# Rainwater harvesting



## MMoore (Aug 8, 2018)

Anyone harvesting rainwater for irrigation?

I have a pretty optimal downspout system in place now to store some rainwater from my roof, im just wondering how to implement it and how much storage to aim for. I have a downspout that runs half of my roof area under my deck through the top deck boards and I could easily put a pad under the deck itself for as much or as little rainwater storage as I can afford.

QUESTION
I doubt I would be able to harvest enough water to irrigate all summer so that's the hardest part of planning this thing to work. Would I need to plumb a system with a float to fill up the tanks to keep the sprinkler running in the event of low water or can you not run the pump and switch the whole system over to run off of domestic pressure from City water?

im just starting my research into how I can make this work.

cheers,
Matt


----------



## corneliani (Apr 2, 2019)

As a starting point, 1" of water over 1000 sqft (90 sq m) is roughly 500 gallons of water. This should give you an idea of how much storage you'll need for 1 deep watering cycle. I've looked into this idea while building my house but the expense of it makes it unreasonable. Trying to calculate a break-even point is impossible. A good pump can run you $500+, and if you add in a controller that will tie in your municipal water can run in the thousands $$. Then you need a good container that won't allow bacterial growth, etc, not to mention a good filtration system to keep organic matter (leaves, etc) out. The one lingering thought I'm still considering is a 'repurposed' storage tank with a submersible pump that could drive a couple big sprinklers for use during droughts, etc. This would allow the accumulation of rainwater over time, in approx 2500 gallon container (for my non-irrigated section) and used as it fills up. But if you already have municipal water the system you're thinking of is much too expensive to implement. Just my 2c.


----------



## ShilpaMann (May 12, 2018)

MMoore said:


> Anyone harvesting rainwater for irrigation?
> 
> I have a pretty optimal downspout system in place now to store some rainwater from my roof, im just wondering how to implement it and how much storage to aim for. I have a downspout that runs half of my roof area under my deck through the top deck boards and I could easily put a pad under the deck itself for as much or as little rainwater storage as I can afford.
> 
> ...


I am doing the same thing, and I will tell you, your roof might be handling a considerable amount of water.

In my area, a "big" storm rains about 6.4in/hr ( https://www.thisoldhouse.com/sites/default/files/rainfall-intesity-chart-01.pdf ) ... but the last 3 years we have had multiple storms that are considerably more than that. Using my roof area, I calculated a "normal" storm (~1 in of rain, which is a small-ish storm out here) to drop about 1,300Gal of water on my ~2,000sq.ft roof. We recently had a storm, which I didn't think to be crazy, and just one of my 4 downspouts filled a 275Gal tank in about 15 minutes (meaning my 1" estimation is low). The storm ultimately lasted about 1/2hr. So, if I was going to try and store that one storm worth of water (admittedly, not my goal), I would have needed about ~2,500 Gal of storage space. My irrigation "system", which waters 4 cannabis plants, my ~200sq.ft garden, and some hanging plants, would take the entire summer to use that single storm.

Now, you're watering your lawn, so that is a lot more water. However, when it rains, you both collect water AND get free lawn watering. So, really, you need to figure out how much water you would need "between storms".

As for pumps, as corneliani mentioned, they can be expensive. However, that depends on how creative you want to be, and if you're willing to build a water tower/have a lot of elevation change in your yard. Assuming you don't want to make a 60' water tower that you pump water into with a high-pressure/low flow pump, a used pool pump will get you at LEAST 25PSI for ~$50 (where I live); this isn't ideal for the pump, as they're a pretty high-flow design, but if you configure your zones properly to ensure the pump is running within spec, it would be serviceable (we used to do it a lot for waterfront properties who could pull free water from the river). At worst, you're down ~$50 + some connectors. The storage tanks, OTOH, are potentially very expensive. I personally sourced some 275Gal tanks for about $30/ea (which is quite cheap), but I ultimately want 10 of them, so that's a lot of cash (and THOSE connectors aren't cheap).


----------

