Prepared by Robert Shibatani
The New Year’s Atmospheric River storms of 2023 have abated and catchments across the State are draining as exemplified by continuing baseflows through their hydrograph recession limbs. River flows are still elevated, but releases have been incrementally curtailed and stage levels continue to drop.
Despite early positive signs, however, the reality of what this storm (or series of storms) brought in terms of drought relief is made eminently clear by reviewing various data sources. The State precipitation indices for the Northern Sierra, San Joaquin, and Tulare Basin regions, for example, provide a comparative inspection of how this water-year (WY) stacks up against previous WYs in terms of accumulated precipitation totals.
Without another similar prolonged wetting front encroaching California this winter/spring, it is doubtful whether the State will even reach average-year precipitation totals for the Northern Sierra. In fact, based on the State’s Northern Sierra Precipitation 8-Station Index, an additional 15.7 inches of precipitation would be required just to meet the 1991-2020 average. Not impossible, but it likely cannot happen without another AR. See Figure 1.
Figure 1.
Northern Sierra Precipitation:8-Station Index, January 30, 2023
Source: California Department of Water Resources
A similar situation occurs south of the delta when reviewing the San Joaquin Precipitation: 5-Station Index which currently stands as 33.7 inches, some 6.2 inches below the long-term (1991-2020 average) as well as the Tulare Basin Precipitation: 6-Station Index which currently shows 24.4 inches, only 3.9 inches below its long-term average.
The 2022-23 WY has been unique in that snow accumulations are much more pronounced in the south and central Sierra Nevada this year than in the northern regions, as is typical. Relative to existing conditions, SWE across the State is significantly above average.
While some have referenced the rather robust cumulative snowpack, closer inspection reveals that the numbers really are not that impressive. As most snow hydrologists will caution; early-season snowpacks typically bear little resemblance to their final snowmelt contribution during the annual freshet.
Reservoir storage across the State has been stymied by capacity limitations. This is a real shame, particularly in following protracted water-short periods, since much of the storm- and floodwater surpluses resulting from the recent New Year ARs have already been evacuated to the Pacific Ocean. That valuable water supply has, for all intents and purposes, been lost.
So, where does reservoir storage currently stand?
CVP reservoir storage is about 5.6 million acre-feet (MAF) as of late last week or, 47% of total system capacity. Reservoirs such as Shasta are filling nicely (e.g., 2.57 MAF as of yesterday). Other reservoirs are, for a number of reasons, languishing in their refill objectives. Trinity and New Melones reservoirs, each over 2.4 MAF in storage capacity, both remain significantly below 1 MAF.
What is the immediate hydroclimatic outlook?
There are a few days of periodic showers and light rains forecast for next week, but nothing notable in the way of major frontal systems on the horizon. Unless California receives another Atmospheric River-type incursion, I doubt that the residual hydrology will be sufficient to avert another “water-constrained” year. In systems with limited capacity, reservoirs have a significantly lower likelihood of filling, particularly when left to the narrowing window of the spring freshet. Under such conditions, it is more by blind luck than anything else that reservoirs fill. Ideally, reservoirs should be filling the ENTIRE rainy season, especially after protracted droughts; when reservoirs cannot due to size limitations, it severely hampers water managers’ ability to establish, let alone retain water security and sustainability.
We are closely monitoring daily CVOC actions to observe how integrated CVP/SWP operations are responding to hydroclimatological threats, both in the long- and short-terms. The bulk of the wet season remains; February and March alone can provide more than ample opportunity to meet the reservoir refill targets for the 2022-23 WY.
