Reservoir Report May 1st

by Maven's Notebook

For a moment it was looking as though we had skipped over Spring and went directly to Summer although temperatures have since returned to those more in line with the current season.  A season that, hydrologically, at least in areas reliant on snow accumulation, is known for one significant event; the spring snowmelt.

As April closes out, we can begin to see the first signs of the spring freshet.  And with California’s many reservoirs either at, or nearing capacity, the next several weeks will illustrate both the strengths and shortcomings of our current water resources system.

At present, Shasta Reservoir is storing some 4.42 million acre-feet (MAF), New Melones 1.51 MAF, Folsom 0.792 MAF, and Millerton 0.162 MAF.  Currently, total CVP reservoir storage is over 8.7 million acre-feet (MAF) or, over 73% of total system capacity.  Surprisingly, with all of rain we’ve received, total CVP storage is only slightly above average for this date (e.g., 104%), which should give an indication of the magnitude of this most recent drought in terms of severely depleting water storage reserves.

Monitoring reservoir releases often represents a quick way of gathering information on how a coordinated system is both currently operating and what it’s future intentions might be.   For example, operators can keep Millerton Lake low (e.g., currently at 162,000 acre-feet storage or less than one-third full), knowing the reservoir, owing to its small size, can fill rapidly.  Considering this year’s robust Sierra Nevada snowpack, particularly, in its southern region, the potential water remaining in SWE makes refill of Millerton a safe bet.  But timing is vital.  Releases from Friant Dam into the San Joaquin River have been high throughout the early Spring, consistently between 8,000-9,000 cfs.  Not surprisingly, for months, releases from Friant have represented the primary San Joaquin River watershed contribution to delta inflow.

On the SWP side, Oroville Reservoir was releasing 20,000 cfs on Saturday, a modest rise from the 15,000 cfs it was releasing earlier last week.  The reservoir recovered storage rapidly this Spring and is still gaining storage.  As of Saturday, storage was over 3.22 MAF, that’s 91% full!

From a precipitation perspective, apart from some north State rains occurring on April 7th and 8th, no additional precipitation occurred across CVP or SWP watersheds in April.  What we are seeing in reservoir operations and instream flows, therefore, are all melt and baseflow triggered hydrologic responses.

As California moves irrevocably towards its Spring peak flow, all eyes are on the CVOC and its management of the CVP/SWP.  Both anecdotal and media-driven warnings are already being voiced along with real-time alerts; Yosemite closed earlier in anticipation of the Spring floods.

Early hydrologic analyses are suggesting that the ablation of the Sierra snowpack and depleting SWE is occurring faster than inflows into the downstream terminal reservoir.  For example, Folsom Reservoir inflow from its upper basin snowmelt should be more closely aligned with SWE declines (even taking into account terrestrial and system runoff lag).  Currently, It’s not.  If continually corroborated in the weeks ahead, it will demonstrate that inbasin storage (e.g., surface and subsurface storage reserves) were taking up more meltwater away from runoff than original assumptions had supposed.  Again, this is an all-too-common oversight of many basin res-ops.

From a water supply perspective, with total CVP storage over 8.7 MAF, adding in Oroville’s 3.2 MAF, provide what will easily be in excess of 12 MAF of carryover for the remainder of the WY.  A big difference from last year …

As noted earlier, apart from some north State rains occurring on April 7th and 8th, for all intents and purposes, what you see in the reservoirs and lakes and what additional SWE can make it into their downstream reservoirs over the next several weeks is all the water you’re going to have for the remainder of the WY.

Manage it wisely …