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RWC Water Supply Rellabffity u Bay Area Water Users Association, Interim Water Shortage Allocation Plan, July 2001. r~ Nicole IV[. Sandkulla, P. E., Bay Area Water Users Association (BAWUA). From this information I generated Exhibit I showing the current level of water supply reliability for RWC. Because the future is unknown, reliability must be expressed in probabilistic terms using the best information available. It is important to note that as part of the 1984 Master Water Sales Contract RWC has a contractual "water supply assurance" of 12,243 a~re-feet per year (AFY). However, this amount relates to a legal definition and not an absolute volume guaranteed. In times of shortage, the SFPUC will provide less than the assurance as illustrated in Exhibit 1. Exhibit 1. SFPUC Water Supply Reliability I1) (~) (3) (4) (6) Hetch Hetchy BAWLIA RWC RWC Future Syatem Demand Demand Demand Allocation Hlutodcal Probability Scenario Cutback Cutback Cutback AFY Probabilit,/ 2010 1 0% 0.0% 0.0% >12,243 NA 2 °5% -6.4% -12.2% 11,584 N^ iNA 3 -10% -12.0% -17.5% 10,891 7.6% 6.3% 4 -15% -18.2% -23.3% 10,124 NA 5.1% 5 -20% -23.6% -28.4% 9,456 2.5% 2.5% 6 >-20% >-23.6% >-28.4% <9,456 0.0% 2.5% (1) The Interim Water Shotrage All~cation Plan addresses system wide cutbacks up to 20%. (2) Values provided by Nicole Sandkulla, Bay Area Water Uaar~ Association. (3) Based on worksheet from Nicole Saedkulia, updated with FY 1999/00 and 2000/01 water use. (4) Based on FY 2000/01 water use totals. (5) Probability of each scenario occ~rlng in a given year based on hydrology from 1921 to 1999. Information taken from Figure 2-5 in SFPUC Water Supflly Master PJan, A4xi12000. The probability of a 5% and 15% cutbadxs not addressed, hence reported as not available (NA). (6) Future probabilities based on interpolation of the 260 and 325 MGD demand scenarios in master plan. Exhibit 1 shows that the frequency and magnitude of water usc cutbacks. From a statistical perspective, given current circumstances in a given year there is a: ~a 89.9% chance of no water shortage or a 5% cutback. Unfortunately, the SFPUC master plan did not identify the probability of a 5% cutback. For San Francisco and most BAWUA agencies, a 5% system cutback will be a minor event met with voluntary conservation. For RWC, in contrast, a 5% systcm cutback will lead to a 12.2% RWC cutback--a significant event requiring some active conservation interventions. Page 2