Drought Recovery for San Francisco?

As winter draws to a close and after all the hoopla about our "Godzilla" El Nino, it looks like water supply for most of California will be about average - as measured by rain, snow and river runoff. For many areas this means significant recovery. For other areas, especially those where groundwater depletion has been significant, recovery will be limited.

San Francisco's surface water supplies in the Tuolumne watershed will recover substantially but probably not completely. Let's take a closer look. (See notes below. San Francisco did not respond to our request for actual data, so this analysis is based on a combination of actual data where available and estimates where actual data were not available.)  

Hetch Hetchy, Cherry, Eleanor and San Francisco's share of Don Pedro Reservoirs can hold a maximum of about 1,200,000 acre-feet of water supply. The City would obviously like to achieve that maximum by the end of the spring (or summer) snowmelt season.

On average, San Francisco uses about 260,000 acre-feet of water from the Tuolumne River - about 236,000 acre-feet is moved to the Bay Area through a series of tunnels and pipelines and about 24,000 acre-feet is lost to evaporation. (See notes below)

The first two weeks of March were very good for San Francisco, bringing the City's share of the Tuolumne River for 2016 so far to 234,764 acre-feet. That is pretty close to the amount San Francisco annually moves from the Tuolumne watershed to the Bay Area. See Figure 1.

fig_1_daily.jpg

These "Miracle March" flows provided the greatest influx to San Francisco's supplies since the drought began - roughly on August 1, 2011 when the Sierra snowmelt came to a halt and San Francisco's surface supplies were full. As Figure 2 shows, supplies recovered almost completely in 2012, somewhat in 2013 and virtually not at all in 2014 and 2015. The sharp rise at the far right of Figure 2 reflects the increase in 2016 to date.

fig_2_storage.jpg

The good news is there is more to come. This year there is a decent snowpack which, when it melts, will provide San Francisco with additional supply. Starting in February, the California Department of Water Resources predicts how much runoff will occur between April and July on Sierra rivers like the Tuolumne. Figure 3 shows these predictions as of March 15, along with considerable variability due to weather for the rest of the year and uncertainty about how much snow is actually on the ground at present.

fig_3_runoff.jpg

We have included our estimate of how this flow will be divided between San Francisco and the Turlock and Modesto Irrigation Districts. Note that the vast majority of any increased flow (roughly 80%) will belong to San Francisco.

Finally, how much water will San Francisco have in storage in July this year? Snowmelt is unlikely to last until the end of the July as it did in 2011 - a very wet year. But even if the rest of 2016 is dry, San Francisco will have about 982,000 acre-feet in storage - an increase over 2015 of about a year and half of supply. Under median conditions, San Francisco will almost entirely fill these reservoirs with a projected total storage of 1,169,000 acre-feet. If the rest of the year is wetter than normal, San Francisco's storage in these reservoirs will max out at about 1,200,000 and any additional flow will "spill" to the Turlock and Modesto Irrigation Districts or possibly to the lower Tuolumne River. 

fig_4_projected_storage.jpg

Our legal case, now pending in Superior Court in Tuolumne County, depends on system improvements necessary to replace the current level of reliability without Hetch Hetchy Reservoir. We look forward to the opportunity to present evidence that it can be done.

Three important caveats to these charts:

  • The capacity of San Francisco's share of its Don Pedro water bank is always at least 570,000 acre-feet, but sometimes can be as high as 740,000 acre-feet. The charts above use the more conservative figure of 570,000. 
  • Tuolumne River flows, along with reservoir storages are readily available to the public and posted on the internet. San Francisco's share of its Don Pedro water bank, and its diversions to the Bay Area, are not. San Francisco did not respond to our request for data, so Charts 2 and 4 rely on estimates of San Francisco's operations.
  • San Francisco's share of April-July runoff relies on the actual daily pattern of runoff. The values shown are estimates based on simple regression of historic data.