# Large Scale Rainwater Collection/Harvest



## Deltahedge (Apr 1, 2020)

Does anyone have experience with large scale rain water harvesting?

I'm not talking about a 50 gallon barrel at the bottom of a downspout. I'm talking about 10k gallons or bigger, stored either above ground (not ideal) or below ground.

There's a local rule that even if I drill a well, I'm only allowed to pump 40% of my water need from the well, and I still have to buy 60% of my water from the utility company.

If I harvest rainwater, that harvested water counts as if it were purchased from the utility company as far as calculating the 40% well water used vs 60% (water harvested + water purchased)

I live in the Houston, TX area, so we have plenty of rain. and with catch basins around the yard, it would be easy for me to harvest 100k gallons each time it rains, but that is probably cost prohibitive. So, I'm considering getting an installation quote to store 40-50k gallons underground.

According to my research, 1" of water on 1 acre is just over 27k gallons. If I were to irrigate 2 acres with 1" per week, I would need 54K gallons a week.

Each time it rains, I don't need to irrigate for about 4 days. Then on Day 5, I could use about 27K gallons to put 1/2" on 2 acres. Then if it doesn't rain again, on day 8 or 9, I would need 27k gallons again. With enough storage, each time it rains, I could get through 8-9 days without having to pump from the well or purchase water, depending on the size of storage.


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## Wiley (Dec 2, 2019)

Deltahedge said:


> Does anyone have experience with large scale rain water harvesting?
> 
> I'm not talking about a 50 gallon barrel at the bottom of a downspout. I'm talking about 10k gallons or bigger, stored either above ground (not ideal) or below ground.
> 
> ...


I'm interested in something like this as well, so I'll be following along. What are you plans for pumping and gaining access to the storage supply? Trash pump?


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## cglarsen (Dec 28, 2018)

Deltahedge said:


> Does anyone have experience with large scale rain water harvesting?
> 
> I'm not talking about a 50 gallon barrel at the bottom of a downspout. I'm talking about 10k gallons or bigger, stored either above ground (not ideal) or below ground.
> 
> ...


I've designed some conceptual RWH systems and have access to a model if you wanted to run some numbers on cistern sizing. Your 27k gal figure assumes 100% runoff from one acre. That would be from an impervious surface like roof or parking lot. You could reliably capture water from your home but I'm not sure about collecting runoff from vegetated areas. You will have naturally occuring bacteria that could cause problems in storage system. Most systems I've seen for crop irrigation collect from rooftops as it is still high quality water at that point.


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## Deltahedge (Apr 1, 2020)

cglarsen said:


> I've designed some conceptual RWH systems and have access to a model if you wanted to run some numbers on cistern sizing. Your 27k gal figure assumes 100% runoff from one acre. That would be from an impervious surface like roof or parking lot. You could reliably capture water from your home but I'm not sure about collecting runoff from vegetated areas. You will have naturally occuring bacteria that could cause problems in storage system. Most systems I've seen for crop irrigation collect from rooftops as it is still high quality water at that point.


I typed that part up in a confusing way. I was talking about the water required to irrigate 0.5" and not the volume of water I would capture.

I'm guessing I would need to make some assumptions on how much rain the ground would absorb before runoff would make it into the catch basins. I can tell you that there is standing water in the backyard over nearly half of it probably 20 days a year. With big rains, sometimes there is standing water for 3 or 4 days, and I just look out there wishing I had a way to get it into storage and off the grass (nutsedge is the only thing that likes the standing water)


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## cglarsen (Dec 28, 2018)

Deltahedge said:


> cglarsen said:
> 
> 
> > I've designed some conceptual RWH systems and have access to a model if you wanted to run some numbers on cistern sizing. Your 27k gal figure assumes 100% runoff from one acre. That would be from an impervious surface like roof or parking lot. You could reliably capture water from your home but I'm not sure about collecting runoff from vegetated areas. You will have naturally occuring bacteria that could cause problems in storage system. Most systems I've seen for crop irrigation collect from rooftops as it is still high quality water at that point.
> ...


Ok, yes, correct. Where you live, assuming sandy loam soils with underlying clay restrictive horizon, you don't see any appreciable runoff from storms producing less than 0.7" unless it comes down really fast. Anything over that threshold will provide stormwater - I can tell you how much runoff in gallons per acre you'd expect. Ex. 2" rainfall would produce about 10,000 gal runoff - assuming you could drain and collect it all.


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## Deltahedge (Apr 1, 2020)

cglarsen said:


> Ok, yes, correct. Where you live, assuming sandy loam soils with underlying clay restrictive horizon, you don't see any appreciable runoff from storms producing less than 0.7" unless it comes down really fast. Anything over that threshold will provide stormwater - I can tell you how much runoff in gallons per acre you'd expect. Ex. 2" rainfall would produce about 10,000 gal runoff - assuming you could drain and collect it all.


Ok, that's a good assumption of my soil. Pretty spot on. I need to go through and look at the data and see how many days a year on average we get more than about 0.7"

And also, to drain a larger surface area, it requires more trenching and underground pipes. But I might also use the underground storage as a staging area for a slower GPM well to be able to hit higher GPM rates into an irrigation system.


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## Amoo316 (Jul 8, 2021)

Dumb question here, but if you drill a well, how does the city have any way of knowing how much water you are using from said well?


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## Deltahedge (Apr 1, 2020)

Amoo316 said:


> Dumb question here, but if you drill a well, how does the city have any way of knowing how much water you are using from said well?


The rule is from a local subsidence district. They require a permit before the well is drilled, and a meter to be installed. As far as I've found so far, all the well drilling companies will only do it by the book with permits. The home owner is required to submit well logs annually along with their utility bill totals. The subsidence district also has the right to come check your meter to make sure it matches the log you submit to them, and to make sure no one has tampered with it.


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## Amoo316 (Jul 8, 2021)

Deltahedge said:


> The rule is from a local subsidence district. They require a permit before the well is drilled, and a meter to be installed. As far as I've found so far, all the well drilling companies will only do it by the book with permits. The home owner is required to submit well logs annually along with their utility bill totals. The subsidence district also has the right to come check your meter to make sure it matches the log you submit to them, and to make sure no one has tampered with it.


Wow, thanks for answering that. I was afraid you were going to say they put a meter on it. The nicest thing I can say about it and keep it forum safe is that is the dumbest thing I have heard. I'll just leave it at that, but I would not exactly be happy about that.


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## rotolow (May 13, 2020)

I recall thinking about this and when I ran the numbers there was no chance I could catch enough rainfall to make up the cost on the cisterns or pumps. I've got ~1.5a here and about 3.5Ksqft of roof. I think the calculation was I would harvest a few thousand gallons every 1 inch or so of rainfall. I have 30K of turf irrigated, so no chance that it would make a measurable impact in my location.

Once it hits the ground you might as well forget about trying to collect it. We might have a handful of rain events every year to get a significant amount of runoff to collect.


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## Deltahedge (Apr 1, 2020)

Yeah. I need to run the math on how much rain I could collect from roof and concrete areas, vs how much rain I might be able to collect from in ground drains in the lawn.


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