This report prepared by hydrologist Robert Shibatani.
As of March 31, 2020
While many other issues are at the forefront of our attention these days, the annual water supply picture always endures. This past month, we received some precipitation that alleviated some of the growing concern over this year’s water supply, but it was not excessive. Still, it did help. If anything, it helped buffet or delay our annual spring depletions.
As of yesterday, total north CVP reservoir storage remained at 103% of their 15-year average, showing little change from last month. That’s not bad, for if you can say that your total north of CVP storage is 103% of their 15-year average on April 1st, then you do have some flexibility. In fact, we still have almost 8.5 MAF in storage which is approximately 75% of the total reservoir system capacity. Water storage levels in the four largest northern California federal and State reservoirs, Shasta, Oroville, Trinity and New Melones, were 101%, 94%, 113% and 121% of each of the 15-year averages, respectively. Folsom and San Luis reservoirs are each about 75% of their 15-year averages.
Despite the mid-month precipitation, totals remain below average across the State. The northern portion of the State has received about half of what it usually gets and the central and southern Sierra Nevada, while still below their annual averages for this date, are not as depressed. Consistent with lower precipitation levels, accumulated inflows from their source area watersheds have been well below normal with Folsom and Trinity reservoirs faring the worst at 45% and 39% of their 15-year average accumulated inflow for this date. The cumulative Northern Sierra 8-Station Precipitation Index as of April 1st, is 24.2 inches, trending definitely towards a “dry” year. This total would have to double for the remainder of the WY to reach the long-term average of 51.8 inches.
The USBR have remained consistent and judicious in reservoir releases staying very close to each reservoir’s 15-year average. With the exception of Keswick which, for some time now, has been releasing almost 1,000 cfs higher than normal.
As with any WY, the snowpack is the ubiquitous water reservoir of the Sierras and Cascades. The State average snow water equivalent (SWE) is 15.2 inches which is about 53% of normal for this date. No noticeable deviations in distribution between the northern, central and southern Sierras are evident.
As reservoir operators know better than anyone, it’s a very delicate balance to manage many beneficial uses given limited storage capacity, a below normal snowpack, and significant ongoing regulatory requirements. April’s weather, basin ground conditions, and ensuing reservoir operations once again will play a key role in determining our water status for the remainder of the WY.
Prepared by Robert Shibatani: Robert Shibatani, a physical hydrologist with over 35-years combined academic, legal, consulting and water advisory expertise, is an international expert witness on reservoir-operations, climate change hydrology, commercial flood damage litigation, and water supply development. He is Managing Partner for The SHIBATANI GROUP International, a division of The SHIBATANI GROUP Inc. and resides in Sacramento, California. firstname.lastname@example.org