Roof water, in a tank, out to a tree on a schedule.

What it is

CIS-1 is the family of active rainwater interventions: an above-ground tank or array of tanks, plumbed to one or more roof downspouts, with the apparatus needed to deliver the stored water back out as drip irrigation. We install only above-ground polyethylene tanks. We do not install in-ground concrete cisterns; the cost differential rarely justifies the additional excavation and waterproofing in Tucson's climate, and access for maintenance becomes a problem after year five. We install only food-grade NSF-61 poly tanks with full-volume UV stabilizers; the dye is the difference between a tank that lasts 25 years and one that fails at 12.

The active part of an active cistern is the way water comes out of it. Most homeowners expect a hose bib at the bottom and a five-gallon bucket; we install that, but we also install a small Grundfos SCALA1 pressure pump (3-15 psi adjustable) feeding a Hunter X-Core controller wired to inline drip emitters at each receiving plant. The pump runs only on a schedule the controller calls, in the cool hours, and stops when the tank's float switch trips below a 10% reserve so the pump never runs dry. Water moves out of the tank in 30-minute zones; a typical 3,000-gallon tank runs three zones of about 80 emitters each.

Schematic

Sizing

The two ways to size a cistern are by demand (how much water you need each year for your trees) and by supply (how much water your roof can deliver). Neither is the right answer alone. The simplified ARCSA-recommended sizing approach (ARCSA Design Manual) sizes the tank to capture roughly 80% of average annual rainfall on the catchment area. In Tucson, that yields about 1.1 gallons of capture per square foot of roof per inch of rain, so a 2,400 sf roof receiving 11.6″ a year produces ~31,000 gal of theoretical capture; we typically size the tank at 10–15% of that — call it 3,000–5,000 gal — to handle peak monsoon storms without significant overflow.

Demand sizing is simpler. We compute the gallons-per-week each receiving plant needs in summer (see garden per-plant water budgets), multiply by 16 weeks of monsoon-and-after, and round up by 30%. A pair of mature mesquites + a citrus + an ironwood, all of them at full summer water, comes to about 1,800 gal/wk × 16 = 28,800 gal — and a 3,000-gal tank refilled four times per monsoon supplies 12,000 gal of that, with the rest coming from passive earthworks and condensate.

Tanks we use

Tanks come from Western Sanitary Supply in Tucson or Tank Depot Phoenix; both deliver to the lot. The "Mocha" colorway from Bushman matches Tucson stucco better than any of the green options. We never install black tanks (algae bloom risk and aesthetic objections from HOAs in the Foothills are equally inevitable).

Controllers & telemetry

The controller is a Hunter X-Core 4-zone or 8-zone with the optional Solar Sync ET sensor for evapotranspiration-aware scheduling. The pump is a Grundfos SCALA1 3-25 pressure-and-flow self-priming with the dry-run protection set to trip below 10% tank level. The float switch is a Levelogger ULS-50.

For tanks 5,000 gallons and up we add a small SCADA tie-in that reports tank level, pump runtime, and zone valve state to a free Adafruit IO dashboard the homeowner can check on a phone. June Walden, our controls technician, sets these up with the homeowner during commissioning. We do not store the homeowner's data, monetize it, or share it.

Bill of materials, typical 3,000-gallon install

Tucson Water residential rainwater rebate at $0.25/gal of installed capacity caps at $2,000 for 8,000+ gallons; our 3,000-gal tank captures $750 of that. We file the paperwork on your behalf.

Permits

Tank above 1,500 gallons requires a City of Tucson Plumbing Minor permit (PMT). We pull this under our R-37R license. Tank pad of stabilized decomposed granite under 12 inches deep does not require a separate building permit; we use 4 inches of compacted DG over native subgrade with 6 inches of pea gravel for drainage, which is well under the threshold. Tanks above 5,000 gallons combined capacity may be subject to Arizona Department of Water Resources reporting under specific Active Management Area conditions; we have not yet hit that threshold on a residential job, but the form is on the ADWR website if you want to read it.