# TH: The Mathematics of Cisterns

```This post is humbly submitted by Dr G Wiz of Baltimore.  Richard has been
kind enough to forward my communications until my IP address is secured.

While attending a session about impervious surfaces(IS) at a
conference, I was amazed to hear the speaker refer to above 26% IS
as red line:  an ecological disaster if you prefer.  Hell, I work
in the big city.  We're 95% IS and above.  The ground water re-
charge here is so bad, I'm amazed that anything can grow.

Urban environmentalists have long awaited a change in policy about
how the government deals with storm water -that's the Dept of
Public Works in Baltimore.  Is all this concrete and asphalt really
necessary?

Meanwhile, I have seen some resourceful uses of existing water
sources.  Some people run a tube from the drip tray of their window
air conditioner to a tree, others run the down spouts in their back
yard to a fish pond.  This brings us to the fantasy of cisterns.

Wouldn't it be marvelous (Marx Marvelous here) if citizens, let's
call them (us) the little people, caught rain water in barrels
and slowly released it back into the ground.  I'm talking about de-
centralized storm water management here and maybe a bit of anarchy.
If we're lucky there might even be mischief.

To the point of this communication:  here's a little arithmetical
ditty for those of us who just don't have enough to think about.

GALLONS OF WATER AVAILABLE PER INCHES OF RAIN

SurfaceWidth X SurfaceLength X InchesRainFall = Q

Q divided by 231 = Gallons of Water

NOTES:

Surface area (Width X Length) is calculated by the "footprint" or
ground dimension.  The square footage of a sloped roof is larger
and perverts the result.

We are assuming no wind, this could effect the amount of water
available -"depending on which way the wind blows."

We are also assuming ideal conditions with no leaks in the system;
fat chance(FC).

So now you know the secret:  1 gallon = 231 cubic inches.

For you folks in the metric system, the concept remains the same.

G Wiz

```