I just missed out on 10,000 Gallons of free water. Best way to build (or buy) a cistern.

Well it happened, 10 inches of rain in 24 hours. I have gutters on multiple buildings that would have caught at least 10,000 gallons of this water but I only have storage for 900 which is about the epitome of lame.

What is the best (re: cheapest over at least 50 years, more or less) way to store a large amount of water? 10,000 gallons would be the minimum. I know how bad plastic and cement are but I really would need to be convinced that the bad would outweigh the good as far as my 5 acres of drought-stricken forest is concerned. I would prefer something else but I just don't see it...

I can't dig a pond because of the slope and property line concerns.

I have been considering this myself for a future investment.  Plastic cisterns are more affordable and definitely lighter and easy to move and place.  Metal containers are considerably more expensive and heavier making then much more difficult to move and place.  When the time comes for me I will have to settle for plastic.

A quick check showed 5000 gallon plastic for about $3800.00, a 3750 gallon galvanized steel tank is about $9400.00, and a 3750 gallon stainless steel tank is about $15,000.00.
https://www.plastic-mart.com/category/232/rainwater-tanks

I am also thinking of building out my water catchment system, and I have been doing research into ferro-cement - with a twist. From what I gather, ferrocement should last for about 50 years, and requires a lot less cement than conventional reinforced concrete. The issue it has is that inevitably the water will eventually get to the steel armature through tiny pores or cracks, and then it starts to rust and will eventually fail. So, the twist is that now you can get fiber reinforced polymer rebar, remesh and lath. I am going to start some testing this winter, and then post a thread when I have some results to share about making water-tight containers.

My plan is to use 12mm FRP rebar from an outfit called Karbon Bar on maybe 2 or 3 foot centers. The same company makes a 6 inch remesh that comes in a 5' by 163' roll. It was about 20 bucks more than an equivalent roll of steel highway mesh, but is so light I can lift it with one hand! The rebar is even cheaper than steel, but since it comes in 20' sticks, youd have to find a place nearby that has it. For the lath, I ordered some basalt mesh in a couple different sizes to try and find one that will let me apply the mortar with a sprayer. It should be possible to apply the cement with hand tools, and there are lots of videos of people in the developing world that make some amazing tanks with the most basic materials.

Anyway, if you cant tell, I am excited about this. I scribbled some notes a while back, and I came up with like 800 bucks in materials cost to make a 5000 gallon tank. If I end up 50% over-budget, it still ends up being a pretty good deal. Also, I have access to my own sand, so that would add maybe a couple hundred bucks.

There are many ways to build your own tank.
If you are researching the subject matter may come under many names;
- cistern
- tank
- storage system
They can be built from steel rings, with a plastic insert, swimming pools, bricks of any sort, solid concrete, ferro cement
And they can be built underground, above ground, as part of the walls of a home.

Its actually a huge area of possibilities.
Or you can buy a 100,000 - 250,000L manufactured tank.

If you had time and a source for stones, I wonder if you could build your own cistern? I'm imagining these old wells that had mortar and stone for walls. If I did my math right (ok, fine... if google did it's math right) then something 10ft across and 20ft deep would hold over 11,000 gallons of water. If you had access to stone and sand and had plenty of time, this would not be very expensive (depending on where the bed rock is). Obviously this would not be for everyone, but I thought I would throw out the idea. You would still need some sort of cover. Wood would probably be fine if it was at ground level or higher. If it was going to be buried, then you would probably need some sort of reinforced concrete for the cover.  

Hahaha thanks Carl, I am excited too! I think we are at about the same point in our projects. I did a little bit of napkin calculation a month ago but I lost the napkin! Anyways, I am very interested in the materials you mentioned. I can't wait to see your testing.

Thanks so much everybody,

Dan