Reliable Input Measurements
Passive Rainwater Harvesting
We "passively" divert rainwater into a dry wash where basins slow and hold the flow so it can slowly seep into the surrounding plantings. The academics call it passive. I am here to tell you building the system was anything but passive. A lot of "A" Mountain "soil" was moved. Which frequent readers know is no mean feat!
We buried half of my beautiful back wall...
- The primary source for the passive system is that we graded the yard so any rainwater that can not be directly absorbed by the "soil" runs off from where it falls into the basins. When a basin fills, it overflows into the next basin. And so on.
- In heavy rains, we add rainwater flow that has been diverted from up the hill at Peggy and Al's basins into the easement at the back of the property. From there we divert it through the grate we put in the back wall and into the dry wash basin system through our side of the compound. Any overflow then ends up going into DnT's side where they also have a basin system.
It's a little hard to see the basins, but this shot is from the top of the dry wash. It runs down around the boulder and saguaro and then continues bending to the right out of sight. The soil that is reddish are walking paths. The more grey rocks and dust ("soil") are the dry wash basins.
Here is a shot from a little farther down the wash after a 0.8 inch overnight rain. We got a little flow through the wall in this event. Cool thing is by lunch time the water had percolated into the ground and the basins were ready for another go round.
Active Rainwater Harvesting
We also actively harvest rainwater off the roof. The gutters are connected to pipes that connect at the other end to the cistern in back behind the dead tree. This is called a "wet system". Which means the piping from the gutter downspouts is part of the cistern. The picture below is quite similar to our system. Like almost exactly. Our tank inlet and overflow are a little different but accomplishes the same thing using what I think is more elegant plumbing. The fact that it leaks a little notwithstanding. We're not using the system for potable water so we've eliminated the first flush and filtration stuff. Our pump system is more basic.
You'll notice the only air gap is between the gutter and collection pipe. Everything else is sealed and "wet". What's cool about this, is it allowed us to put the cistern in a convenient spot way out back, AND, you can tap into the system anywhere and have the same water availability as anywhere else in the system. What's uncool about this is if you have to work on any of it, you have to drain all of it. I've been able to do everything so far without draining it.
the Quail Manor cistern is rated to hold around 2,800 gallons. It's on the order of 10 feet in diameter by 7 feet tall. These things take space! Since the overflow is positioned so water in the system will always be below the bottom of the gutters on the roof, the capacity is actually closer to 2,250 gallons. Interestingly, the piping from the roof to the cistern holds around 90 gallons of water before the cistern actually starts to fill. So our total capacity is on the order of 2,340 gallons.
Here is an extreme close-up of the cistern.
The cistern is wrapped in the same mesh as the Quail Manor Fences. You can't really see from this perspective, but vines are growing on the mesh. In the fullness of time the vines will obsure the cistern and provide a bit of shade. The cistern is painted to protect the plastic from the sun. Tucson sun is death to plastic and has to be painted.
The grey pipe is the plumbing from the Quail Manor roof. The guys did not get it very plumb. To fix it I'll have to drain the cistern pretty close to empty. Maybe before the next monsoon...
- The upside down "U" in the foreground is the overflow. Water can get no higher than the top of the upside down "U". Which coincidently is the same elevation as the bottom of the lowest gutter on the roof. Lasers were involved. We needed every inch of capacity. We got the cistern as low as we could during the grading process. It rests on a nice bed of sand that in-turn rests on a nice layer of caliche . Once you hit caliche, you're done digging.
- We learned the hard way that once it starts to overflow, the overflow will siphon the cistern down to pretty much empty. We fixed that with a vacuum break in the horizontal pipe at the top of the "U". In other words we drilled a small hole so some air could get in and prevent the siphoning from happening in the first place.
- The overflow dumps into the top of the passive dry wash, so even if we overflow, the water still gets used. The dump pipe is the one just on top of the ground in front of the hose.
- The pipe coming out of the ground just behind the overflow dump comes from the south gutter on the garage end of the house.
- The pipe running around the right side of the cistern is coming from the gutter on the north side of the roof. A fun fact. It is not buried because we (the cistern guys) ran into some caliche as they were trenching. Unfortunately AFTER they cut several irrigation lines that had been pointed out before they started.
- The two flows collected from the gutters join in the tee that then goes in the bottom of the cistern. This tee is where there is a small leak. Gluing big diameter pipe is a little challenging, but it appears they dry fit all the tees and whatnot and forgot to glue a couple of pieces. They own up to it, but would have to drain the cistern to fix. So it leaks...
