Rain Collection

May 26, 2013

in Water

WATER is usually the single most critical survival factor. Finding a water source and being able to collect it, store it, purify it and distribute it is always a high priority. Being able to accomplish that task by collecting rain water can be an excellent solution, depending upon circumstances.

So, the first thing we need to know is how much rainfall there is in our area and then, how much of it can we collect and store. Rainfall averages are easy to obtain from weather related sources, but it may be important to look at minimum historic levels if we live in an arid or semi-arid area.


  • How long can it go without raining in my area
  • How much does it rain in an average month
  • What is the average amount of rain each time it rains
  • How many square feet can I collect rain from
  • What percentage of the rain will I collect
  • How much water will I end up with
  • How much water can I store
  • How much water do I need over time
  • How long will my water storage last between rain events

Collection – the most often discussed and used rain collection device is the roof on our house. The formula for calculating rain collection from a roof is:
Volume of rain (gals) = area of roof (sq. feet) x rain (inches) x .623 x eff%
[note – efficiency percentage (eff%) allows for incomplete rain capture due to evaporation, first flush diverters or other reasons]
1000 sq feet roof surface x 1″ rain x .623 x 100% = 623 gallons
1000 sq feet roof surface x 1″ rain x .623 x 75% = 467 gallons

In most cases, the rain water will be collected in some manner from downspouts that are fed by a gutter system. We must pay attention to what percentage of our roof surface is gathered by our gutters and what percentage of that we can realistically expect to get in our collection containers. A rain water diverter can be installed in the downspout or the downspout can feed directly into a rain barrel or other container. In an emergency, the simplest setup would involve just cutting off the downspouts and placing buckets or tubs under them to catch water. This could entail a lot of effort emptying the buckets into larger reservoirs during steady rain.

Here is an excellent series of video tutorials on rain water storage using IBC tote containers:
Rainwater Collection System Series

If you want a step by step video series on how to build a rainwater retention system this series is for you. I go into every detail I did to make my rain water collection system. I include what to do and what not to do.

Storage – With each IBC tote holding around 275 gallons (they may vary in size), using a two tote system as described in the videos above, would give us around 500 gallons of storage capacity. That should be enough to get us from one rain fall to the next, even during extreme drought conditions. But our water needs will vary according to how many people we supply and how well our water conservation discipline is applied.

Each normal sized bathtub holds around 50 gallons and a garden/soaking tub can hold 100 gallons. (see the water calculator to estimate volume based on size) We need to calculate our water needs over time for a period that will last between rain events. Then we can add up all of our storage capacity: bathtubs, totes and other tubs, buckets and barrels. Then we know if we need more storage capacity and how much. A 50 gallon rubber/plastic tote can cost as little as $20 and can be used for storage or other purposes until it is needed for water storage.

Purification – Most rain water is fairly pure before it hits our roofs. But once it on the roof, if may become contaminated by a variety of substances. Newer asphalt shingle roofs tend to “shed” particles that may be toxic while older ones do less of that. Roofs can accumulate dust, bird excrement and other undesirable things. This may vary with the type of roof and the environment and the current weather conditions. First flush diverters are designed to discard the initial rain runoff in an attempt to avoid collection of contaminants. But they also waste some water. We will need to consider filtering and purification needs for any rain water that we intend to use for drinking.

Selection and Consequences of Roofing and Gutter Materials for Rainwater Harvesting

Water Basics
Basic Water Calculator
Using Chlorine Pool Shock to Disinfect Water

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