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 1500-gallon 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 1100-gallon tanks are the correct storage for the 525-sq.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 conservative-adjusted 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.

 DVD Soon!

Watch this space for the coming DVD on how to install your own rainwater harvesting system.