- The hose bib at the bottom of the cistern is pretty worthless for attaching a hose to. No pressure (more on that later). Frequent readers may recall it IS very useful for draining the cistern. Since this picture was taken, I removed the handle so the hose bib is decoration. Until I need it.
- You can just see at the top of the cistern a black cover. That is an access port. Pretty sure my fat ass is not going in there...
- This picture does not show them but there are a couple of air vents in a couple of places at the very top of the walls so water will flow easily in and out of the tank.
Inches of Rain is How Many Gallons of Stored Water
The Intensity of a rain event has a dramatic effect on how much water we collect. A slow steady rain, typical of winter rains. will not trigger runoff from up the hill through the back wall at all. But it will fill the passive basins quite nicely.
Further we "actively" collect less rain off the roof. This is VERY dependent on the intensity of the rain event. Not certain what the physical science is behind why. Suspect it might be a surface tension thing that there is more water clings to the roof material so less flow. Maybe someone out there will have some ideas. if you do, send me an e-mail.
The active system calculation estimates are based on a "runoff coefficient". Pretty much a fudge factor based on the roof material. Metal, like we have, is supposed to be the gold standard. During design, we used a "standard" coefficient of around 0.8 to be conservative. Our experience suggests in monsoon, no way am I going out there, type events, this is actually a little low. We're now using 0.85 for this event type. With lighter, you'd just hunch up your shoulders and jog to the car kind of rain, it is closer to 0.35. A good steady, you'd put on a good rain coat to go out in it, moderate rain comes in with a coefficient of around 0.58.
Given the size and type of the Quail Manor roof, in theory an intense monsoon type rain of about 2.8 inches will completely fill the pipes and cistern from empty. Our experience suggests it takes more like 6.5 inches with low intensity rain events. Truth is probably in between somewhere. There is an overflow if we collect more water than the cistern can hold. The overflow dumps excess water out of the cistern into the passive rainwater harvesting system. So it all gets used.
If you're interested, you can use the cells below to estimate the volume of water any rain event adds to the Quail Manor collection cistern. Enter or scroll to the amount of rain produced by the event (from Peggy's log above) into the cell. The estimated gallons collected in the cistern will be calculated automatically as you change the rain event amount in which ever model you pick. Note the calculator won't work in some versions of Internet Explorer. If it does not work, probably time to do something about your computer! Or, try another browser!
Monsoon Intensity, OPTIMISTIC Estimate
Light Rain Intensity, PESSIMISTIC estimate
Moderate Rain Intensity, Most Likely Estimate
Alternately, there is a spreadsheet containing all my rainwater harvesting computations. Converts inches of rain to gallons and inches of water in the cistern for the variety of estimating models I've come-up with. It also contains the rain and usage events and gauge readings over time in a tab called Cistern Level. It will be the repository for future seasonal analysis as we collect more data.
Look around all you want. If a cell is bright yellow, it is for data entry. If, gasp, you find an error in logic or a bug, let me know ASAP.
But Please. I put a not a little work into this thing. Don't be evil. I think I have stuff locked down, and I do make copies, but please don't change anything except the yellow Rainfall or Observed Gauge Change cells. If you change something accidently, you're probably not evil. Let me know and I'll sort it out.
How Much Rain is in the Cistern Now
We continue to enhance the cistern system to make it more useful. What we're learning is that the good stuff consistently comes out of Australia. Or they come up with it and we copy. If you're interested there are a couple of good sites I find myself going back to again and again. RainHarvest Systems and Rain Brothers Amazon has a shocking amount of this stuff, but I tend to shop at one of the others since they put the work into sorting what's out there.
Measuring the water level is on the one hand pretty simple, but to turn that into information - less so. We've been through several techniques for measuring just how full it is and then translating that into how much "free" water that means we have. Most recently we've landing on a simple gauge and camera set-up. The fact we have a wet system simplifies things a bit. We can measure anywhere. So I connect everything at the hose bib installed by the garage where it's handy, I have room and electricity.
Here is a shot of the gauge system. The gauge is a connected to a float that bobs up and down in the water. Since I did not want to cut any holes in the system, I connected a standpipe to the hose bib and float the gauge bober in that. You can't see them, but in the cap that the gauge attaches to are a couple of vent holes so water can flow unimpeded in and out of the standpipe.
By the way. The aluminum thing in the foreground is what's called the rainhead. It filters out chunks that come off the roof so they don't get into the pipes or cistern. There is one on each gutter. A look down from the cistern access port suggests its working!
