An acre foot is the amount of water it would take to flood an acre to a depth of one foot. An acre is, ballpark number only, a patch of land 100 feet by 400 feet. If you were standing on that patch of land with water about halfway up to your knees — well, you’d be having a direct experience of the acre foot.
An acre foot is 325,000 gallons. If you pay a water bill that shows how much you use, you can figure how long that much water would last you. The rule of thumb, that we journalists borrowed from “water experts” here in California was that an acre foot was enough to supply two average households for a year. That’s for use inside and outside the home for three or four people say, and it comes out to about 460 gallons a day for each household. Of course, there’s a lot of variation. An inner city apartment dweller uses a lot less than someone whose lawn looks like a fairway at Augusta National. Someone in a cool coastal area — Berkeley, for instance — uses less than someone in a much hotter area on the other side of the hills, lawn or no lawn.
The acre foot is a basic unit of life in California. Yes, weather people and water management officials count inches of rain and snow in the winter. But those units are incidental in a place that needs to capture and store an immense amount of water to irrigate roughly 15,000 square miles of crops and to supply 36 million people. The acre foot is the fundamental currency of reservoir storage and water delivery.
Up and down California, the federal government, the state government, electric utilities, county and city water companies, and irrigation districts have built reservoirs. They’re big bathtubs that together hold something like 42 million acre feet; that would be enough to submerge the entire state of Wisconsin under a foot of water. They reservoirs are expected to fill up in the winter and spring with runoff from the rains and melting snow running down from the Sierra Nevada. Then the water is pumped out during the dry season to help the fields and orchards thrive and to keep the showers and garden hoses flowing. Some water is even set aside for the fish that swim in dwindling numbers through the maze of waterways between reservoir, farm, and town.
A wet winter here–what people like to think of as normal, with nature’s tap switched on when we get into the middle of autumn–keeps our big bathtubs full and the water running where it’s needed. But there are other kinds of winters, too. Very wet ones, where the system simply can’t hold all the water coming down the rivers. And dry and very dry ones, where the water level in the reservoirs falls and keeps falling if two dry years come back to back.
We’re in what appears to be our third dry–or drier than “normal”–year in a row. Three of the biggest federal reservoirs in Northern California– Shasta, Oroville, and Folsom–all fell to critically low levels in December and January. Together, the three reservoirs can hold about 9 million acre feet; that’s about enough to supply a year’s worth of household water for New York, Pennsylvania, New Jersey, Massachusetts, Connecticut, Rhode Island, New Hampshire, Vermont, and Maine. That’s every U.S. resident east of the Great Lakes and north of the Mason-Dixon Line.
Remember,: that’s the capacity of just three reservoirs. There are dozens of others. During the last five weeks, copious rains have fallen and the water has started to rise in some of those big bathtubs. More about that later.