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50% of average runoff expected this summer

April 12th, 2012 by Greg, Information & Restoration Specialist

As of April 1, the winter precipitation in Lee Vining added up to 45% of average, and Mono Basin snowpack measured 47% of average. The forecast for April–July runoff is 50% of average.

An average March for precipitation in the Mono Basin brought to an end the 2011 Runoff Year (April 1, 2011–March 31, 2012). The preliminary runoff total for last year is 142% of average. The table below shows a preliminary tally for each stream and how much water flowed past the aqueduct to Mono Lake from April 1–March 31, 2012, causing a 1.7 foot rise:

CREEK UNIMPAIRED RUNOFF RUNOFF IMPAIRED
BY SCE
FLOW TO
MONO LAKE
Lee Vining 146% 157% 132%
Walker 133% 133% 132%
Parker 124% 124% 123%
Rush 141% 136% 155%
Total 142% 143% 143%

The difference between the left column and the middle column is water stored or released from Southern California Edison’s hydropower reservoirs, and the difference between the middle column and the right column is water diverted or released from Grant Lake Reservoir.

These preliminary numbers show a few interesting things. The amount of water diverted from Lee Vining Creek was roughly equal to the water exported through the Mono Craters Tunnel, which was roughly equal to the water released from storage in Grant Lake Reservoir that was sent down Rush Creek. This amount was about 10,000–11,000 acre-feet.

The last runoff year (April 1, 2011–March 31, 2012) was a good year for Rush Creek for two reasons:

  1. The new streamflows that are recommended in the Synthesis Report were tested until October. These streamflows fix the problem where water is difficult to divert from Lee Vining Creek, causing a disproportionate amount of the diversions to come from Rush Creek or out of Grant Lake Reservoir storage (which would eventually come out of Rush Creek). Unfortunately, the peak flow recommendation was not met because the aqueduct infrastructure is currently unable to reliably deliver high enough flows in wetter than normal years. But despite the low peak flow, the high volume of water going down Rush Creek was a good thing.
  2. The previous winter DWP “borrowed” about 5,000 acre-feet of water from the 2011 Runoff Year in a Water Board-approved effort to delay the spill of Grant Lake Reservoir until spring, when incubating trout hatch and are less sensitive to high flows. This meant there was that much more water left in the Mono Basin this past runoff year. This water flowed down Rush Creek to Mono Lake.

An almost 150% of average year being followed by a 50% of average year is very similar to 2006–2007, when Mono Lake rose 1.8 feet the first year and fell 1.5 feet the second year—resulting in a net rise of 0.3 feet. If you want to keep track this year, we post lake level updates at least monthly. You can also check the Real Time Data Frames Page, which has been updated for 2012 with details on flow requirements for each stream.

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