California’s April 1st Snow Report made national news. Governor Brown attended a manual reading at the Phillips snow course at 6,800 feet in the Sierra Nevada that showed no snow readings for the first time in 75 years of readings in early April. Electronic readings show that snow pack was at 5% of normal levels statewide, ranging between a low of 3% of normal in the North Coast and Sacramento regions to a high of 7% and 9%, respectively, in the South Lahontan and North Lahontan regions (see table).
Summary of Statewide Snowpack Readings (April 1, 2015)
|Region||Water Content||% Normal|
|San Joaquin Valley||2.4”||6%|
Snowpack traditionally peaks in April before it starts melting providing runoff for the state’s reservoirs and water supplies for the summer and fall months. The 5% of normal snowpack for April 2015 is only one-fifth of the 25% of normal snowpack last year. In normal years, runoff from snowpack accounts for about 30% of surface water supplies. In its press release, DWR states, “California’s historically wettest winter months have already passed and the drought is now firmly rooted in its fourth consecutive year.”
Will DWR maintain its current 20% State Water Project Allocation announced on March 2nd? The odds are not favorable.
Precipitation in March was minor (see chart “Northern Sierra 8 Station Precipitation Index for Water Year 2014-2015, Year to Date”). The storms in early February (that bumped DWR’s announced SWP Allocation in March) are in the distant past. Along with the amount of water in storage in Lake Oroville at the beginning of the water year, precipitation for the water year through April is a key driver of final SWP allocations.
The dismal precipitation in March does not portend well for the water year through April. JOW’s forecasts have fallen from 43.4 inches based on precipitation through January and 42.3 inches based on precipitation through February, to only 34.7 inches based on precipitation through March (see Chart “Northern Sierra 8 Station Precipitation Index (October through April)). Despite the decline in forecast, it remains above actual precipitation through April of only 28.1 inches last water year.
The updated forecast projects an expected Final 2015 SWP Allocation of 14%, a 30 percent decline below DWR’s current announced 20% allocation. Models, of course, have underlying variability in their predictions (see the Probability Distribution of Forecasted 2015 SWP Final Allocations chart). The probability of a zero allocation is 15%. There is about a 75% chance that the final allocation will be below the current 20% allocation. On the flip side, there is about a 25% chance that the final allocation will be above the current 20% allocation.
There are two factors to keep one’s eye on as we wait for the Department of Water Resources determination of the Final SWP Allocation for 2015.
What will Mother Nature provide in April? Will precipitation follow the beneficence of February or the harshness of March? What will be the balance between the needs of man and the environment?
Written by Rodney T. Smith, Ph.D.