Subject: Calcultations for rain water catchment

From: "Steve Diver" <[email-masked]>
Date: Mon, 20 Oct 1997 16:00:29 +0000

Some useful figures and calculations for rain water catchment: 

1st
Inches rainfall x Square feet of surface area = Amount

2nd 
Amount x 0.625 Factor 

Factor = (12-inch depth per cubic foot and 7.5 gallons per cubic 
foot of  water) or (7.5 divided by 12 = 0.625)

Example: 

30-inches per year is equivalent to 18.75 gallons per of water 
collected from each square foot of surface area.  

30-inches of rainfall collected from the roof of a 1,600 square 
foot home = 30,000 gallons water 

30-inches of rainfall collected from the roof of a 2,500 square
foot home = 46,875 gallons water 

Storage tank capacity will depend on total rainfall, surface area 
collected, seasonal rainfall patterns, and seasonal water use patterns.

If ya'll got any other handy calculations and figures, please 
share them. 

Steve Diver 

--
[email-masked]

---

From: [email-masked]
Date: Wed, 22 Oct 1997 06:37:31 -0400 (EDT)

Hi Steve:

I don't understand where your figures come from or how you used them, but
somehow we you got to the right answer!

The basic formula for volume of container with vertical sides (rainfall
should be considered as a vertical accumulation of water) is h x w x l = V
where the letters stand for height, width, length and volume.  Needless to
say, it is necessary to use the same unit of measure.  This is where most of
the world, using the metric system, have it much easier than the USA.

In the rooftop example, the calculation is much simplified if you use 2.5
feet instead of 30 inches.  You have l x w, the area, already so you multiply
that times 2.5 and you get 6,250 CUBIC FEET of water.  It is a simple matter
now to convert to gallons, multiply times the number of gallons in a cubic
foot, 7.4.  (I've won more ice cream sundaes betting on this number than I
care to recall.)  This gives us 46,250 gal, close enough.

People need to remember a couple of other things.  

1)  The area of the roof is irrelevant.  It is the horizontal area covered by
the roof that you measure.  The steeper the roof, the more area the roof has
to cover the area under it.  However this does not cause any more rain to
fall!  You would be surprised at how many people get confused by this.

2)  In storing water, it is important to have some idea of the weight of the
water in order to build a strong-enough container.  Water weighs 8.3 lbs. per
US gallon.  (An Imperial gallon is about 20 percent larger.)  

3)  The most conservative strong container that can be readily home-made is a
cylindrical tank in which the height is about equal to the diameter.  The
circular cross section is strong, having no corners--the stress is even
around the wall, although of course it gets more as you go deeper in the
tank.  It can be easily figured in pounds per sqaure foot by dividing,
logically enough the weight of the water in pounds by the area.  This is the
pressure at the seam where the side walls come out of the pad.  This is your
critical area as the joint is potentially weaker than other parts of the
tank.  So you reinforce it, give it extra coats of waterproofing, whaterver.
 Also the footing of the storage tank needs to be very strong and reinforced
with steel.  

4)  Generally if you divert all water from a roof to one tank you save money
over using additional tanks.  This is because the area of materials you use
increases in proportion to the square of the linear increase whereas the
volume increases in proportion to the cube.  If the height is fixed, for
example as just below the eaves, then the savings is more.  This is offset by
the modularity factor that is a severely undertaught issue in permaculture
design.  While one tank saves materials and money, with two tanks you can
clean one and still have water.  Modularity vs. scale is a case-by-case
design decision and enters into all aspects of design.  (There are elegant
solutions in some cases that partly offset this issue, for example hexagonal
arrays.  However this does not apply here.)

We generally try to get at least seven years of monthly rainfall records.
 Averages are interesting but we want to look at extremes.  How many months
rain do we need to store in order to have enough water in hand for the
longest period on record with less rain than we need?  For this we look at
monthly rainfall and subtract monthly use.  The difference is what we can
accumulate against drought.  The tank needs to contain all the water we will
need when we go into an absolutely dry perioid.  However if the rainfall
tapers off over the season, we might need a tank double that size to hold
enough going into the dry period.  Because water is essential, we have to
store all we can.

