The Weather Service says we might get up to a quarter-inch of rain today. It got me thinking: how much is that?

An inch of rain is just what it says, enough water on the ground to make a depth of one inch. But that’s just one axis of the problem. How much water is that, really?

Let’s imagine a square yard of land, three feet long and three feet wide. Rain falls to some depth, like a quarter-inch or half-inch. If we have to put everything in inches that’s a 36-inch by 36-inch plot, or 362 or 1296 square inches.

If our precipitation is one inch, that’s 1296 x 1 or 1296 cubic inches of water.

If our precipitation is a half-inch, that’s 1296 x (1/2) or 648 cubic inches of water. Thus a quarter-inch of rain would be 324 cubic inches.

But that’s no help. No one thinks about water in cubic inches. We need pints and quarts and gallons! A quick trip to Wolfram Alpha reveals that a gallon of water is 231 cubic inches.

So an inch of rain on a square yard (1296/231) is 5.6 gallons, or five gallons plus five pints. And a half inch is 2.8 gallons and a quarter-inch is 1.4 gallons, or one gallon plus one quart plus one pint.

Let’s translate that to an acre. An acre is 660 feet by 66 feet (do you know why?*) or 43,560 square feet. That’s 220 yards (one furlong) by 22 yards or 4840 square yards.

4840 multiplied by 1.4 gallons gives me 6776 or almost 7000 gallons of water. I live on about one-third of an acre here in town and that means we should get well over two thousand gallons today! The water trucks you see at construction sites carry anywhere from 2000 to 4000 gallons, so that’s a way to visualize the amount.

It has been raining steadily since before sunrise so I think we might get more than is forecast, or at least be closer to the upper end.

California is perpetually short of water. The notion of seasonal drought is a quaint anachronism—supply will always fail to meet demand and that means drought is a permanent condition, just temporarily (and locally) relieved by precipitation.

Some of this water will make its way to the streams and some of it will fill lakes and ponds and some of it will recharge aquifers and some of it will be taken up by plants (even this late in the year) but most of it will evaporate and/or find its way back to the ocean. And the cycle will start all over again.

And some time around May the rains will stop and we’ll have to make do with what’s left over to get us through October and back to the wet season. I remember telling my Irish cousin that we could go six months without rain in California and she was literally open-mouthed with astonishment. She kept shaking her head and saying “can you imagine that?” to her kids. They get over 100 inches of rain per year in Galway which means they average well over a quarter-inch per day.

Now THAT’S a lot of rain!

*An acre is 10 square chains, that is, a piece of land 10 chains long by one chain wide. A chain is 66 feet so 10 x 1 is 660 x 66 and thus 43560. A square mile is 640 acres. Can you see why? (Hint: a mile is 80 chains long.)

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