While rains have brought some relief to a drought-stricken California, the fact remains that the state’s groundwater supplies have plunged and last winter’s modest rains and snow have brought no relief.
That’s seriously bad news for the state’s Central Valley, the source of many of the foods gracing America’s tables.
And the odds are that this winter will bring a return of La Niña conditions, meaning lower than average precipitation and an intensification of the drought.
This past winter, most water agencies across California were counting on the strong El Niño to produce surplus water, helping to increase groundwater and make up for what’s been pumped out due to the severe drought. Unfortunately, precipitation during the winter of 2015-16 was barely above the long-term average in the state, despite stormy weather in the northern part of California.
Recent patterns in groundwater
The drought was somewhat alleviated in Northern California, thanks to these rains. However, new evidence suggests that the groundwater level in California’s Central Valley will continue to decline this year. We examined about 55 years of data from nearly 500 wells, and also used estimated water storage from Gravity Recovery and Climate Experiment (GRACE) satellites.
Historically, drought and reduced groundwater storage occurred almost hand-in-hand in the Central Valley. When drought conditions ended, groundwater storage would normally rebound – this is the relationship we see in records from about 1960 – 2000. But our recent study found that this relationship has changed over the last decade and a half.
In the data from the past fifteen years or so, scientists found that groundwater storage continued to decline for a full year after drought has ended. So, whereas previously when drought ended, groundwater resources would begin to recover, now groundwater continues to decline, even through a wet period. It will take more research to understand exactly why this is happening, but it’s possible that the recent tendency toward more intense, longer-lasting droughts in this region has changed the way rainfall and snowmelt are taken up by the soil and recharge groundwater.
California’s groundwater has been used to supplement the water supply for households, agriculture, and industry for many years, and there’s been a downward trend in groundwater storage since at least the middle of the 20th century. However, this trend doesn’t explain the recent change in the effect of drought on groundwater supply.
The effect of ENSO
It’s well known that precipitation in California is somewhat tied to the El Niño/Southern Oscillation (ENSO). Roughly speaking, El Niño tends to bring rain, and La Niña tends to withhold it. [Although this past winter is an excellent example of how what’s expected doesn’t always happen.]
There’s more, after the jump. . .
Several recent studies have found that drought conditions in California have become increasingly more intense and longer-lasting . Some of our research has linked this tendency to the combination of strong ENSO events and global warming. We’ve found that strong ENSO events modulate California’s climate not only through the peak of El Niño and La Niña, but also their transition phases when they dissipate, and before another El Niño or La Niña potentially forms.
Specifically, our research suggests that in the year prior to El Niño, there is often a ridge of high pressure in the Gulf of Alaska. This relationship has strengthened over the past 20 years, with the ridge becoming stronger. This ridge can then tilt conditions toward drought in California.
So, in short, if La Niña tends to drought, and the ramp-up to El Niño tends to drought, there’s a likelihood of more drought time than wet time, even if El Niño itself leads to wetter conditions. If there’s an increased duration of drought AND groundwater recovery now lags the end of drought, groundwater resources could have a hard time recovering at all.
The implication is that current groundwater storage in the Central Valley will likely continue to diminish further in 2017, even though the recent El Niño somewhat mitigated the drought. Currently, there is a 55-60% chance of La Niña developing by the end of 2016 which, based on past cases, would tend to reduce winter precipitation in California. This forecast provides all the more reason to continue our careful management of water resources.