SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

.� .y Attachment A - 2- Comments on Evaluation of San Joaquin River Flows at Stockton April 2001 Using the assumptions detailed above, Equation [1] is used in the Report to determine the StocktonNemalis flow ratio during the winter months (i.e. no barriers in place): Equation f 11: StocktonNernalis ='0.5-C(Pumping/Vernalis) In determining the minimum likely flow, the Report proposes that the minimum StocktonNemalis flow ratio is 0.15 during the winter months. Therefore, the following assumptions are made: • 50% of Vernalis flow passes Stockton when Pumping/Vernalis ratio is less than 1 • C = 0.07 (This value was back calculated, the Report actually suggests using 0.05 in Figure 13.) • Pumping/Vemalis ratio = 5 StocktonNernalis = 0.5—(0.07 x 5) = 0.15 The following assumptions could easily be supported, and are only minor changes: • 45°% of Vernalis flow passes Stockton when Pumping/Vemalis ratio is less than one • C =0.07 • PumpingNemalis ratio =6 Stockton/Vernalis = 0.45 —(0.07 x 6) = 0.03 Using the above assumptions with the minor changes, the StocktonNemalis flow ratio would be 0.03, resulting in a minimum monthly flow of 30 cubic feet per second(cfs), compared to 150 cfs estimated in the Report. Increasing the PumpingNernalis ratio to 7 results in a negative ratio (reverse flows). This is an example how results can be drastically affected with small changes in model assumptions, leading to potentially significant variability with overall model results. 2. The average annual SJR flows at Vernalis averaged more than 7,000 cfs during the study period, while the minimum 10-year return flow at Vemalis is approximately 1,000 cfs. It is questionable to assume that StocktonNemalis flow ratios determined using this data could accurately estimate SJR flows at Stockton during a critically dry year. 3. The Report's calculated StocktonNemalis flow ratio exceeded the actual StocktonNemalis flow ratio in 20 percent of the months from November 1995 through September 1999. This is during above average wet years and during the same time period in which data were used to calculate the ratios. 4. The Report states on page 14, "The Pumping/Vernalis flow ratio has been less than 7 during the 5 years of available UVM measurements, and the Vernalis flow has remained greater than 1,500 cfs, so the estimates of the Stockton flow at extremely low Vernalis flow and high Pumping/Vernalis ratios remain uncertain." This critical situation that would correspond to low flows at Stockton, an( rt 'c ss not , ' taken into account a the determination of the Stockton/Ve, _. :low ratio.