Tag: water volumes

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Evaporation

When you read California’s daily water-storage reports and pore over the columns of data–for instance, from Lake Shasta, the state’s largest reservoir–you happen across this one: EVAP. It’s a record of how much water escapes the lake by changing state from liquid to gas. We’re schooled very early on in our science education about the hydrologic cycle. You know, the process by which water evaporates from oceans, lakes, rivers, and your scotch on the rocks, is transported into the atmosphere to fall as rain or snow, and then is evaporated and carried into the sky once more. Still, it’s one thing to know that a theoretical process is operating out there in the world somewhere and another to see its monumental workings in a statistical read-out.

Go back to Lake Shasta. I’ve been to the dam that holds it back, and I’ve driven past it dozens of times. It sprawls in an endless series of bays and inlets — old river courses — among the mountains that wall off the northern end of the Central Valley. The lake is usually ringed by a collar of brilliant orange-red soil, the upper margin of which marks the reservoir’s high-water mark. That ring is a sort of drought gauge: the more of it you see, the lower the lake is and, generally, the drier the state is.

One thing I don’t think when I drive past Shasta is that I’m watching a massive machine pumping millions of gallons of water into the sky. But that’s what it is. According to the summary from the Department of Water Resources California Data Exchange Center, yesterday the lake lost about 316 cubic feet of water per second through evaporation. Here’s what that is in household terms:

–A cubic foot of water is 7.48 gallons, so the lake was leaking 2,363.68 gallons into the atmosphere every second; three seconds’ worth at that rate would be more water than we’ve ever used in our Berkeley household in an entire month.

–Every minute, 141,820.8 gallons evaporated. In rough terms, the evaporation rate was 1 acre-foot every two minutes and 15 seconds. That’s a generous annual supply of water for two water-guzzling U.S. household.

–Every hour, 8,509,248 gallons of water — about 26.1 acre-feet — departed the lake. That’s enough to submerge a football field to a depth of 20 feet.

–For the day (and the evaporation rate is a 24-hour average), the lake lost 204,221,952 gallons, or 627 acre-feet. That’s a minor part of the reservoir’s net change for the day–the overall level fell by 7,271 acre-feet, mainly through water released for power generation–and it’s a tiny fraction of the lake’s current storage, about 3.15 million acre-feet.

Last stat for now: the process of evaporation is highly dependent on local weather, just like the grade-school lesson on the water cycle would suggest. When it’s warm, more water evaporates as the surface layer of lake water heats up. When it’s cool, the process slows. For the past month, the highest daily evaporation rate was 362 cubic feet per second, in the midst of a spike of very hot weather. The low point was 14 CFS, during a stretch of cool, rainy weather.

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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.