Written by Robert Shibatani

This past month has seen the much-anticipated ablation of this year’s record-breaking snowpack.  The big resulting news was that it did not translate into the Statewide record-breaking flood that many in the media were predicting, at least not yet.

No, what has happened is that many Sierra watersheds, particularly in the central and northern parts of the State have simply used the natural storage reservoirs of the landscape to buffet this year’s historic spring freshet.  The surface depressions, shallow perched aquifers, moisture-rich mosses/wetlands, bogs, as well as deep seepage losses, etc., all contributed to both store and attenuate this year’s runoff pulse.

As noted in last month’s Report to Maven, the very real possibility of a dissipating snowpack without necessarily, seeing a corresponding increase in runoff was suggested.  This in fact occurred.  The massive late season SWE (e.g., over 60-inches) recorded only two months ago dissipated rapidly.  Average regional SWE for the north, central, and southern Sierra Nevada snow regions are now only 19.8, 25.5 and 20.8 inches, respectively.  In the runoff-intense upper American River watershed as an example, despite record-breaking snow totals, Folsom Reservoir has been discharging about 15,000 cfs for the better part of a month, far less than what early-season runoff forecasts were predicting.

With a dry May, all the reservoir storage gains have resulted from antecedent runoff; for example, Trinity Reservoir 369,000 acre-feet (AF), New Melones Reservoir 306,000 AF, Folsom Reservoir 102,000 AF, etc. keeping in mind that storage gains and reservoir inflows are not the same.

Across the CVP, reservoirs are nearing capacity; total CVP reservoir storage is 9.63 million acre-feet (MAF) or 81%.

Shasta Reservoir is storing some 4.46 million acre-feet (MAF), New Melones 1.82 MAF, Folsom 0.907 MAF, and Millerton 0.231 MAF.

If there was any doubt as to the hydroclimatic robustness of this winter/spring season, a quick look at the accumulated reservoir inflows to date can prove informative.  Shasta and Folsom reservoirs alone have already received 4.7 and 3.8 MAF of year-to-date inflow, respectively, both above their 15-year averages by 19 and 78%, again respectively.  Unsurprisingly, given the strength of the precipitation record in the southern Sierras this year, Millerton inflows have already exceeded even those observed in 1983, their wettest year on record.  New Melones Reservoir will likely follow suit this month …

I think that overall, CVOC has managed the system admirably, particularly in how it has and, continues to oversee the much anticipated (and originally feared …) spring “flood” without difficulty.  Unlike other spring peak flow years triggered by some atmospheric event (e.g., hot spell, late season tropical storm system), this year has been more subdued, ambient conditions resulting in no precipitously rising hydrologic peak.

As the Spring hydrologic season runs through its recession limb, we are comforted by the fact that stored yield in our federal/State water facilities are near capacity.

Accordingly, from a water supply perspective, we have little worry for the remainder of this water-year and, if we remain judicious, likely for water-year 2023-2024 as well.

An oft undesired result of water-rich years, however, is ambivalence.  For in times of plenty, chasing down the root causes of various criticisms is not a popular activity of government water departments.

Yet, federal reservoirs “spilled” over 236 thousand acre-feet in May alone.  This occurred, not surprisingly from Folsom Reservoir, bringing the yearly total “spills” to 830,691 AF.

Can we continually afford to waste over 800,000 AF?

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