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.

4 Comments

Filed under Current Affairs, Weather

4 Responses to Evaporation

  1. Lydell

    Does California desalinate or is it too costly? Maybe if approached with Mother Palin’s You Betcha Spirit… After all, this country came up with the spork and the boner pill.

  2. Is Shasta a particularly shallow reservoir? You describe it well. Such a sprawl. Maybe we need reservoirs with less surface area?…. (Brings to mind Tahoe. A natural lake, of course, but it must lose a far smaller percentage of its volume to evaporation, eh?)

  3. Dan

    Lydell: Some desalination is going on in Southern California. Santa Barbara and San Diego counties, I think. We can use the You Betcha spirit to save up urine and distill it for home and garden use. We’ll be featured on The Daily Show any time now.
    Pete: Don’t know about the depth of Shasta, but no, I don’t think it’s real deep. And wherever the water can heat up, the evaporation rate goes up, too. My recollection is that Lake Tahoe has a volume of 120 million acre feet — about 28 times Shasta’s capacity. But its surface area is roughly 191 square miles, about four times that of Shasta (47 square miles). A deeper lake, colder water, cooler weather on average and less surface area relative to volume would all seem to add up to a lower evaporation rate.

  4. Lydell

    Twit will always be a noun to me, so….congratulations.

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