Cloudcroft, New Mexico, is perched at the top of the Sacramento Mountains at nearly 9,000 feet above sea level. From the right vantage point, you can see White Sands in the Tularosa Basin as well as Alamogordo nestled against the foothills. On a clear day, you can see the Organ Mountains this side of Las Cruces. On a clear night, you can see the lights of El Paso reflected off the atmosphere.
Cloudcroft has spent many a year struggling with its water supply, fed by springs and water wells. Given Cloudcroft's home at the top of the mountain, the springshed and aquifers beneath are not that big and are susceptible to drought. At one point, Cloudcroft pined to have the nation's first direct potable reuse plant (and the second in the world!) before a bad concrete pour (and rain?) put the project on the back burner.
Some locals suggest rainwater harvesting as a solution to the village's water woes. Rainwater harvesting is collecting rain from a roof and storing it in a tank for later use. A non-potable system doesn't require treatment, but a potable one does, although that treatment is often just a first flush, a filter, and a UV light.
As a rainwater radical, I am in full support of rainwater harvesting. Our cabin in Cloudcroft has a 3,000-gallon potable system fed by a 2,000 square-foot roof while our house in Austin has a 5,000-gallon non-potable system fed by a 440 square-foot roof. Cloudcroft receives 26 inches of precipitation each year on average while Austin sees 36 inches.
Many installers use rules of thumb or average rainfall amounts to determine tank size. Very few calculate the reliability of supply from a rainwater harvesting system. For example, for our system in Austin, our goal was to not use any city water outside. The state's method for determining tank size is based on average monthly rainfall and suggested that a tank of 2,000 gallons was adequate for the roof of our two-car garage.
However, there's no such thing as average in Austin (I might modify that to "the Western United States"). So I modified the state's spreadsheet calculator to use daily rainfall from Camp Mabry to assess reliability for data collected from 1930 to present. My spreadsheet showed that a 2,000-gallon tank would go dry 4 out of every 10 summers. Given our goal, that's not good. However, with a 5,000 gallon tank, the tank would only go dry twice over the nearly 100-year rainfall record. We installed a 5,000-gallon tank (and, after a dozen years in service, it has yet to go dry).
I recently worked with a graduate student at Texas State University to develop a spreadsheet that uses daily rainfall data to calculate reliability for a defined roof area, storage volume, and daily use. We found that you could have a reliable source of rainwater anywhere in the state, including El Paso, that gets you through the worst drought on record. I have now modified this spreadsheet for Cloudcroft.
Because there isn't a single weather station with full coverage for Cloudcroft, I had to conjure one out of available data. Therefore, I pulled in three different records to have one full record for the village for a period of time that stretches from 1930 to present. There were still gaps in the data (days with no reported measurements), so I filled those gaps with 10-year back averages for those days (94.1 percent of days had a precipitation measurement). Based on a cumulative rainfall chart, there is not a trend away from long-term mean rainfall.
Unlike the vast majority of Texas, Cloudcroft has a proper winter with frozen precipitation (such as snow and sleet). The weather stations include equivalent rainfall amounts (how much rain there would be had the frozen precipitation fallen as rain). However, all of this "rain" probably doesn't make it in the tank, and probably not on the day it fell. The snow may sublimate (evaporate) straight from the roof into the sky or it may simply blow off or slide past a gutter as an avalanche (Look out!). Given this, the spreadsheet likely overestimates reliability. Having said that, this past winter it seemed as if much of what fell frozen from the sky did wind up in our tank. So there's that (and temperatures are rising).
I see reliability as key to rainwater harvesting, although most do not calculate it. For example, if your rainwater tank runs out, you will then switch over to the back-up supply (the village supply) or haul water. If your tank runs dry, it means there's a drought, which means you (and your neighbors) suddenly increase the demand on village water supplies at the worst possible time, when it is also struggling with drought. So it's important to design rainwater harvesting systems to be reliable where community supplies respond to rain. As an added bonus, if the village runs out of water, you will have some!
So what does my model show for Cloudcroft?
In short, if Chief Brody was hunting rainwater systems instead of sharks, he would have said "We're gonna need a bigger tank--or a bigger roof!"
A water-efficient person (who, indeed, bathes every day) uses about 25 gallons per day. So that requires either a 3,000-gallon tank with 2,000 square-feet of catchment or a 5,000-gallon tank with 1,000 square-feet of catchment. Unfortunately, the addition of more reliable water is not linear, meaning we need even more roof area or tank storage to support two water-efficient people. We need about 10,000 gallons of storage with 2,000 square-feet of catchment to support two people (50 gallons per day), more than three times the storage to support one person. For a 1,000 square-foot roof, it's impossible to have enough storage to support two people.
