by Jay Lund

California’s 2021 calendar year is over, but its 2022 Water Year (which started October 2021) is already three months old and still early in its wet season.  So far this wet season is actually wet.

It is a good time to assess the condition of the present drought and whether it is likely to end with this wet season.  And under such conditions, what are water management activities and policy initiatives we should be doing? 

A Wet Wet Season?

For the first time in three years, the wet season is wet (173% of average for the end of December), so far.  For the Sacramento Valley, based on DWR’s Northern Sierra 8-station Index, this October was the 2^nd wettest in this 102-year record.  However, November was much drier than average, the 31^st driest November in 102 years.  December rebounded strongly as the 23^rd wettest on record.  Overall, this water year through December was the 16^th wettest on record, which is about what the wettest year on record (2017) was at this time.  But the drought year 2013 had a bit more than this precipitation at by this time of year (Figure 1), so it is unwise to presume we are out of the drought yet (based on precipitation alone).

This year can quickly return to dry conditions. The forecast is dry for the next couple of weeks into January.

Figure 1. Historical Sacramento Valley 8-Station Index Water Year Precipitation vs. October-December Precipitation (inches) (Data from CDEC)

Snowpack is also doing well so far.  Northern California’s snowpack is at 145% of average for this date, so far.  This is nice for skiers, and improves prospects for refilling reservoirs.

What about likely 2022 streamflows?

Prospects for water year precipitation seem good, and at the risk of counting atmospheric rivers before they hatch, what might this imply for 2022 water year streamflows?

Figure 2 shows the effectiveness of Sacramento Valley precipitation in producing runoff over 102 years of data.  In wetter years, a higher proportion of precipitation becomes runoff, although there is scatter in this relationship. 

Drier years can produce about half as much runoff per unit of precipitation as wet years, probably because more initial precipitation goes to replenishing soil moisture and evaporation before it can runoff to streamflow or recharge groundwater.  It also seems that runoff efficiency has diminished in the last two decades, probably from the warming climate increasing evaporation and evapotranspiration rates. Alas, the plot does not include water year 2021, where runoff forecasts (and estimated runoff efficiencies) greatly over-estimated actual runoff.

Figure 2. Sacramento Valley Runoff Efficiency from Precipitation, historical results, open circles are data since 2001 (CDEC data)

Wetter conditions help streamflows from both more precipitation and higher runoff efficiencies, but probably by less than we would have estimated in the past.

Climate change aspects

It seems likely that we are seeing two aspects of climate change in this and recent years.

1. Warmer temperatures are a) increasing evaporation and evapotranspiration, depriving streams and groundwater from rain and snowmelt (a newer story, Pascolini-Campbell et al 2021) and b) making more precipitation fall as rain instead of snow, and shifting runoff from spring snowmelt to winter rain runoff (by now, an old story, Lettenmaier and Sheer 1991).

• Variability in precipitation seems greater, with more large storms and more periods without storms (Swain et al. 2018).  This increases extremes in California’s already extreme-prone hydrology.

October’s immense 2-day storm which largely refilled depleted soil moisture might be a consequence of both processes together.  California’s hydrology is becoming more variable both seasonally and between years, worsening conditions for drought, flood, and wildfire.

Water storage conditions

Reservoirs have begun to rebound, but most are not refilled.  Shasta, Oroville, New Bullards Bar, San Luis, and most other large reservoirs are filling, but some still have a long way to go.  Folsom reservoir is doing very well and has begun prudently releasing modest amounts of water to protect from possible floods – a normal and highly-precedented operation, even during drought.

You can see how reservoirs are progressing at this wonderful USACE website: https://www.spk-wc.usace.army.mil/plots/california_new.html

Groundwater lacks digested systematic monitoring regionally and statewide, so its changes are harder to assess.  But, from past experience, Sacramento Valley groundwater seems likely to be refilling well so far.  Refilling San Joaquin and Tulare basin groundwater is much more difficult, as these are much drier basins, with tremendously much more recent and historical overdraft and drought drawdown.  Indeed, depleted groundwater conditions in much of the southern Central Valley will likely accelerate actions under SGMA to reduce groundwater pumping.  Under SGMA, the additional pumping during the drought has increased the recent overdraft debt that must be repaid by 2040.

Floods coming?

Small amounts of local flooding are common in California in the winter (although flooding never seems small if it is happening to you).  For several years, California has seen little flooding, but with wetter conditions so far this year, and the filling of Folsom reservoir and potential filling of more reservoirs, flood operations (and perhaps actual flooding) later this year is plausible.  Still, even with the Oroville spillway failures in the wettest year on record in 2017, there was remarkably little actual flood damage (except the Oroville spillway) and almost no loss of life.

How this drought endures, even with a wet year

This drought, whether it ends this year or later, will have several enduring impacts:

• Extended agriculture groundwater scarcity. More reductions in groundwater pumping will be needed in wetter years to restore groundwater levels to 2015 levels to comply with SGMA requirements.  Agricultural costs of this drought will therefore extend for years, and perhaps decades.
• Depletion of endangered and other native fish species.  Salmon and smelt populations continue to decline and will be harder and longer to recover after this drought.  For Delta Smelt, the time to recovery might be never.  (Urban and agricultural drought impacts have long galvanized effective preparations for future droughts.  Drought responses have been less effective for ecosystems.)
• Restored attention to rural drinking water systems.  Hopefully the drought will accelerate addressing this problem, with faster connections and upgrades for more poorly performing systems to larger, better systems, improved support for counties for rural drinking water, and better stabilization and compensation for rural water problems through SGMA.
• New efforts to strengthen integrated water management portfolios.  This drought has again proven the effectiveness of integrated portfolio approaches to water management for cities and agriculture, and their vulnerabilities when portfolios are limited in scope and integration.  Portfolio solutions cannot solve all problems, but they can always make problems less problematic.  Previous droughts greatly strengthened preparations for this and future droughts.

Jay Lund is a Professor of Civil and Environmental Engineering and a Co-Director of the Center for Watershed Sciences at the University of California – Davis.

Further reading

Lettenmaier, D.P. and D.P. Sheer (1991), “Climatic Sensitivity of California Water Resources.” Journal of Water Resources Planning and Management, Vol. 117, Issue 1 (January)

Moyle, P., K. Börk, J. Durand, T-C Hung, and A. Rypel, “2021: Is this the year that wild delta smelt become extinct?,” CaliforniaWaterBlog.com, posted on January 10, 2021

Pascolini-Campbell, M., Reager, J.T., Chandanpurkar, H.A. et al. A 10 per cent increase in global land evapotranspiration from 2003 to 2019. Nature 593, 543–547 (2021). https://doi.org/10.1038/s41586-021-03503-5

Pinter, N., J. Lund, and P. Moyle, “The California Water Model: Resilience through Failure,” CaliforniaWaterBlog.com, posted on August 1, 2021

Rypel, A. et al., “A Swiss Cheese Model for Fish Conservation in California,” CaliforniaWaterBlog.com, posted on January 24, 2021.

Swain, D.L., Langenbrunner, B., Neelin, J.D. et al. Increasing precipitation volatility in twenty-first-century California. Nature Clim Change 8, 427–433 (2018). https://doi.org/10.1038/s41558-018-0140-y

Data

Reservoir levels, especially for flood operations: https://www.spk-wc.usace.army.mil/plots/california_new.html

Precipitation and snowpack information: http://cdec.water.ca.gov/snow_rain.html

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