Saturday, July 27, 2013

Rainwater Catchment Systems

I have been doing quite a bit of reading on Rainwater Catchment Systems.  The best information I have found so far was a very nice pdf file by the Texas Water Development Board.  Here is a link to the pdf file.  The manual is 88 pages long with a lot of good information in it.  The only thing you really need to look for is to find a table showing the Average Annual Precipitation for your area that you live in.

Here is the table for my state Oklahoma.

Where I live is in the area between the 24 and 28 inch per year range.  Now to keep it simple, (remember when doing calculations to use the K.I.S.S. method, 'Keep It Simple Stupid' ) I will use the 24 inch to use a conservative number.  Now I do realize that we are in a drought right now but also keep in mind that these tables are an average not a per year basis.

So here is a few pictures of the old cistern that is on the property.

 As you can see it has not been used in quite a while.  We have owned the place since 1982 and it was not in use then.  It appears that the walls are a thin concrete with a plaster over them.  There is one section down towards the bottom where it has cracked and fallen in.  That broken piece is probably a foot or so but there is a section a few feet long that is pushing out.  There is about 18 inches of water in the bottom and a decent level of debris in the bottom.

The cistern measures about 7 foot in diameter by 14 feet deep.  Keep in mind that is to the debris level not the true bottom of that tank.  Using these measurements that gives it a 3454 gallon capacity.

Next the roof footprint of the house is roughly 30 ft x 71 ft.  That comes out to 2130 square feet on the footprint.  Now with rainfall you can get .62 gallons per inch on every square foot.  That comes out to 1320 gallons of water for every inch of rainfall.  Now a person will not be able to get 100% of the water off of the roof.  The professional contractors that install systems use between 75-90% catchment when designing and installing systems.  I'll be conservative and use 75%.  That still gives the roof 990 gallons of water per inch of rainfall.

With this system  it would catch 23,760 gallons of water in a year that is usable.  They say a household uses anywhere from 24-49 gallons of water per day on average.  One person would use anywhere from 8760 to 17885 gallons per year.  So as you can see water usage is a huge thing in this.

Right off the top of my head I think we are going to need to find another source of water, a well for example to supplement the rainwater.  As I continue to research and crunch the numbers it should be interesting.

The first question is whether or not the old cistern is worth refurbishing or if we need to go with a new tank catchment system?


  1. If she holds water she is worth using in my opinion. My rain water/cistern combo is over 100 years old and lined with limestone and I still use it for watering the garden etc.

    Figure I can treat the water if it comes to needing it and will be useful even if I install a new system.

  2. Thanks, that was one of my thoughts. It looks like any above ground tank purchased would run around 1k for a 1,500 gallon tank so I would have to spend over 2k just to come close to replacing what it can hold let alone have any more capacity.

    What kind of a catchment system do you have on yours?

    1. All I have are two 100 gallon tanks that are fed from the gutters on the house. All my water use comes out of the tanks as they are elevated a few feet above the use areas. If they are full and more rain is coming I empty them into the cistern and the plan is to put a hand pump on the cistern to carry water back up to the tanks if I need it and we are low. My next water project is repairing my old farm pond dam and then I will look at a viable rain catchment system for the barn.