To make it a remote sensing internet connected "smart" gauge, we point a web camera at it. I'm as smart as the average Australian! I can't figure out yet how to have the live feed from the camera show-up here, so here is a link if you want to see how much water we have in real time. You'll need account information. Call Steve and he'll pass it along. If you don't know his contact information, send an e-mail and we'll see if we can figure something out. Here's a link to snapshots we take after getting rain or using our bounty of "free" water.
So. Is the gauge calibrated? Of course.
- The red pointer is cistern empty. There will still be a little water left in the lowest pipes, but we'll define this as empty.
- The green pointer that is a little hard to see for the glare is cistern full.
- The smaller black pointer that is at about 14.00 in this picture is the current water level.
Now. What do all the marks mean? Well, that required a spreadsheet.
Between Full and Empty on the gauge, there are 4 major marks. I'll call those Major Marks. Each Major Mark is 25% of capacity. Beautiful. Between each Major Mark are 4 minor marks, which actually means there are 5 minor increments, the 5th increment being the next Major Mark. So each minor mark is 20% of a quarter cistern capacity. This maters because I needed a way to record gauge readings. The format is Major.Minor. So what you see above is 2.2, the second Major Mark and the second Minor Mark. This will be useful as you read on.
Previously, I have computed cistern capacity in gallons, inches, gallons/inch, gallons/cistern ring. Then I related all that to the marks on the gauge.
Doing the math, that says each Major Mark on the gauge is good for about 590 gallons or 17 inches of water. Each Minor Mark is good for about 120 gallons or 4 inches of water. If you're reading this, you might actually be an engineer or at least capable of performing mathematical reasonableness tests. I know I just said each Minor Mark is 20% of a Major Mark so 5 times a Minor Mark should equal a Major Mark. Do the math and my numbers don't tie. But. I did some rounding so the per Mark numbers did not seem ridiculously precise (we are measuring with what amounts to a string and a hose here!). So, the numbers are close to tying, but not exactly.
What this tells us, and that I have cross-correlated between the mathematics, the gauge, and the open a hose and raise it up to the level where water stops coming out technique; the math works and the gauge - accepting the fact the needle is fat and the camera is cheep so a little imprecise to read - is pretty accurate. Brings me joy.
The Rainwater Harvesting spreadsheet contains all the cistern level information. Go to the Cistern Level and Cistern Trends tabs that with Peggy's actual rain measurement, I will maintain (if I remember) so we get the charts and graphs. Same request as before, look around all you want, but don't change stuff. Let me know about failures in logic or implementation. But don't be evil. Evil bad. Steve no like Bad.
Here is the history of water level in gallons (capacity is the horizontal green line) and percent full since the gauge system was implemented. These charts, and others, are in the spreadsheet Cistern Trends tab. If you find any logic bugs or think of other analysis I'd love to hear about it. Send me an e-mail. The big drop on 4-Jan-2016 is when we learned about the siphoning. AFTER we sucked the cistern pretty empty. The passive system downstream was full full full!
Atmospheric Pressure doesn't Move Much Water Down a Hose
The active rainwater harvesting "wet" system is all well and good. Water accumulates in the cistern. But. Any idea how far atmospheric pressure alone will push water down a hose. 10ish feet is how far. And then what comes out is a trickle. Your run of the mill city water pressure varies, but is on the order of 70ish psi. Your run of the mill atmospheric pressure varies with elevation, but is on the order of 14.7ish psi. 14.7ish psi is good for pretty much useless flow at the end of a 10 foot hose. To use cistern water we (SWITBO) were patiently filling and hauling a whole bunch of 5 gallon buckets. Good for Home Depot where we bought them, but not good for getting any good out of the freaking expensive "free" water.
I lack SWITBOs patience. After pondering and researching, we now have pump system that will cycle automatically so we can attach a hose and use the "free" cistern water like you would city water. It runs on the order of 60 psi and changes the game. There are more elegant ways to accomplish 60 psi in the hose, but this is good enough for now.
Further, we have a conceptual design for disconnecting irrigation from the city water altogether and irrigating with cistern water only. If rain does not keep the cistern full, it will fill automatically from city water so the irrigation continues to work. Not free water anymore, but at least we use the free stuff first. Of course my conceptual design came from the Australians. It's basically a toilet float. Going to wait another season or so before we implement.
Genus! Is this beauty in simplicity or what? Could not be simpler way to keep enough water in the cistern to run irrigation. We do need to figure out how much water is enough for a full circuit, but one problem at a time.