MONTHLY RESERVOIR REPORT for May 1, 2021

The monthly reservoir report is authored by hydrologist Robert Shibatani exclusively for Maven’s Notebook.

Continued warnings about California’s pending dry-year and potential extended drought are headlining the news.  Rain has been sparse over the past month; less than half-an-inch at Shasta and Trinity reservoirs, zero at Folsom Reservoir, and about one-tenth of an inch at New Melones and Friant reservoirs.  By contrast, direct reservoir evaporation for the month was 7.8 inches at Friant, 5.9 inches at Folsom, and 6.4 inches at Shasta.

Water-year accumulated precipitation levels are low.  Total precipitation recorded at Shasta Dam and Huntington Lake to date for example are only 42 and 46% of average, respectively.  Totals recorded at Trinity Reservoir, the upper American River basin at Blue Canyon and at New Melones Reservoir are about 16, 31, and 17 inches, respectively, representing about 56, 52, and 66% of their water-year averages.

As of yesterday, north CVP storage stood at 5.815 MAF or 51% of total north CVP capacity.  This represented a storage level equivalent to about 70% of the 15-year average.  Most CVP reservoirs lost storage over the month with Shasta losing over 96,000 AF, San Luis over 94,000 AF, New Melones over 75,000 AF, Berryessa over 27,000 AF and Folsom losing over 3,200 AF.  Not all reservoirs, however, lost storage.  For example, Trinity and Millerton actually gained approximately 14,000 and 41,000 AF, respectively, primarily all from snowmelt runoff.

Warm temperatures rapidly depleted SWE across the State.  As of yesterday, Statewide average SWE was about 5.3 inches, representing only 24% of normal for this date.  As with other years, the Central Sierra was reporting the highest SWE at 7.4 inches (or 30% of normal), followed by the Northern Sierra at 4.9 inches (or 23% of normal) and the Southern Sierra at 2.8 inches (or 14% of normal).  By June 1^st, effective Statewide SWE will be zero.

All reservoir storage levels are increasingly trending below their 15-year averages with Trinity, Shasta, Folsom, and San Luis at 73, 63, 50, and 59%, respectively.  New Melones is the only CVP reservoir with current storage close to their 15-year average at 97%.  Oroville Reservoir is storing about 1,488 TAF or 59% of its 15-year average.

With continuing dry and warming conditions, we can expect to increasingly see further drought and water curtailment declarations.  Also, it is anticipated that a fairly active water transfer and temporary urgency transfer season will unfold and groundwater will once be significantly relied upon.  This year’s July-September period will be troublesome; exactly how bad will depend on what we do in May and June.

Prepared by Robert Shibatani

Robert Shibatani, a physical hydrologist with over 35-years combined experience as 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. robert@theshibatanigroup.com