Written by hydrologist Robert Shibatani

Summer is here and while temperatures across much of the State have returned to “normal” this week, the recent incredible extreme heat alerts (and those occurring elsewhere in the country) have reinforced the notion to everyone that a rapidly changing climate is here to stay.  Water storage across the State continues to decline and the wildfires with the likes of the Lava, Tenant, and Salt fires have started to burn and portend an ominous fire season.

As of yesterday, north CVP storage stood at 4.531 MAF or approximately 40% of total north CVP capacity.

Current storage in CVP reservoirs represents about 61% of the 15-year average.  Not as critical or dire as some would like to believe given the proper historical context.  Still, water storage in Shasta, Folsom, San Luis, and Oroville reservoirs are well below their 15-year averages at 53, 41, 36, and 48%, respectively.  Only New Melones and Trinity effectively enjoy more “normal” storage levels for this time of year (e.g., 83 and 69% of their 15-year averages, respectively).  With approximately 288 TAF in storage Folsom Reservoir water levels stand at about 385 ft msl, this is still well above the centerline of the urban water supply intake at 317 ft msl and above the 307 centerline for the power penstocks.  Floating the barge pump is not yet necessary, but may very well be later this summer or early autumn.

Scorching temperatures combined with low humidity and windy conditions during June were perfect to drive the vapor pressure gradients and evaporate surface water.  With no rain, computed evaporation losses became increasingly important.  Evaporative losses were high across the State with the primary reservoirs Shasta, Trinity, New Melones, San Luis, Folsom, and Millerton all losing in excess of 11-12 inches of water, with San Luis recording 18-inches of evaporative reservoir water loss.

All reservoirs lost storage over the month of June.  Shasta Reservoir lost 242 TAF, San Luis 217 TAF, New

Melones, 146 TAF, and Trinity, Folsom, and Millerton losing 98, 73, and 14 TAF, respectively.  Computed inflows were significantly lower than reservoir releases through the power penstocks (e.g., Shasta Reservoir’s computed inflow/power releases was 135/366 TAF, Trinity was 16/107 TAF, New Melones was 23/163 TAF, and Folsom, 49/106 TAF. Add in the evaporative losses and one can see how the overall reservoir water budgets were affected.

As in previous months, we have been watching reservoir releases closely.  No peculiar releases have been observed.  All releases have been below or well within the acceptable 15-year median range.

This upcoming July 4th weekend will send many long-awaiting recreationists back out onto the waterways. Water levels and flow rates are clearly lower than normal.  The spring freshet has run its course and water temperatures are rapidly rising.  As we enter the sun’s summertime anvil, the next two months will determine how we allocate our diminishing supplies, whether critical shortages will exist, and how we enter the next WY.

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.

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