Water: The Midwest View

[Other posts on water: 

Big Bathtub II: 'Wasted']

Spotted the following letter today on the Chicago Tribune's (Tom Skillings's) weather page: 

Dear Tom,
The level of Lake Michigan is up 13 inches from last year. That's great, but could you 
express that in gallons of water?
Dan Fridley
Dear Dan,
The quantity of water that circulates through the Lake Michigan hydrologic system is 
truly staggering. And expressing that volume in units as miniscule as gallons yields 
numbers that are so huge as to be practically incomprehensible, but here it goes. 
A 13-inch increase in the level of Lake Michigan's 22,300 square miles amounts to 
5.044 trillion gallons of additional water (5,044,000,000,000 gallons). And that's not 
all. Lake Michigan and Lake Huron are essentially one lake; their water levels rise and 
fall in tandem. Thirteen inches of water added to the level of Lake Michigan means 13 
inches added to the 23,000 square miles of Lake Huron as well, and that amounts to an additional 5.202 trillion gallons (5,202,000,000,000 gallons).

So to summarize the arithmetic: Lakes Michigan and Huron, total surface area 45,300 square miles, have risen a foot and an inch in the past year. The total increase in water volume is 10.2 trillion gallons.

There is no doubt that is a lot of water. But it is an abstraction, proof that in the wet eastern two-thirds of the United States, water is, most of the time, something that's just there, like leaves on the trees, mosquitoes, corrupt politicians and bad beer. In fact, this immense amount of water, these trillions of gallons, are a trivial amount in the Great Lakes context, where volumes can be calculated in hundreds or thousands of cubic miles.

But before we get to that, let's put those 13 inches of Michigan/Huron water to work: let's frame them in the California context. 

In California and anywhere in the West where water means the difference between nothing and abundance, the working unit is the acre foot: the water it takes to submerge an acre a foot deep. An acre foot is 325,851 gallons, and that is said to be enough water for two average American households to keep their toilets flushed and lawns green for a year. 

The extra 13 inches of Michigan/Huron water: It comes out to something like 31.3 million acre feet. California's total reservoir capacity is said to be about 42 million acre feet. So that foot and an inch here–the incidental effect of increased runoff in their basins–would fill California's collection of monster lakes and catch basins three-quarters full. What a gift to a dry place. 

Lake Michigan has an approximate volume of 1,180 cubic miles, and Lake Huron 849. A cubic mile of water is just under 3.4 million acre feet. So the 13 extra inches of water in Michigan/Huron added about 9 cubic miles to their volume, or a little less than 0.5 percent (that's not too shabby, actually). All California's reservoir capacity would be satisfied with roughly 13 cubic miles, about 0.75 percent of the volume of the two lakes (and while we're throwing Great Lakes volume numbers around, the combined volume of Michigan, Huron, Erie and Ontario is about 2,538 cubic miles; the volume of Lake Superior is 2,900 cubic miles). 

When you see numbers like this, which may be close to meaningless without more context, you think you can understand the envy and ambition of Westerners who think the Great Lakes would solve all their problems. It seems a little crazy, until you travel up and down California and see how much has been invested in large-scale plumbing to make water go places and do things that seem to defy nature and physics.  

Big Bathtub I: The Acre Foot

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.