Most economical catchment surface besides roof?

Greetings! This is my first post here. Thanks so much for the opportunity to be involved in this community!

We're planning a rainwater harvesting system to be used as our sole water supply (at least initially) here in AZ.

According to my rough calculations, that means a minimum of 5500 sq ft of catchment surface. That's to start. Ultimately I'd like to be more in the neighborhood of double that.

Since we're going to be living 'small' (I can't go so far as tiny), our combined home / garage footprint won't come anywhere close to that.

What I'm trying to figure out is what type of structures, and what materials, would lend themselves to large permanent catchment surfaces for the least $$ / sq ft. ?

Obviously, it would be optimal if they served some other purpose. Like a generic shade structure, gazebo, greenhouse, plant shade structure, solar panel mounts (we'll have some, but not that many lol!), etc.

Snow loads would be present, but light (maybe 2-3" at any one time tops).  Wind could be a problem during storms / microbursts. During the monsoon season we get a lot of rain, all at once, so that would be something to consider as well.

If anyone has any ideas, I'm all ears. Thanks in advance for your insight & assistance!

- Roland

Roland, there is a bit of maths to go through to give an answer.
I live in an area that has about 450mm annual rainfall and I use 20,000L tanks.
I see that 'much of Arizona is characterized as arid to semiarid, with annual average precipitation ranging from less than 4 inches
in the southwest to around 40 inches in the White Mountains in the east-central region.'
So I will start with some questions;
- What area are you in?
- What is the annual average rainfall?
- What is your water consumption/
    - wasteful or frugal
- Tanks need to be big enough to clean and purify the water and hold about 3 months supply.

The tank volume varies according to;
- consumption rate
- rainfall intensity
- length of dry spells
- area of roof

Roland,

I would like to recommend watching some of Brad Lancaster's videos:

 Embed this video

His Books:

https://permies.com/wiki/51855/Rainwater-Harvesting-Drylands-Brad-Lancaster

If you've got a monsoon season, can you do something temporary, just to catch the bulk of that water? I'm thinking just laying down a tarp or whatever kind of material you're comfortable with, and having it funnel into either your main tank or a intermediate one. You'd need some open area and a slope for that. Then you can roll up your catchment surface and store it away until next year.

I am thinking similar to Jan.  Some 6 mil plastic with some careful contouring of the ground may be your best immediate bet. It doesn’t have to be in place all year.  But, for something more permanent, you might want to walk though with John Daley.

Hi Roland,

Welcome to Permies.

Thanks so much for all the replies, and the warm welcome! I so much appreciate it!

To be clear, I think I've got the math side of things pretty well sorted out. Granted, once put into actual practice I may find out I'm wrong, but I'm designing things so I can add capacity / catchment area incrementally if needed.

Also, I don't need this to be functional now... there's no rush. I'm planning ahead for building permanent structures, not something make-shift to get me by. Right now I'm mostly trying to determine if it's feasible for us to make a rainwater-only homestead work within our anticipated budget.

Going to start going through the links & videos posted. Thanks a bunch!

Every time I’ve run the numbers for lowest life time cost (20-30 years minimum) the winner has been a basic agricultural pole barn. Greenhouses or shade structures are cheaper up front but replacing the plastic adds up over time.

It really depends on what else you need that you can “share the cost” with. If you plan to grow a lot of stuff that needs shade, the shade structure probably makes the most sense. If you need a shop or barn or secure storage, the barn probably wins.

I do know someone that built a HUGE pole barn roof with no sides and then parked an rv in it while they built a straw bale house under another corner. It didn’t stop the wind but provided the only shade on the property plus a place to store stuff and work during the rainy season.

I'm mostly trying to determine if it's feasible for us to make a rainwater-only homestead work within our anticipated budget.

The Rainwater part is much more economical than wells etc.
Rainwater only needs a small RV 12V pump to circulate a lot of water and that is easily solar / battery powered.

R Scott wrote:Every time I’ve run the numbers for lowest life time cost (20-30 years minimum) the winner has been a basic agricultural pole barn. Greenhouses or shade structures are cheaper up front but replacing the plastic adds up over time.

It really depends on what else you need that you can “share the cost” with. If you plan to grow a lot of stuff that needs shade, the shade structure probably makes the most sense. If you need a shop or barn or secure storage, the barn probably wins.

I do know someone that built a HUGE pole barn roof with no sides and then parked an rv in it while they built a straw bale house under another corner. It didn’t stop the wind but provided the only shade on the property plus a place to store stuff and work during the rainy season.

This proves very useful! Thank you and again, everyone else that's posted!

But yeah, that is/was our delimma... what 'useful permanent surface' could be built for the least amount of $$ that would do the job as a catchment surface.

We do want / need structures such as a detached garage, and yes probably a greenhouse too. But we don't necessarily want/need/can afford any of the structures - even combined - to be nearly as big as they'd need to be to satisfy the whole of our collection requirements. I was looking at steel buildings yesterday that were WAY TOO BIG for anything we'd ever need to place inside them, just because I needed the roof space. That just didn't seem like the smartest way to go about things lol.

I really dig the example you cite. That might just be a very workable solution for us, though I think we might want 2 identical structures rather than 1 huge one.
Did the straw bale home ultimately comprise only one corner, or did they 'frame out' the entire space?

They ended up with several little straw bale bedroom cabins as the kids got to be teens.  Scattered along the most storm protected corner.  The cabin roofs were giant pallets covered with bales and thin cob plaster. They just needed air/bug barrier because the big roof protected from water.

What I'm trying to figure out is what type of structures, and what materials, would lend themselves to large permanent catchment surfaces for the least $$ / sq ft. ?  

