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Building rainwater catch system for irrigation

 
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Hi there.  My wife and I are building a garden this winter/spring and part of it will be a fairly large rainwater catchment system.  I've been working on the design for a bit now, including both the catchment, storage and delivery of the irrigation water.  The garden will be about 2500 sq ft and in the Pacific Northwest where we live we get plenty of rain *most* of the year.  Typically we get ZERO rain from late June to early September so our goal is to collect and save enough to get us through that period.

Design Strategy

First, we don't want a system that makes our property look like a jungle gym playground with overhead pipes running all over the place.  We want to build one that is mostly hidden and out of sight.  Second, from my research, using purpose built rainwater collection equipment for this project would bankrupt us.  So we will be putting this system together with off the shelf parts normally used for other things.  The pumps are inexpensive and used for shallow wells and basement sumps.  The irrigation side is all off the shelf lawn irrigation parts and the plumbing will be mostly PEX and some PVC.

The Plan

We have a pole barn beside the garden location with a 1200 sq ft metal roof and gutters.  According to my calculations that should gather about 750 gallons from a 1" rainstorm.  We get an average of 52" of rain each year so we have PLENTY available to collect.

The gutter downspouts are now all plumbed underground to a single pipe that runs out into the property behind the barn.  That pipe that carries all the rainwater will have a diverter valve (4" septic drain field diverter) installed inline.  When the valve is in the 1st position the water will "bypass" being collected and run out where it currently does.  In the 2nd position the rainwater is diverted into a sump made from a buried, plastic 55 gallon drum with the top removed and a lid fabricated.  In the sump is a 1/2 hp sump pump with a float switch and check valve.  The pump has an 1/8" metal screen wrapped around it for filtering the small amount of debris we get in those gutters.  There is a screen filter on the outlet of the pump (from a swimming pool pump) before the water is dumped into the first tank.  The 3 tanks are poly, 2500 gallons each and will be connected together on the bottom.  Each tank will have a valve/union on the outlet with flexible sch40 PVC connecting it to the "pump manifold" piping to help eliminate plumbing issues caused by flexing of the tanks.

On the outlet side of the tanks I will have a pair of 1 hp pumps (shallow well pumps) both isolated with valves and with spin down filters on the inlets.  Both will not be used at the same time but the ability to have a ready spare is important so one will be isolated while the other is in use.  The pumps have a pressure switch and will provide more than enough flow for our needs and at a pressure of 55 psi.  The irrigation system will be built using a wifi enabled irrigation controller that can control 16 zones.  Each raised bed will be its own zone and have a dedicated irrigation valve and a single 3/4" feed to be distributed in the bed via poly drip hoses.  There will also be 2 hydrant style hose connections in the garden fed from this system.

I attached a crude drawing of my plan.  The pumps are all here, the sump system is half installed and the large tanks are on order and due in about a month.  The irrigation controller is on the way and I still need to order all the valves but we're making progress.  I won't need to start collecting water seriously until the end of February so I'll have time to do some testing, etc once i get it all installed.  

We're open to suggestions as we've never done this before


 
pollinator
Posts: 2369
Location: Bendigo , Australia
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Great idea!
Can you put the tanks near the position you are planning to put the 55 gal sump?
I doubt you will have a big enough pump to shift all that rain when it comes.
I suggest you fill the tanks from the top and have  2inch flexible connections between them separate from the discharge pipes.
This will ensure they all get filled and the pump is not fighting to get the water into the tanks.

Use large diameter pipes 2 inch from the tanks to the pumps and also run the 2inch or maybe 11/2 " depending on flow rates  completely around the garden [ full loop ] so pressure drop off is minimised,
then smaller pipes in loops through the garden. Keep them all above ground so you can see them.
I suggest you dont go down to 1/2 " pipes until very close to the plants.
Some solenoid valves need very high pressures to actually work, better check they are ok to use, or fit a bigger pump.

 
Jason Nault
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Thanks for the reply!  A lot of the things you suggested are actually what I was planning but didn't include in the original post, so many details!  

The sump has to be where it is and the tanks will be about 20' away.  The tanks need to be on compacted, flat ground as they will weigh about 63,000 lbs when full so they will sit beside the building.  The sump is slightly lower in elevation because gravity is how the water from all 6 downspouts gets to the sump.  The sump pump we're using is rated for 4000 gph so unless we get 5" of rain in a single hour I doubt it will overrun the pump.  If it were to be overran, the excess water would go out the sump's overflow and back into the pipe that takes it out to the back of the property.  We would lose some water but we have far more than we need, we get 52" of rain per year and need about 10" to fill the tanks.  ‚Äč

The tanks ARE to be joined at the bottom using flexible PVC and the sump pump will flow in to the top of 1 tank, then be distributed slowly to the others.  

We aren't using anything smaller than 3/4" and that is just the feeds to the raised beds from the irrigation valves.  We DO need to bury the pipes as we do get a few freezes each winter and we don't want anything to be visible or a tripping hazard.  

The valves we're using are rated for 15 - 150 psi and the pump will pressurize the system to 55 psi so we should be fine there.  The irrigation controller we're using only activates one zone at a time so the most we'll be able to use at once is a 3/4" flow (going through drip irrigation so likely even less).  Not worried about flow or pressure yet but if i have to upgrade the pump that would be easy enough.  I've read that people run whole houses with this pump so I'm optimistic it will be ok for our purposes.

Discharge pipes from the tanks will be 2" and the discharge pipes from the pumps will be 1 1/2".  That 1 1/2" will run through the garden and the valve boxes (4 valves each) will tee off of that.  My wife is handling the irrigation in each raised bed, I will leave a 3/4" NPT fitting in each that can be adapted to whatever type of irrigation that raised bed requires.

Hoping to get the diverter valve, sump and pump installed this weekend.  I'll try to post some pics when I have something worth looking at
 
pollinator
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Location: Denmark 57N
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I only have one concern, the pump can move a lot of water and drops it into tank 1 but only gravity is redistributing the water to tanks 2 and 3? I fear that the water will not move around fast enough and you'll end up with an overflowing tank 1 at peak rain, unless the pipe connecting them is a similar size to the down pipe the water originally enters the system from.
 
Jason Nault
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Thanks, that is a valid concern.  The pipe entering the sump is 4" and the pump outlet is 1 1/2" so there may be an issue there.  The nice thing is it doesn't often rain super hard here, it's more like a constant drizzle for 8 months with a few periods of heavier rain tucked in there.    I may go with a 2" pipe from the inlet strainer to tank 2 instead, that way the flow will be slowed a bit.  This is still a bit of a trial and error thing so hopefully this works, can always modify it if it doesn't  It's looking like I'll have some time to get out and work in the dirt later today or tomorrow for sure.  We also heard from our excavation guy and he'll be out in a week or so to get the garden space leveled and get started on a small retaining wall.  The 10' x 12' greenhouse arrives in a couple weeks and the tank just after that.  After a year of planning it's finally starting to happen!!!
 
John C Daley
pollinator
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Location: Bendigo , Australia
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A couple of extra points picking up on Skandis comment.
The discharge pipe could be altered to fill each tank with T pieces.
If the sump pump discharge pipe was say 2 inch instead of 11/2 less power would be used because the frictional losses would be less. Those loses occur because of water speed.

I would encvourage the use of ball valves rather than gate valves, they break from time to time.
 
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