Calculations
Here you will find
answers to the following questions:
How much water can I collect
from my roof?
What size tank do I
need?
How much water do I need per
year?
How large must my catchment
be?
Here are some
formulas:
1. For every square
foot (horizontal) of catchment, 1 inch of rain
will produce .62 gallons of water.
Thus, for every 10 inches of
rainfall, each sq. ft. of catchment yields 6.2
gallons.
A catchment of 100 sq. ft.
will produce 62 gallons for every 1 inch of rain, 620 gallons
for every 10" of rain.
2. Tank size can be
smaller than total rain needed per
year.
If you are using the rainwater
as the tank fills in the rainy season, your tank will supply
more than its capacity per year.
The tank need not contain a
full year's rainfall, especially if your area has more than one
rainy season per year.
Example: I have about 4000 gallons of tank
capacity, but they can supply over 7000 gallons per year, by
refilling as I use the water.
3. Water needs are personal,
however it is wise to calculate with a wide safety margin,
because rainfall varies from year to year. If your area
averages 10 inches of rain per year, figure that some years may
be twice that, other years half that. Design for the lesser
amount, and be glad when you have more.
If your rainwater system only provides
drinking water, figure one gallon per person per day  or 365
gallons per person per year. But you may have visitors, so why
not plan for them?
4. Catchment size determines how much
water you can harvest. Each horizontal sq. ft. of
catchment catches .62 gallons of water per inch of rain.
Regardless of tank size, you can only store what you can
catch.
Example: one of my
catchments is about 1260 sq. ft. Each inch of rain yields 750
gallons of water, or half the capacity of its storage tank.
Another inch of rain and it's full  1500 gallons.
For every gallon of water
you want to harvest per inch of rain, your catchment must have
1.6 sq ft.
If you plan to grow food,
using stored rainwater, calculate how much water you think you
will need, then double that.
You can often add more catchment and
more tanks to a surface catchment system. I have
three catchments and three tanks. If your needs change, you can
1) Enlarge existing catchments; 2) Plumb another tank to your
system; 3) Create another entire system.
In other words, if your tank never
fills, add catchment. If your tank always overflows,
add a tank. If you don't harvest enough water, enlarge your
system or add another system (catchment and tank).
My 1500gallon tank fills after only
two inches of rain, because the catchment feeding it
is large: 1260 sq.ft. Adding another similar tank would allow
me to store most of the water that now overflows and is
lost.
My two 1100gallon tanks are the
correct storage for the 525sq.ft. upper
catchment, but they don't both fill in a
dry season, only the upper one fills. That's because they
are sized for my conservativeadjusted average yearly rainfall
of 7" (it's actually 8.9"). I will extend the catchment area by
perhaps 400 sq.ft., to mostly fill both tanks in a dry
year.
Rainater harvesting is not
precise. Your area's yearly average rainfall is just
that  an average. One year's rainfall total may be 20",
another year 6", yielding an average for the two years of 13".
If you design your system for 13", a dry year may leave you
short of water.
Locate the average yearly rainfall
for your area for the last 20 years, and note the
driest years recorded. My nearest town recorded 1.82" for 2002
 the driest for 24 years. I could have designed my systems for
that, making everything larger, but as that was the only year
out of 24 when the average was below 2", I opted for a more
optimistic 'adjusted' average of 4.5" (reduced from my previous
7"), about half the official average of 8.9". Why? Because half
of those 24 years had rainfalls well below the average, as
would be expected. Some years had 14", and the most was 18"+ in
1993.
Design for the driest years
and be glad when you get more. It will take several years for
you to see whether or not your system is sized correctly for
the available rain. What's important is whether you have enough
water, not whether your tanks are filling completely. I'm
putting in a veggie garden and fruit trees, so I'm adding
catchment to my upper system. I'm in the desert, and it's hard
to have too much water there.
