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Friday, February 12, 2010

Water water everywhere, Part 3

Water is something we each use in small quantities. But because there are lots of us and we use little bits of water all the time, we have to store it in huge quantities.

When people in the USA talk about water, we use ounces, cups and gallons as our units of measure. In most other places we'd use liters. But what good are cups and liters when it's raining and you want to figure out how much water is going into the reservoirs? Do you say

We just had 52 gazillion cups of water added to our lake? or 800 trillion windshield-wiper sweeps of rainfall?

No, water keepers use larger units. One common water storage unit around here is the acre-foot. That's the amount of water needed to cover an acre (43,560 sq ft) one foot deep in water.

For people who like Olde English units, that's 66 feet (one chain) by 660 feet (one furlong) by a foot deep.

In metric, it's an acre-foot = 1,233.5 m(cubic meters)

That's a big number. You might ask How many gallons?

One Acre Foot = 325,851 gallons
One Acre Foot = 43,560 cubic feet or 435.6 HCF

The US water folks used to say an average family uses one acre-foot of water in a year. However in the Western US, where conservation is much more important (mostly desert!) we tend to use about a quarter of that, or one-fourth of an acre-foot.

Since our family's average consumption per month is 9 HCF, what percentage of an acre-foot do we consume in a year?

9 x 12 = 108 ÷ 435.6 = .248 acre-foot per year. That's about the average.

A few months back, I did a blog about our yard, called A Square Root.  We calculated that our property covers about 1/3 of an acre. So here's my question:

How long would it take to get a foot of rainfall on our property, and how long would it take us to use it up?

The San Diego County Water Authority says average rainfall in our area = 9.9 inches. Let's just say 10 inches.

If we caught all the rain that fell in 1.2 years, we had a place to save it, and none evaporated or got to water the plants, we would have a foot of rain times 1/3 of an acre, or .33 acre-foot.

Our indoors consumption is .248 x 1.2 years = .297 or about .3 acre-foot. In theory that means we might be able to be self-sufficient when it comes to water, but I wouldn't want to try it!

Our plants would certainly resent it. Even though we mostly have California natives so we don't irrigate, they need every drop that falls. Thank goodness we have the water authority to catch, store, clean and distribute our drinking water.

(Although it was a bit of a mess when they recently decided to replace the old pipes under our street!)

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