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Attachment A Comments on Evaluation of San Joaquin River Flows at Stockton April 2001 The Jones & Stokes report,Evaluation of San Joaquin River Flows at Stockton, dated April 2001 (Report)was prepared for the City of Stockton to evaluate the minimum likely San Joaquin Rive_ (S,:,'1 flow in the vicinity of the Regional Wastewater Control Facility's (RWCF' lischarge. A US Geolgical Survey(USGS) flow monitoring station was installed in the vicinity of the RWCF discharge in August 1995. However, a critically dry rainfall year has not occurred since installation, thus making the direct measurement of the critical low flow impossible. In fact, the years of record (1996 to present) would be considered above average wet years. The next closest flow monitoring station is upstream at Vernalis and has been in place for over 70 years. However, a large portion of the flow in the SJR at Vernalis is diverted into Old River and other delta channels so it is difficult to determine the flow at Stockton based on the Vernalis flow. Unfortunately, there is not a direct correlation. The Report attempts to determine the relationship through monthly StocktonNernalis flow ratios. The Report predicts the monthly StocktonNemalis flow ratios by analyzing SJR flow data at Stockton, SJR flow data at Vernalis, export pumping data, and delta barrier operations. The Report contains good information regarding barrier operations and export pumping. However,the flow estimations inherently contain a number of assumptions that impact the conclusions. Specific comments regarding the Report are discussed below: 1. The Report uses assumptions that have a significant impact on model results. Some examples are the assumptions made in determining the minimum likely flow during the winter months (December through March). This flow is extremely important because the Report proposes that the minimum likely flow occurs during this time and is used later in a dilution model. • The first assumption is that when the Pumping/Vernalis ratio is less than one, at least 50% of the Vernalis flow passes Stockton. This not a false statement, but a case could also be made that only 40% of the Vernalis flow reaches Stockton, based on Figure 13 of the Report. Figure 13 is a plot of PumpingNernalis Flow vs. StocktonNernalis Flow and contains widely scattered data. • Another assumption is that a PumpingNernalis flow ratio of 5 should be used to determine the minimum likely flow during the winter months. The winter Pumping/Vemalis flow ratios from December 1987 to March 1992, averaged more than 6, with a maximum of 8.4 and a 75`h percentile of 7 (see Table I at the conclusion of Attachment A). Based on the data evaluated (mean of 6 and standard deviation of 1.4), the Pumping/Vernalis ratio is just as likely to be 7 as it would be 5. Higher PumpingNemalis ratios result in lower estimated flows at Stockton. • Finally, a coefficient, C, is estimated to determine the Stockton/Vernalis flow ratio when the Pumping/Vernalis ratio exceeds one. The estimation of the coefficient is very subjective. Figure 13 is used to estimate this coefficient, by drawing a line through the widely scatter-9 data. In addition, th instant during above average wet years may or may not be tl r Xy years