Assessment of Solar Radiation, Lake Evaporation, Higher Rainfall,
Water Additions, and Reduced Pumping on Lake Levels at Lake Merced

David R. Dawdy, Hydrologist
January 8, 2006

In response to the questions raised by John Plummer in his memo of January 5, the impact of the various points he raises can be assessed roughly.  The first is the amount of rain, which would raise the lake level by some 8 inches. However, part of that would travel downstream in the aquifer. The 8 inches of greater rainfall would also generate more runoff from the contributing area.  Perhaps all the extra rainfall would flow into the lake from the impervious contributing area, which is some 18 acres (Geo-Resource) as opposed to  250-300 acres of the lake.  This 12 acre-feet would raise the lake another 0.5-1 inch. Other inflow from the pervious areas may contribute a similar amount.

The average solar radiation in KWH per day for each month sums to about 48, with   the average lake evaporation of 26 inches at Lake Merced according to the Task 3 Technical Memorandum of May 2004.  Therefore, the difference in the solar radiation for the two years can be converted almost directly into inches of lake evaporation by multiplying by 0.55.  The major difference is in September 2004 of 5.3 to September 2005 of 3.6, a difference of 1.7 KWH. In addition, May, the second largest difference, is 5.9 for 2004 and 5 for 2005, adding another 0.9 KWH for a total of 1.4 inches. In fact, if we merely add the monthly values for each year we obtain 49.7 versus 47.1, for a total estimate of about 1.4 inches less lake evaporation in the 2005 water year.  All of that major deviation occurred  in March and September, as stated.  The lesser evaporation in August, September, and October may partly account for the decrease in the rate of decline of  the  lake level.

The increase in the lake level by addition of water in 2003 should  have a negligible effect at this time. With a time response of 6 months, a reasonably conservative value for a sand and gravel aquifer, the remaining effect of that addition would be one third raised to the fifth power, which is well under one percent of the increase remaining.
  
Similarly, the Daly City additions should have a small effect because the acre feet added to the lake would have to be considerable to raise the lake level when the area of the lakes is some 250-300 acres.
 
This gross order-of-magnitude analysis accounts for about a foot of the added level.  Another unaccounted for increase would be from the reduction in pumping by the golf courses.  The water pumped was used for irrigation, so that the added water supplied to the golf courses does not increase infiltration.  However, there would be a rebound of the cone of depression.  That cone is a pressure level, and ceasing pumping has an immediate effect in that the pressure decrease as a result of pumping rebounds quickly.  This changes the pressure difference between the upper and lower aquifers which, in turn, decreases the percolation downward to the lower aquifer.  Even more important is the fact that the confining layer thins toward San Bruno Mountain.  Thus, there may be a more direct connection between the upper and lower aquifers in the vicinity of the golf clubs to the east.  This could have a direct connection to a more rapid response of the upper aquifer to the reduction in pumping.  Also, this effect may also contribute to the slower decline in the lake level this last year.  This circumstance of a reduction in pumping and a subsequent unexplained higher lake level should present interesting data for calibration of the groundwater model.      

David Dawdy, Hydrogist
January 8, 2006