San Antonio is on the edge of two water realities. The Edwards Aquifer is the city's primary source and it is under continuous pressure — pumping limits, drought-stage restrictions, and irrigation bans show up almost every summer. SAWS water rates are tiered, and the upper tiers are where most landscape irrigation lives in July. Meanwhile, roughly 32 inches of rain fall on every property in the city every year, and most of it leaves over the curb.
The math says you are paying for water you could be catching. The question is whether catching it is worth the install. Here is how we run that calculation.
The math — start here.
The catchment equation is fixed and simple:
Gallons captured = roof area (sqft) × rainfall (inches) × 0.623 × efficiency factor
The constant 0.623 is the conversion: one inch of rain on one square foot of horizontal surface equals 0.623 gallons. The efficiency factor accounts for first-flush losses, gutter overshoot, evaporation off a hot roof, and overflow during intense storms. For a well-built system in San Antonio we use 0.85. Cheaper systems with undersized gutters or no first-flush diverter come in closer to 0.70.
For one inch of rain on 1,000 square feet of roof:
1,000 × 1 × 0.623 × 0.85 = 530 gallons
For an average San Antonio year (about 32 inches) on a typical 2,500 sqft roof:
2,500 × 32 × 0.623 × 0.85 = 42,364 gallons per year
That is real water. The catch is that it does not arrive evenly. About 60% of San Antonio's rain falls between April and October, and most of that comes in 1- to 3-inch bursts. The August and February dry windows can run 4 to 8 weeks. Your cistern sizing has to address that timing, not the annual average.
Why harvest at all.
Three reasons stack up, in this order:
- Cost trajectory. SAWS rates have risen in nearly every recent rate cycle and the tiered overage rates have widened. A property using 25,000+ gallons a month in summer is in tier territory where each additional 1,000 gallons costs noticeably more than the first 1,000. Catchment trims off the top of that bill.
- Edwards Aquifer pressure. SAWS triggers drought-stage restrictions (Stage 1 through Stage 4) based on the Edwards Aquifer J-17 well index tracked by the Edwards Aquifer Authority. Under Stage 2 and beyond, irrigation is restricted to specific days and hours. A cistern is exempt — rainwater you own does not count against drought-stage rules.
- Independence. A 5,000-gallon tank with a working pump runs your irrigation through a 6-week dry stretch without touching city water, and runs your most-loved plantings through a Stage 4 emergency without breaking the law.
The system — what the components do.
A complete residential rainwater system, in flow order:
- Gutters and downspouts. Size for the storm. Standard 5-inch K-style gutters handle most San Antonio roofs; 6-inch is worth the upgrade on steep pitches or roofs over 2,500 sqft. Downspout count matters more than diameter — undersized downspouts overshoot in a 3-inch storm and the catchment efficiency collapses.
- First-flush diverter. The first 10 to 30 gallons of any storm carries the dust, pollen, bird droppings, and shingle grit that accumulated on the roof during the dry stretch. The diverter routes that first flush off to a slow-release chamber and lets the cleaner water that follows go into the tank. This is not optional on a potable system and it is still worth installing on irrigation-only systems.
- Screened inlet. A fine-mesh screen at the tank inlet keeps leaves, insects, and debris out of the water column. Mosquito-proof to code.
- Cistern. The tank. See sizing below.
- Overflow. Where the water goes when the tank is full. Best practice: route the overflow to a bioswale or a rain garden so the excess soaks in instead of running to the curb.
- Distribution. Either a pump (for pressurized irrigation) or gravity feed (for drip on a slope below the tank). Pumps are usually 1/2 to 1 HP for residential systems, on a pressure tank to handle drip demand.
- Treatment, if potable. Sediment filter, carbon filter, UV sterilizer, in that order. Each is a quarterly maintenance item.
Cistern sizing — for a half-acre Hill Country lot.
Sizing is the part most homeowners get wrong. The temptation is to size the tank to the annual catchment, which produces an enormous and expensive cistern. The right move is to size to the longest dry stretch you want to ride out.
For a typical half-acre Hill Country property irrigating garden beds, fruit trees, and a small native lawn — call it 3,000 sqft of irrigated area — drip-zone demand runs roughly 800 to 1,500 gallons a week in peak summer. A 6-week dry window is the planning target.
1,200 gallons/week × 6 weeks = 7,200 gallons
You do not need to size to the full 7,200 because storms usually break the window before week six. In practice, a 3,000 to 5,000 gallon cistern handles a half-acre Hill Country property reliably. Properties with larger irrigated areas, orchards, or any livestock should size up.
Above-ground or buried.
Above-ground tanks are cheaper, easier to inspect, and easier to repair. They take up visual real estate, which is why most homeowners eventually want them screened with planting or a slat wall. Polyethylene tanks come in 1,500, 2,500, 3,000, and 5,000 gallon sizes and last 20 to 30 years if kept out of direct UV.
Buried tanks vanish from the property visually and stay cooler in summer (which slows algae growth and helps water quality). The cost penalty is significant in Hill Country caliche — excavation for a 5,000-gallon buried tank can add $3,000 to $6,000 over the above-ground equivalent. Reinforced fiberglass or concrete is the right material below grade; polyethylene burials are a long-term failure.
Potable vs irrigation-only.
If the system is irrigation-only, you can skip every component downstream of the tank that has anything to do with health code. Tank, pump, drip distribution, done. Most residential systems land here.
If the system is potable, the Texas Water Development Board's Rainwater Harvesting Manual is the governing document. You need a food-grade tank lining, the full filtration and UV train, and an air gap at any connection to the city water backup. The install is not the hard part — it is the quarterly maintenance discipline. Most homeowners who install potable systems for resilience reasons end up running them on the city water backup most of the year and using the tank for the dry stretches and for outages.
What you really save.
An irrigation-only system on a half-acre property typically displaces 20,000 to 35,000 gallons of city water a year. At SAWS rates with tiered pricing, that translates to roughly $300 to $700 a year in direct water bill savings, plus exemption from drought-stage restrictions on the irrigated area. On a system that costs $5,500 to $7,500 installed, simple payback is 8 to 14 years. SAWS rainwater rebates can shorten that by 1 to 3 years depending on the current rebate level.
That payback math is fine but it understates the value. The real return is independence — knowing that the garden does not die when SAWS calls Stage 4, and knowing that the water on the orchard came from the sky and not from the aquifer you live on top of.