Obviously, as always in permaculture design, conservation is the first
resource.  We can save water a lot easier than we can make it rain.  This
would seem to be self-evident, but a look around the state of Florida where I
live shows that people are living way beyond their means as far as fresh
water is concerned.  For example we had a dry "rainy season" this year and
the water table dropped DOUBLE THE ANNUAL AVERAGE RAINFALL	!  Obviously,
someone is stealing a lot of ground water from the Earth.  If transporation
and evaporation were double the annual rainfall transporation would drop to
zero, we would be in a desert.  

Another factor in roof catchment design is to really work out well what to do
with SURPLUS water.  Anytime we have concentrated rain water, we need to do
some really good design and meticulous implementation or we get water erosion
and water waste.  Obviously the simplest solution, in areas of moderate
rainfall, is a swale that can handle all the runoff.  This is part of the
roof catchment design and must be stressed as such.  Here we have exposed
water table (read "swamp") close at hand so we can simply add to it, or we
can move water through swales and get some use first, for example growing
strawberry guava which likes a lot of moisture and does well in this climate.
 Or lemon grass. etc. 

In cold temperate climates, additional factors come into play.  

1)  What do you do with runoff when the ground is frozen solid?   Generally,
people use dry wells that go below the frost line--maybe four feet deep for a
nominal frost line of 3 feet.  There may need to be a dispersal system as
often in areas that were recently (10,000 years ago) glaciated there is a
hard-pan at that depth.  
2)  Winter catchment collection has special problems.  We will assume a well
insulated roof that is designed for the consequent snow/ice loads.  The
problems include:
\	a)  The water accumulates or blows off the roof as it is in solid form.  So
although precipitation may be adequate, available catchment may not.
	b)  Do we want to drink this stuff?  Snow is particularly "rich" in
pollutants.  This is because, like a rain drop, a snow flake forms around a
particulate in the air.  Many, if not most, particles in the air are now
pollution, particularly in winter when we are unlikely to get good pollen
dispersal.  However a rain drop is orders of magnitude more massive than a
snow-flake, so the pollution in the snow melt is orders of magnitude more
concentrated.  Yum.  
	c)  On the other hand, there is likely to be less organic pollution (bird
shit) on the roof in winter.  Trees are more comfortable than a windy, ice
encrusted surface and the bird population will be down to the most hardy
jays, woodpeckers & flickers, chickadees, nuthatches, etc.  This is just as
well, as any flush system in use to divert the first rain to an irrigation
system is likely to be disabled by snw.
	d)  If the catchment tank freezes solid, you are screwed.  Again, this
argues in favor of a large tank of circular dimensions (no corners to burst
apart as ice expands).  In areas where the temperature may not climb above
-20 degrees F for a week at a time (such as where my daughter lives in
Vermont), it is well to bury your tank.  While it then would be conservative
of materials to make a spherical tank, since you don't have to balance it--it
is imbedded in Earth--you would probably have to buy this so it makes more
sense to us a cylinder.  However you may chose to lay the cylinder on its
side.  Storing the tank under an attached solar greenhouse has obvious
interactive thermal advantages.  Moreover, you don't have to bury it as deep
and you have emergency access even in winter.   While a catchment tank high
enough to give at least a useful trickle by gravity is good, in this case
water must always be pumped to use.  A hand pump, e.g. a pitcher pump, is
adequate for most families and can be adapted to charge a pressure tank for
showers,e tc.  It is well to have this set up, even if an electric pump is
usually used, as ice storms can take out electricity for extended periods and
will likely increase as climate further destabilizes.  I suppose I need to
say that you install your catchment tank BEFORE you build the greenhouse,
preferably right after you pour your footings and/or foundation. (!)

Have fun!

For Mother Earth, Dan Hemenway, Yankee Permaculture Publications (since
1982), Elfin Permaculture workshops, lectures, Permaculture Design Courses,
consulting and permaculture designs (since 1981), and now correspondence
courses via email.  Next starts in Oct. 1997. Internships available.
Copyright, 1997, Dan & Cynthia Hemenway, P.O. Box 52, Sparr FL 32192 USA
 [email-masked]  

If its not in our food chain, we're not thinking.