Over the last 18 months, California has experienced one of the driest, wettest and wildest rides in its recorded water history.
As the 2015-16 water year opened in October 2015, drought had driven the state's reservoir and groundwater levels to all-time lows. Entire towns were left without water. Reports of lakes turned to puddles, of wells running dry by the thousands, and of the cracked ground above depleted aquifers sinking several feet a year dominated state headlines.
Then came the deluge. Since last fall, a steady stream of "atmospheric river" storm systems has been battering the coast, the Sierra Nevada and almost everywhere in between, restoring reservoirs and the snowpack to their highest points in years.
All winter, Californians have been asking one question: Is the drought finally over? The federal monitor shows just a few lingering tan and yellow patches in Southern California, but for scientists, the beginning and end of drought conditions are exceptionally difficult to pinpoint. Still, after only a few more serious encounters with the "Pineapple Express," Gov. Jerry Brown may well declare the state's 3-year drought emergency over.
Which leads us to the second most frequently asked question of this unusually wet winter: What's our water future? The answer has been clear for a while: It's going to be a lot like our water past, but more so — California is, was and will be chronically water short.
The drought has underlined three important realities that aren't going to change.
First, the way municipalities use water can be sustainable, even as their population grows, as long as they embrace conservation, water recycling and reuse, and a diverse portfolio of management options. However, agricultural water use at today's scale in California is not sustainable. Agriculture is literally sucking the state dry.
Food production requires nearly unfathomable volumes of water, and has resulted in the long-term decline of the total available fresh water in California. The great thirst of our highly productive agricultural sector has never been and will never be satisfied by the annual winter storms that feed the state's rivers and reservoirs.
The shortfall is met by pumping groundwater at rates that greatly exceed those of replenishment. As a result, groundwater levels in much of the state, including the once-vast reserves beneath the Central Valley, have been declining for nearly a century.
It is essential to understand that wet winters like the current one will not reverse this long-term decline. Historically, even the wettest multiyear periods result in only a modest uptick in the otherwise steady loss of Central Valley groundwater.
Consequently, agriculture in California has to adapt to this dwindling supply. Farmers and ranchers will face more of the kinds of difficult decisions the drought has already forced, such as fallowing fields as groundwater levels drop, or worse, taking land out of production.
Next, we must recognize that the classic definition of water as a sustainable resource — that is, using only the surface and groundwater available on an annual, renewable basis — is no longer tenable for the entire state. Instead, water sustainability in California must now refer to efforts to slow the rate of disappearance of the state's groundwater reserves.
The landmark Sustainable Groundwater Management Act, passed in 2014 in Sacramento, acknowledges and confronts the declining availability of fresh water in California. Its requirements, however, will never result in the recovery of statewide groundwater levels, even if important efforts to enhance groundwater recharge and construct additional storage are pursued.
Finally, it is simply impossible to effectively plan for California's water future without knowing a lot more about how much water the state has, how much it needs and how these amounts are changing with time.
The amount of groundwater remaining in the state's aquifers hasn't been adequately measured; it must be quantified by exploration. This includes characterizing how its quality degrades with depth, and estimating the costs and environmental consequences of pumping and treating this deeper, lower quality groundwater.
Estimating California's diverse water needs — for food and energy production, for domestic and municipal supply, for the environment and for economic growth — requires precise measurement, as well as a partnership between water management entities and the research community so that advanced, science-based tools can help establish trade-offs among allocation options.
Climate change and population growth are the primary drivers of changing water supply and demand, but other factors will also be important in managing the gap between the two. For example, personal water-use habits, greater agricultural efficiency, new technologies like potable reuse and desalination, and changes in water pricing, rights and policy will all affect the state's water availability and needs.
At the beginning of this month, and with a few weeks of winter still to come, the snowpack in the southern Sierra measured 201% of average. That's a lot of snow and great news for a parched state. But the long-term disappearance of groundwater will persist, and water scarcity is California's once and future reality.
Embracing this distinction, understanding its causes, working to mitigate them and monitoring our water down to the last drop are the essentials of the new, post-drought era of California water.
Jay Famiglietti is a hydrologist and former professor of Earth system science and of Civil and Environmental Engineering at UC Irvine. Michelle Miro is a hydrologist and doctoral candidate in civil and environmental engineering at UCLA.