Structure: driveway
Material or catchment surface: hard packed dirt
Like yours, our climate is very dry (11” water / year). When sloped land is extremely parched and packed, the water does not infiltrate but rolls down hill.
I take advantage of this effect with the design of the 3500 sq foot driveway sloping about 2% from street to collection basin. The hard pack drive is surrounded by 45 degree berms that funnel the water in the driveway to the basin. If your driveway is hard pack, nearly impervious, and has a collection feature at the low point, this could add to your total collection surface.
Street runoff, as seen in the Brad Lancaster video linked by Anne Miller, diverted into the driveway can increase the catchment.

I guess its best to just start, and get rolling.
Otherwise you may analyse it so much you get overwhelmed

Amy / Anne,

These are strategies I never even considered. Thanks so much! Still doesn't negate the need for other catchment surfaces in our case, but with some planning should hopefully shrink that demand.

John, I won't get overwhelmed analyzing. I prefer to do as much research & planning up front as possible. Sometimes it's excessive, but it saves me big-time in the regrets department.

I'm on the border of AZ and NM, we are building here. We erected a metal garage a couple years ago, and had a bigger metal building erected last year that will become our home.

In looking into lots of different building methods, one of the factors that effects cost is how far/close you are to a place to get building materials. We are very far.  We compared using concrete block, metal, and earthbag building to build a garage. We didn't compare wood, because it's both a fire risk, and extremely termite prone in the desert. At this moment in time, I think wood may be even more expensive than metal.

Earthbag building would be the cheapest of those methods we considered, but would have taken us way more labor than we could commit, we realized.

Concrete block and metal tied in cost at the time, but concrete took more labor and of course requires water availability.

We went with metal. We got bids from a bunch of companies. Regional ones had the best pricing for what they provided.  One of the awesome things with metal buildings is that they deliver them almost anywhere.

I also looked into the cost of carports from the companies selling both "red metal" and "white" or tube metal buildings, and we visited locals who already had those carports.  The carports are nice and held up in our incredible winds, and are priced very competitively.  I would still reinforce a carport more than the kit comes with for added safety.

Here are two companies who sell metal carports in AZ:

Mueller red metal buildings carport kits

Absolute Steel metal carport kits 90mph wind rating

I'd recommend getting these types of buildings with a snow load rating if you can, just so you can feel comfortable adding things to the roof if you wanted to later on.

In the desert, you could also turn a carport like this into a greenhouse. We have one of these lean-to greenhouses, and it's wonderful.  We grow things in there all year. Right now it's full of starts, tree seedlings for our permie projects, and all sorts of plants in the ground like ginger, galangal, peppers, and herbs. In winter we grow cilantro, radishes, turnips, peas as annuals, and still keep the perennials in there, like the peppers, ginger, etc.  Then start all sorts of spring and summer annuals nice and early in the year!

Some people think these shaded greenhouses wouldn't work, but in the desert they seem to work better for many things, in my experience.

You've already received so much great information. I thought I'd just chime in from an area with similar rainfall pattern to suggest that you make separate storage tanks for different uses of water. You might have one small roof catchment that if a very high-quality surface that you make sure to keep clean and well maintained and then direct through a filter into a take of water for human consumption also using a small high-quality filter after removing it from the storage tank (like a Berklee filter).  This is your drinking and cooking water. Maybe dishwashing as well. Then you have another second quality water for showering and clothes washing (and other household cleaning perhaps.). Then thirdly you use road/driveway or patio catchment for water that can be used for livestock and irrigation purposes.

Also in addition to Brad Lancaster's books and work--don't forget to use Art Ludwig's classic Create an Oasis using Greywater to cascade your water use through your landscape.

I second Melissa's points above.  You could use a pond for irrigation water, and a roof for household water, for example.  Flexibility really maximizes the possibilities.  

There is also a really good book by Art Ludwig about water storage which includes instructions for building ferrocement water storage tanks. These tanks can be made to look very nice, to even match natural surroundings. Or look like an urn!

Water Storage by Art Ludwig

I have extensive experience with catching rainwater.
I can say there is not need for separate tanks for different uses.
If you use a 20,000L tank it has the volume to allow settlement and self cleaning of the water.
I do not use filters for water I am going to drink!
If you use 'first flush' devices the roof  does not need to be kept clean, the device separates any 'dirt'.
My signature has a link to more information.

In my county in WA it is illegal to catch rainwater off of anything other than an existing roof on an existing structure that "WAS NOT BUILT SOLELY TO COLLECT RAIN" so check your laws!

Washington State Rainwater guidelines.
From Rainwater-collection guidelines

"When does rainwater collection require a permit?
Under our current policy, you don’t need a water right permit to collect rainwater, with a few conditions:
Rainwater must be used on the property where it is collected.
Rainwater can only be collected from existing structures that have another purpose other than collecting rainwater.
If we find that rainwater collection is negatively affecting existing water rights in an area, local restrictions may be developed to govern new systems.
However, we do not expect the collection of harvested rainwater to cause problems.
If you are planning to use rainwater as your primary drinking water source for new building construction, you'll need to check with your county to see if it is allowed."
Benefits of rainwater collection
Rainwater harvesting can have multiple benefits, such as reducing stormwater runoff that can sweep pollutants into streams and cause erosion and flooding.
It can lessen the strain on combined sewer systems during rain events, helping reducing the risk of system overflows running directly into streams, lakes, or marine waters.
In some areas of the state, such as the San Juan Islands, rainwater may be the only viable water supply for new construction.
Some municipalities reduce stormwater utility fees for commercial buildings that incorporate rainwater harvest systems.  
While rainwater collection might not replace your primary water supply, it can help keep your gardens green, while diverting water away from your house,
and depending on how much you reduce your public water use, you may see savings on your water bill.