San Francisco Public Utilities Commission Hydrological Conditions Report For March 2016

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1 San Francisco Public Utilities Commission Hydrological Conditions Report For March 2016 J. Chester, C. Graham, A. Mazurkiewicz, & M. Tsang, April 7, 2016 Snow Surveyor Chris Graham crossing Huckleberry Lake in the East Fork of Cherry Creek drainage on March 30, 2016 as storm clouds close in at the higher peaks of Sachse Monument. 1

2 Current Tuolumne System and Local Bay Area storage conditions are summarized in Table 1. Table 1 Current Storage As of April 1, 2016 Current Storage Maximum Storage Available Capacity Percentage Reservoir Acre- Millions of Millions of Millions of of Maximum Acre-Feet Acre-Feet Feet Gallons Gallons Gallons Storage Tuolumne System Hetch Hetchy 1 254, ,830 86, % Cherry 2 155, , , % Lake Eleanor 3 8,638 21,495 12, % Water Bank 345, , , % Tuolumne Storage 764,232 1,201, , % Local Bay Area Storage Calaveras 4 33,567 10,938 96,824 31,550 63,257 20, % San Antonio 49,934 16,271 50,496 16, % Crystal Springs 55,387 18,048 58,377 19,022 2, % San Andreas 18,486 6,024 18,996 6, % Pilarcitos 2, , % Total Local Storage 160,208 52, ,688 74,192 67,480 21, % Total System 924,440 1,428, , % 1 Maximum Hetch Hetchy Reservoir storage with drum gates de-activated. 2 Maximum Cherry Reservoir storage with flash-boards removed. 3 Maximum Lake Eleanor storage with flash-boards removed. 4 Available capacity does not take into account current DSOD storage restrictions. 1,000 SFPUC SYSTEM STORAGE as of April 1, Up Country Storage in 1,000 Acre-feet Total WY2016 Up Country WY2016 Local WY2016 Total WY Local Storage in 1,000 Acre-feet 400 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Water Year Up-Country Storage WY2016 Total System Storage WY2016 Total System Storage WY2015 Local WY2016 Figure 1: Monthly system storage for water year

Hetch Hetchy System Precipitation Index 5/ Current Month: The March six-station precipitation index was 8.15 inches, or 149.4% of the average index for the month.

Cumulative Precipitation to Date: The accumulated six-station precipitation index for water year 2016 is 34.2 inches, which is 96.1% of the average annual water year total, or 115.

3 Hetch Hetchy System Precipitation Index 5/ Current Month: The March six-station precipitation index was 8.15 inches, or 149.4% of the average index for the month. Figure 2: Monthly distribution of the Hetch Hetchy Six-station precipitation index as percent of the annual average precipitation. Cumulative Precipitation to Date: The accumulated six-station precipitation index for water year 2016 is 34.2 inches, which is 96.1% of the average annual water year total, or 115.8% of average annual to date. Hetch Hetchy received 8.0 inches of precipitation in March, a total of 33.8 inches for water year 2016 to date. The cumulative Hetch Hetchy precipitation is shown in Figure 3 in red. Figure 3: Water year 2016 cumulative precipitation measured at Hetch Hetchy Reservoir through March 31 st, Precipitation at the Hetch Hetchy gauge for wet, dry, median, and WY 2015 are included for comparison purposes. 5/ The precipitation index is computed using six Sierra precipitation stations and is an indicator of the wetness of the basin for the water year to date. The index is computed as the average of the six stations and is expressed in inches and in percent. 3

4 Tuolumne Basin Unimpaired Inflow Unimpaired inflow to SFPUC reservoirs and the Tuolumne River at La Grange as of March 31 st is summarized below in Table 2. Observed Flow Table 2 Unimpaired Inflow Acre-Feet March 2016 October 1, 2015 through March 31, 2016 Median 6 Average 6 Percent of Average Observed Flow Median 6 Average 6 Percent of Average Inflow to Hetch Hetchy Reservoir 80,346 39,024 41, % 202, , , % Inflow to Cherry Reservoir and Lake 77,750 37,980 41, % 184, , , % Eleanor Tuolumne River at La Grange 341, , , % 784, , , % Water Available to the City 193,160 27,949 67, % 285, , , % 6 Hydrologic Record: Hetch Hetchy System Operations Draft and releases from Hetch Hetchy Reservoir during the month of March totaled 81,039 acre-feet to meet SJPL deliveries, instream release requirements, and reservoir management goals. The instream release schedule at Hetch Hetchy Reservoir for the month of March was year type A (normal to wet conditions). This year type is based upon accumulated precipitation from October 1 st, 2015 through February 29 th, The instream release requirement from Hetch Hetchy Reservoir was 124 cfs throughout March. The cumulative precipitation through March, 2016 at Hetch Hetchy Reservoir meets the criteria to maintain a water year type A (normal to wet conditions). The Hetch Hetchy instream release requirement for April is 139 cfs. 39,237 acre-feet of draft was made from Cherry Reservoir during the month of March to meet instream release requirements and to meet reservoir management goals. 27,552 acre-feet of water was transferred by gravity flow from Lake Eleanor to Cherry Reservoir in March. The required minimum instream release from Lake Eleanor and from Cherry Reservoir was 5 cfs during March. Instream flow requirement from each reservoir for the month of April is 5 cfs. Regional System Treatment Plant Production The Harry Tracy Water Treatment Plant average production rate for March was 32 MGD. The Sunol Valley Water Treatment Plant average production for the month was 35 MGD. Local System Water Delivery The average March delivery rate was 156 MGD which is a 3% increase above the February delivery rate of 151 MGD. 4

