Date: May 01, 2013
May 2013 - Irrigation Technology

The April 1 survey showed that water content in California’s snowpack is only 52 percent of normal, with the spring melt season already under way.

“With most of the wet season behind us, this is more gloomy news for our summer water supply,” said California Department of Water Resources Director Mark Cowin after surveyors took their measurements at the end of March.  After a record dry January and February in much of the state, DWR decreased its water delivery allocation estimate from 40 to 35 percent of requested amounts from the State Water Project (SWP).

The 29 public agencies that purchase SWP water requested just over four million acre-feet of water for this calendar year.  Collectively, the agencies supply more than 25 million Californians and nearly a million acres of irrigated agriculture.

Pumping restrictions imposed during this past winter to protect Delta smelt and salmon are another reason for the low water delivery estimate, according to the DWR press release.  Though November and December of 2012 were unusually wet, restrictions prevented DWR from pumping more than 550,000 acre-feet of water from the Sacramento-San Joaquin Delta to the San Luis Reservoir.  Consequently as of early April, San Luis — a summer supply pool for both the State Water Project and the federal Central Valley Project — was only 63 percent full.

“This is the kind of conflict we are working to resolve through the Bay Delta Conservation Plan,” Cowin said.

The Bay Delta Conservation Plan would reduce harm to fish from altered stream flows caused by the south Delta pumps serving the SWP and Central Valley Project.  The comprehensive plan’s large-scale habitat restoration would also improve Delta conditions for fish and wildlife.

The November and December storms built California’s snowpack water content to 134 percent of normal by January 2, when DWR and cooperating agencies conducted this season’s first manual survey.  Manual surveys and electronic readings have recorded the water content decline since dry weather set in.  Statewide, the season’s second manual survey on January 29 found the snowpack water content at 93 percent of normal for the date.  On February 28, the season’s third manual survey found the snowpack water content at 66 percent of average.  This latest survey — finding snowpack water content at only 52 percent of normal — is particularly significant because this is the time of year the snowpack normally is at its peak before slowly melting with warming weather.  The season’s final survey on or about the first of May will check the rate at which the snowpack is melting.  Snow normally provides about a third of the water for California’s homes and farms as it melts into streams, reservoirs and aquifers.

While there is still an outside chance that April or May storms can add to the state’s water supply, at this writing April remained dry through more than half of the month and prospects were dimming daily that the year would be anything but below normal.

Specifically the late March survey revealed that electronic readings indicate that water content in the northern mountains was at 55 percent of normal for the date, and 55 percent of the April 1 seasonal average.  Electronic readings for the central Sierra showed 57 percent of normal water content for the date and 57 percent of the April 1 average.  The numbers for the southern Sierra were 40 percent of average for the date and 40 percent of the April 1, full-season average.  The manual measurements supplement and check the accuracy of the real-time electronic readings from sensors up and down the state.

Despite the dwindling snowpack, most key storage reservoirs are above or near historic levels for the date thanks to November and December storms.  Lake Oroville in Butte County, the State Water Project’s principal storage reservoir, was at 108 percent of its average level for the date (83 percent of its 3.5 million acre-foot capacity).  Shasta Lake north of Redding, the federal Central Valley Project’s largest reservoir with a capacity of 4.5 million acre-feet, was at 102 percent of its normal storage level for the date (82 percent of capacity).

Reservoir storage will meet much of the state’s water demand this year, but successive dry years would create drought conditions in some areas.

In calendar year 2012, the final SWP allocation was 65 percent of requested deliveries.  The initial delivery estimate for calendar year 2011 was only 25 percent of requested SWP water.  However, as winter took hold, a near record snowpack and heavy rains resulted in deliveries of 80 percent of requests in 2011.  The final allocation was 50 percent in 2010, 40 percent in 2009, 35 percent in 2008, and 60 percent in 2007.  The last 100 percent allocation — difficult to achieve even in wet years because of pumping restrictions to protect Delta fish — was in 2006.

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