California’s Driest February and Coming Drought?

 

Feb 2020 precip

Source: DWR CDEC

 

By Jay Lund

February has been amazingly dry in California, if anyone hasn’t noticed.  No precipitation at all in February, a dry forecast, about 51% of seasonal Sacramento Valley precipitation (a bit less for the San Joaquin and Tulare basins), and only about half (45-57%) of normal snowpack for this time of year.  Unless March is wet, this dry year seems likely to advance the onset of the fire season and threaten forest health this year.

Reservoir levels are still not bad for this time of year.  Many are fuller than average, perhaps reflecting some snowpack loss.  Some other reservoirs are a bit low.  This is inherent in the first year of a drought, low precipitation and snowpack, but mostly ok reservoirs.

Groundwater has recovered somewhat from the previous 2012-2016 drought, better in the north, but less in the state’s more overdraft-prone areas in the San Joaquin and Tulare basins.

USBR recently released a sobering contract allocation:  100% north of the Delta and 100% for San Joaquin Valley settlement contractors, but only 15% for Westlands and 20% for more reliable Class 1 Friant water contracts (zero for Class 2).   These folks, and others in the San Joaquin and Tulare basins, will be looking to buy water and are likely to pump more groundwater.  In the height of the 2012-2016 drought, these areas pumped about 6 million acre feet (maf)/year or more, on top of an average annual overdraft of almost 2 maf in these regions.

Several dry years will be tougher, again, on farming, and deepen groundwater depletion, making it tougher to comply with SGMA’s call for recovering 2014 groundwater levels by 2040.  This will increase interest in Delta and upstream diversions, with implications for Delta and environmental flow discussions and policies.

What is the likelihood of 2020 being a drought year (below normal, dry, or critically dry)?  This seems quite likely. The plot below has Annual precipitation vs. Precipitation before March 1 for 101 water years.  Given how unusually dry February and the rest of the year has been, March and April are unlikely to save us from some form of dry year.  (Still, in the 4th year of drought, 1991 had a “miracle March”, with three times average March precipitation, but this is unlikely).

Feb vs annual precip

Is 2020 is the start of a multi-year drought?  This is much less likely, but more likely than we’d like.  The dryness of subsequent years in California have pretty low correlations, overall.  By definition half of years have less than the median runoff.  Of 112 years of Sacramento Valley runoff records, 56 years had less than median runoff, 30 times had adjacent 2 years with both less than median, 18 times had 3 sequential below-median years, 10 times of 4 sequential below-median years, 4 times of 5 sequential below-median years, and 2 times of 6 sequential below-median years.  This understates correlation a bit because longer droughts can have rosier years interspersed, but it makes the point that multi-year droughts are far from certain after one dry year, and that drought-year correlations are not terribly high on the scale of a few years.  Recent apparent changes in climate make historical statistics less firm, of course, but are likely better than a blind guess.

Is 2020 a continuation of a longer drought, from 2012 or even 2007?  Given the diverse aspects of California’s water system, this is undoubtedly true for some areas and in some aspects.  Lovers of drought statistics will revel in this question, some of which will be interesting and even useful.  From a surface reservoir perspective, no, because essentially all reservoirs have refilled.  From a groundwater perspective, one can argue we are in more than a century of drought, without refill, in some areas.  (When tortured enough, drought statistics can confess almost anything.)

What to do now?  Hope for the best and prepare for the worst, as should be done every year in managing water in California’s highly variable hydrology.

Given the high likelihood of a drier year and the likelihood of a drought, it is not a bad time for state, federal, and local agencies to prepare and digest some lessons from the last drought, and maybe prepare some drought exercises (“dry runs”, so to speak) to local, state, and federal agencies get better acquainted.  Many agency water leaders retired (or fled) at the end of the 2012-2016 drought (who can blame ’em).  It may already be time, after 4 wetter years, for the next generation of water managers to cut their teeth on drought management.

Whether 2020 is a drought year of not, California will be seeing another major drought.  Given the difficulty and centrality of Delta operations during drought, now might be a good time for the state to develop a multi-agency Delta drought plan.

Don’t panic, and don’t be complacent.  Prepare carefully.

Further reading

Lund, J.R., J. Medellin-Azuara, J. Durand, and K. Stone, “Lessons from California’s 2012-2016 Drought,” J. of Water Resources Planning and Management, Vol 144, No. 10, October 2018. (open access)

The banality of California’s ‘1,200-year’ drought, September 23, 2015, Jay Lund

Jay Lund is in absentia Director of UC Davis’ Center for Watershed Sciences.  Currently, he has fled California on sabbatical, only to find drought and viral uncertainty in Eurasia, as well as cool old aqueducts and great water experts even outside of California. 

 

About jaylund

Professor of Civil and Environmental Engineering Director, Center for Watershed Sciences University of California - Davis
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