For our cabin (3,000 gallons of storage and 2,000 square feet of roof area) with full-time occupancy, the lowest tank level would have occurred on July 3, 2011, for 23.8 gallons per day of use:
This means that our system is barely enough for one person let alone two people, occasional guests, and eight cats (although cats, like modern ovens, are self cleaning).
Given that the rainy season in Cloudcroft is during the summer's monsoon season, it may be that a smaller catchment and roof could amply support multi-person summer cabin use (that, at least, is what we are hoping). This is also when the Village's demand for water doubles, so it is also the critical time for needing new water. So I further modified the spreadsheet to allow for seasonal occupation.
For our cabin (3,000 gallons of storage and 2,000 square feet of roof area), after we retire from our jobs in Central Texas, we plan to become "Summer Cloudcrofters" in July, August, and September. If we only use the cabin these three months out of the year, our reliable daily use would rise from 23.8 gallons per day to 73.5 gallons per day. That's plenty for two people (and eight cats). The lowest of the low occurs after the summer of 1973:
We've often wondered if 3,000 gallons of storage (which is not a lot) was enough storage for our summer demands, but it clearly is.
If we were renting out our cabin during July, August, and September with 50-percent occupancy, our daily use could grow to 147 gallons per day.
It seems that tourist season runs from May through September (when the kids are out of school). Reproducing the plot above with occupancy only for the five months from May through September results in:
This seasonal-only use produces an almost doubling of reliable daily use. Nice.
So what does that look like for our cabin? Full occupancy during those months results in a reliable supply of 46.5 gallons per day. If we doubled our storage from 3,000 gallons to 6,000 gallons, we could pull 82 gallons per day without risk of running dry. That system (and use profile) would hit its low on July 19, 1989:
Note that the defining drought is different for each of the examples above. That was one important finding of our research: the defining drought is a function of catchment, storage, use, and rainfall.
some conclusions
So, in the end, rainwater harvesting is a viable source of water for Cloudcroft, especially for the summer, monsoon months. Given that water demand doubles in Cloudcroft during the summer, this is good news. It's also good news that large tanks are not needed for storage during the summer months. For example, for our planned use of our cabin, we would have a drought-proof supply of water for the two of us with only 2,000 gallons of storage. Having a larger tank (3,000 gallons) allows a safety factor for a drought worse than the drought of record, a leak, or thirstier-than-expected felines.
some other considerations
Because of Cloudcroft's climate, you ideally want your system inside the thermal envelope of your house or in a structure with enough heat to keep the system from freezing during the winter months. You might be able to winterize an outdoor system every early fall and then bring it back to life every late spring.
There's chatter that the folks in town that have rainwater harvesting systems don't use them. I don't know about that, but we've had some challenges with ours. One is that our system pulls from the bottom of the tank, a big no-no for a potable system. There is always dust and whatnot that gets into your tank from harvesting off of a roof. A potable system has a first flush, where a certain initial flow of water is diverted away from your tank. But there's always sediment that gets in. When you pull off the bottom, you are pulling that sediment into your drinking water system. The first time I fired our rainwater system up I watched our filter turn from White Sands into the Malpais in less than 10 seconds. Potable systems really need a floating intake that keeps the intake six inches or so below the water surface and off the bottom (something we'll [hopefully] get installed this summer).
Another thing we need is a water-softener bypass. Rainwater is the closest thing to distilled water that nature provides. That means that it is "aggressive" in wanting to dissolve stuff, so when it goes through a water softener, it dissolves a lot of salt. Rainwater is already soft, so there's no need for water softening (and you can avoid salty ice cubes...).
Rainwater harvesting is not cheap. By my rough estimates, it comes in at about $15,000 per acre-foot of firm yield per year (that's about 325,000 gallons per year). That's expensive, at least by Texas urban standards. One thing rainwater has going for it is that it is scalable, meaning that if you want a little, you can get a little, or if you want a lot, you can get a lot. Many other, more affordable water supplies come in large tranches. Furthermore, when you don't have a lot of choices, rainwater may be all you have.
Finally, I wonder about liability for rental units. A rainwater harvesting system places the safety of drinking water in the hands of the homeowner. Given that, there could be greater liability when you rent a place out. One thing to consider, at least for new construction, is using city water for drinking water (for the faucets) and using rainwater for everything else (showers, laundry, washing machines, dishwashers, toilets). This would be more difficult for existing structures.
If you want to play with this spreadsheet on your own, here it is:
Note that this version is for when the rainwater is used from May 1 through the end of September. If you'd like a different version, please let me know in the comments below!
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