by Jay Lund
Yes, California will have another multi-year drought. California has immense hydrologic variability, with more droughts and floods per average year than any other part of the country. California’s water users, managers, and regulators should always be prepared for droughts (and floods). Eventually, California will have a multi-year drought worse than any we have ever seen.
More immediately, since the 2020 water year was dry in northern California, will this current 2021 Water Year be dry enough to put us into year two of a multi-year drought? (California’s water infrastructure and management system almost eliminates one-year drought effects for most uses in most years. This contrasts with the Eastern US, where rainfed corn or soybeans can die from a few weeks of drought.)
So far, we are early in the 2021 Water Year (October 2020 – September 2021) with essentially no October precipitation and almost none in November. Although early in the wet season, this water year’s Sacramento, San Joaquin, and Tulare precipitation, so far, are the lowest, or among the lowest of record. And the 2-week forecast shows no substantial precipitation. Last year was definitely dry for northern California, so most of California’s reservoirs are drawn down a bit. People also keep talking about El Nino. Let’s look at some historical statistics on how these factors affect the likelihood that we are entering a multi-year drought, and what we should do about it.
Does a dry October and November mean 2021 will be another dry year?
Mostly no, at least not by itself. The figure below plots annual northern California precipitation against October + November precipitation for each year since 1921. There is only a little correlation. The Sacramento Valley precipitation index today is about 6 inches below average. Statistically, on average, we would expect this year’s total precipitation to be about 7.7 inches below average (statistically translating into about 3.5 maf or 20%, below average annual runoff). We have a dry start to the 2021 water year, but the year can still become wet. Given the lackluster forecasts, adding a dry December would make a dry year more likely, but still not guaranteed.
Will 2020 being dry make 2021 drier?
Precipitation in northern California does not seem correlated across years, at least not historically, as shown by the plot below. The trend line is essentially flat for precipitation.
However, a drier year in terms of runoff tends to make the next year a little drier in runoff, as shown for a similar plot below for runoff instead of precipitation.
So this year is likely to have a little less runoff because last year was dry, even if we get average precipitation this year. (But precipitation this year seems unlikely to be affected by last year’s precipitation.)
Why is there a between-year correlation difference between precipitation and runoff? Runoff is slightly correlated over time because precipitation accumulating in groundwater is likely to increase flow into streams in the next year.
What about El Niño/La Niña?
Below is a plot of the historical El Nino index against northern California annual streamflow. The conclusion is pretty clear – La Niña/El Nino has little to do with annual runoff from northern California. (The ENSO index plotted here is average of December-April for each water year.)
Although ENSO may signal significant weather changes elsewhere in the world, it seems to have little predictive capacity in Northern California where most of the state’s precipitation occurs. ENSO has better predictive value for Southern California (Schonher and Nicholson 1989).
So what should we do?
There is a significant probability that this year will be the second year of a multi-year drought, since last year was dry, and this year seems likely to be dry. At this early time in the water year, the chances seem roughly even, but remain quite substantial.
Given such odds, it makes sense to prepare for another dry year, and perhaps several additional dry years. Reservoir water levels are already below average from last year being dry, and groundwater will be a little lower as well. So we have less water in reserve than last year at this time.
What to do?
If your agency or interest has a drought plan, you are likely to be looking it over and updating it.
If your agency or interest does not have a drought plan, what state do you think you live in? This is a good opportunity to develop a drought plan before you are desperate. This applies especially for the Delta and many critical environmental and regulatory interests.
But this year could be wet or contain floods as well. So, as usual for this time of year, Californians should be prepared for both floods and drought.
Finally, most technical people know Murphy’s Law, that “Anything that can go wrong, will go wrong (and at the worst possible moment).” The inverse of this pragmatic “law” would hold that those who just made or updated a good drought plan should be less likely to see a drought. (So update your flood plans as well!) It is always good to be prepared.
Jay Lund is a professor of civil and environmental engineering and Co-Director of the Center for Watershed Sciences at UC Davis.
You can play with these data yourself, cheerfully supplied from DWR’s CDEC: http://cdec.water.ca.gov/reportapp/javareports?name=8STATIONHIST
Capitol Public Radio recently had a great story by the same title: https://www.capradio.org/articles/2020/11/30/is-california-heading-for-a-multi-year-drought-the-odds-arent-in-our-favor-experts-say/
Klemes, V., 2000: Drought prediction: A hydrological perspective. Common Sense and Other Heresies: Selected Papers on Hydrology and Water Resources Engineering, Canadian Water Resources Association, 163–176
Schonher, T. and S. E. Nicholson (1989), “The Relationship between California Rainfall and ENSO Events,” Journal of Climate, Vol. 2, Nov. pp. 1258-1269.
Impacts of El Niño and benefits of El Niño Prediction, National Oceanic and Atmospheric Administration