5 Local Precipitation Well above average precipitation was measured across the local area watersheds. The March rainfall summary is presented in Table 3. Reservoir Table 3 Precipitation Totals at Three Local Area Reservoirs for March 2016 Percentage of Water Year Month Total Average for the to Date 7 (inches) Month (inches) Percentage of Average for the Year-to-Date 7 Pilarcitos % % Lower Crystal Springs % % Calaveras % % 7 WY 2016: Oct through Sep Snowmelt and Water Supply Precipitation during the month of March 2016 in the Bay area triggered runoff to the local reservoir system resulting in an increase in local storage. The storm events also triggered high flows in the Tuolumne River basin. Inflows to the Tuolumne Reservoirs were % of normal for the month. This is due to warm storm events which melted lower elevation snowpack. The combination of rainfall runoff and snowmelt brought unimpaired flows on the Tuolumne River at La Grange above Modesto and Turlock irrigation districts water entitlements for most of the month. As a result Water Available to the City was 284.5% of normal for the month of March, resulting in a water year total of 285,181 acre-feet (Figure 5). The storm events during March did bring a mix of rainfall and snowfall in the high country. Snow surveys during the last week of March and first days of April indicate the snowpack above 7,000 feet ranges from 86% to 115% of normal, while the snowpack below 7,000 feet ranges from 35% to 100%. The overall condition of the snowpack in the Tuolumne Basin is 89% of normal April 1 st conditions. The Tuolumne Basin Water Supply Forecast model was executed using the measured snow course, precipitation, and runoff data. The forecast indicates that the median amount of runoff at La Grange this year is 106% of the longterm median (Figure 4). The median forecast for the April-through-July runoff is about 1,145 TAF, compared to the long-term median measured runoff for the April-through-July period of 1,080 TAF. For natural flow at La Grange, there is an 80 percent chance that the April-to-July natural runoff will be between 1,420 TAF and 970 TAF. The slight reduction in the total inflow volume since the March 1 st forecast is due to the measured snowpack conditions. Much of the precipitation which occurred during March fell as rainfall, which has already been observed as inflow during the month of March. April is the last month of the typical wet season. The National Weather Service predicts the chance of above normal precipitation conditions in the 6 to 14 day forecast as well as above normal precipitation for the month. The NWS is currently forecasting a rainfall event during the April 8-10 period with accumulations in the 0.5 to 0.75 inch range. 5

Figure 4: Water year 2016 conditions for the Tuolumne River at La Grange and for the 80% water supply forecast range (triangles represent the 90% and 10% forecasts, the open diamond represents the

6 Figure 4: Water year 2016 conditions for the Tuolumne River at La Grange and for the 80% water supply forecast range (triangles represent the 90% and 10% forecasts, the open diamond represents the median forecast). Figure 5: Calculated unimpaired flow at La Grange and the allocation of flows between the Districts and the City. 285,181 acre-feet of water has become available to the City during water year

7 cc HHWP Records Gambon, Paul Levin, Ellen Ritchie, Steve Briggs, David Graham, Chris Mazurkiewicz, Adam Sandkulla, Nicole Carlin, Michael Hale, Barbara Meier, Steve Tsang, Michael Chester, John Hannaford, Margaret Moses, Matt Williams, Mike Chiang, Jiayo Kelly, Harlan Patterson, Mike DeGraca, Andrew Jue, Tyrone Perl, Charles Dhakal, Amod Kehoe, Paula Nelson, Chris Dufour, Alexis Lehr, Dan Ramirez, Tim 7

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