from the Department of Water Resources

What a May! 2009 predicted as "Dry" year after late season rains

May 5, 2009 -- Winter is the bulk of the rainy season in CA. January is the biggest single contributor to Northern Sierra precipitation, on average. Here
is the normal precipitation contribution by month in the Northern Sierra (hydrologic water year runs from October 1 to the following:

October 6%
November 13%
December 17%
January 18%
February 16%
March 14%
April 8%
May 4%
June 2%
July less than 1%
August less than 1%
September 1%

May is not normally a big producer, just 2.1" out of a seasonal 50".
But these rare late season large storms are great for supply. They can
help make up for two things we had going on this season; a very weak
January (8th driest for the state, 11th driest for the Northern
Sierra), and lower than normal snowpack. Without January, the snowpack
did not build as much as usual. And spring dry sunny spells make quick
work of what is left. Snowpack represents about a third of our water
supply.

One week rainfall totals (April 29 to May 3rd, 2009):
Feather River Basin (7 station average) 4.1"
Shasta Dam 4.64"
Sacramento 1.17"
Blue Canyon 7.30"
St. Helena 3.92"
Ben Lomond 2.43"

But spring rain is all good, increasing supply at a time reservoirs do
not have to operate for flood control; more can be captured now
(although other supply and environmental requirements must be
followed).

These storms are contributing to runoff, with higher flows. But the
total impact won't be known till the end of May, when monthly data is
compiled. The portion of the storms that fall as snow (and these are
very cold storms for MAY!), will gradually work its way into rivers and
streams, and reservoirs. Snowpack is not measured again this season,
although the automated gauges will provide guidance. The snow level
the last few days has been around 7,000'-8,000' in the Northern Sierra,
and is projected to be about 9,000' with this next strong storm.

Seasonal precipitation (separate tally than snow) and percent of
average to date thru this morning:
Northern Sierra 43.1" 94%
Southern Sierra 36.5" 98%

Statewide snowpack (tracked by automated sensors), shows:
North: 71% Central: 72% South: 55%
Statewide: 67%.

For a nice new graphical map showing the regions of the snowpack
sensors, and the percent of April 1 average go to:
http://cdec.water.ca.gov/cgi-progs/products/swccond.pdf

The course measurements than are conducted by surveyors are more
accurate than the sensors. May 1 readings are being compiled. On
April 30, the date of the media snow survey, Phillips station along
Highway 50 was at only 35% of long-term average. But that is just one
site. The statewide May 1 average will probably come in closer to the
sensor data, in the 70% of average range.

Current forecast for WY runoff this year is about 70% of average. That
will be updated several times before the water year ends September 30,
including later this week incorporating May 1 snow course measurements.

As of now, here are the forecast river region supply indices:
Sacramento Valley Index Dry
San Joaquin Valley Index Dry
(of 5 classification levels: Wet, Above Normal, Below Normal, Dry,
Critical)

* Water Year 2007-08 (WY Runoff: 58% of average) Both Sacramento and San
Joaquin River Water Supply Indices classified as "Critical"

* Water Year 2006-07 (WY Runoff: 53% of average) Sacramento River Region
classified "Dry," with the San Joaquin River Region as, "Critical"

Depending on how dry the ground is, this water year may provide greater
runoff than the last two dry seasons. Perhaps not average, but this
third year could be better than the last two. Trouble is, three below normal
runoff years combine to create bigger deficits in large, snowmelt fed reservoirs. Shasta is at 76% of average for this date, and Oroville at 71%. Some smaller reservoirs have been able to fill, but Shasta and Oroville represent the water supply of the Central Valley and State Water Projects, for which allocations remain near record lows.

Pumping restrictions play a role in San Luis reservoirs level, which is
51% of average. Shasta and Oroville will see boosts from these current
storms.

Remains to be seen if this is a bull-market rally, with more dry years
ahead. The third year of the 87-92 drought looked closer to average
over the Sacramento River, but things turned critically dry again. All
wet and dry stretches have their own characteristics. Right now, we
are tracking for the 10th driest 3-year stretch in CA record (about a
hundred year record). These storms could improve that situation.