Rainwater harvesting – arthur j weaver

On the Taos Plateau of northern New Mexico where I live, nature gives us, at best, about 14 inches (356 mm) of precipitation annually. Nearby areas in the warmer Española basin and Río Grande del Norte may only receive half this amount. With rare exceptions, most people seem to believe that the only way to have enough water for their home is to have a deep well. This is categorically false.

The basic formula for calculating how much rainwater you can harvest is simple: 0.623 × flat roof area (ft²) × annual precipitation (in) = gallons, where the ‘flat roof area’ is the footprint of your roof projected on to the ground – basically the area covered by your building. For example, the roof on my home has a footprint of 24′ × 40′ = 960 ft², and ground-mounted solar arrays add another 417 ft² for a total of 1,377 ft² of collection area. Assuming 14 in of precipitation per year, this should yield 0.623 × 1377 × 14 = 12,010 gallons of water annually. That works out to about 33 gallons per day. I’m collecting data and will share it here to validate the calculation. More complicated calculators are available.

How much water is ‘enough’ is up to you – ultimately, it’s a value judgment. 33 gpd is more than enough for me. Rainwater harvesting is a simple, environmentally benign, ecologically balanced way to obtain water provided that this limited and precious natural gift is used thoughtfully. It is far less likely to be contaminated by heavy metals and industrial pollutants than well water. And like solar energy, it’s free once you have your harvesting system in place. But don’t take my word for it . . .

Consider the words of an expert:

For example, in my desert city of Tucson, Arizona (where we get an average of 11 inches [280 mm] of annual rainfall), more water falls as rain on our city in an average year than the entire population of the city (over half a million residents) consumes of municipal water (the bulk of which is imported/pumped in at great cost from the Colorado River over 300 miles away) in a year. So, we’d typically have more free local water than we need if we’d consciously harvest it, rather than wastefully drain it away. And we don’t need any fancy equipment or large infrastructure to do so.

A typical catchment system involves collection of rainwater, snow melt, dew, and frost from roof surfaces (and solar panels) into above ground or buried cisterns (think of a cistern like a battery – its capacity is limited and its sizing depends on usage and generation; mine is 3,450 gal or about 1/3 of estimated annual harvest). Rainwater, of course, is already naturally purified having been distilled by the hydrological cycle. However, since raindrops coalesce around dust particles, they probably contain bacteria, viruses and other biological stuff that hangs out on dirt (the same is true, of course, for well water). More recently, evidence has been accumulating that PFAS, particularly trifluoroacetic acid (TFA)¹,²,³ may also contaminate rainwater. As such, various methods for filtration and purification of the collected water should be considered.

How much purification is necessary? Necessary to meet baseline water quality standards which are typically (and alarmingly) minimal? Or, necessary to alleviate one’s anxiety about every possible contaminant? I think somewhere in between is reasonable. My approach is: 1) start with a minimal, low-cost filtration-only system, then 2) test actual collected rainwater both for common water contaminants (coliforms, heavy metals) and any others that concern you (TFA?), and finally 3) have a purification system designed and installed to remove identified contaminants.

I would be happy to work or collaborate with anyone on design of a suitable catchment, filtration and purification system. Feel free to contact me with questions. Water is Life.

Updated: 11 May 2025