Water Tank Planning

A couple of months ago I did some back-of-the-envelope planning for the water tank, and figured that 5500 gallons would hold us through dry years with plenty of water. I did those calculations based on monthly water use, and some very very rough estimates of how much water the irrigation would use.

Then I started thinking about water as a flow over time, and how we would use the water, and I realized that that was too simplistic a calculation. We use water every day, not once a month, and the tank is being filled constantly. Maybe if I took that into account, we could use a smaller tank (cheaper, fewer logical problems with installation, etc.). So I spent some time reworking my calculations.

I started by really carefully determining how much water we would use. Here are the zones I designed for the irrigation system.


I took those numbers and worked them into a schedule.

The bottom chart shows the zones and when they would be watered and how much water. The upper chart is the week in hours, with gallons of water in use each hour. On the right and bottom you can see I was calculating totals and averages so that I could even out the water flow over time. The more regular your system, the smaller your tank needs to be to accommodate the swings in water level. The greyed-out hours are when I don't plan on irrigating, ever. (6am turned out greyish, too, but should be a sort of lovely lavender.)

Most irrigation is planned for the morning to reduce problems with rot, and also because it allows water to build up overnight without worrying about things like toilet flushes or loads of laundry, so when the big irrigation events happen (like watering the lawn) the tank is as full as it will be.


With those numbers, I made a sheet that dealt with flow through two weeks, by the hour. I threw in some gross estimates for using the water for flushing toilets and washing clothes, just to see how those would change the chart. I have two variables on this table: the flow into the sump and the tank size. Our actual flow into the sump now is 1gpm, but I worked with lower flows to account for drier years.


Then I charted the flow over time. I raised and lowered the theoretical flow into the tank until the system was no longer sustaining itself. So it is possible for us to maintain this level of continuous water supply with a 550 gallon tank and a flow around a quarter what we have now. And this is with what some would consider obscene overuses of water.


Now to read some more about pumps.

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posted by ayse on 05/18/06


Interesting analysis. One comment: the last chart shows that you have around 150 gal extra all the time. Did you size that to be a safety margin? 150gal out of 550gal seems like a lot.

I know nothing about sizing water tanks, but if this was inventory management, I would say you have too much water sitting around all the time. The bottom seen in that plot, in an ideal world, should reach almost zero. That would mean your tank is perfectly sized. Because the world is not ideal, you need the safety margin, which right now for you is ~150gal. You only need to figure out if that is enough or too much. Or maybe 150 gals is what you need for other stuff at your home, not sure if that's the thinking.

Sorry if I said the obvious here or if I am missing something important. Just wanted to share my 0.02.

No, you're right. But the thing is that water tanks come in specific sizes, and the next step down from 550 gal is 405 gal, which is just a leetle too small to be a decent margin of safety. Or we could make our own ferrocement tank, but I suspect that will not happen.

Also, with water tanks with a submersible pump (as opposed to gravity water tanks on a tower) you don't want the tank to get completely empty -- the pump needs to be underwater to operate properly.

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