Texas Hill Country Rainwater Harvesting: Key Points From the TWDB Manual

Rainwater is common in the Hill Country. These points come from the Texas Water Development Board manual (3rd edition).

How much you can collect

The manual gives about 0.62 gallons per square foot of collection surface per inch of rain. Installers typically assume 75 to 90 percent capture. Its example: 1.97 inches of rain on a 2,500 sq ft roof at 85 percent yields about 2,595 gallons. The catchment area is the roof footprint, regardless of pitch.

First flush and storage

A rule of thumb is to divert at least 10 gallons per 1,000 sq ft of roof. The manual also cites a preliminary Australian study at 13 to 49 gallons.
A shortcut is to size storage for a three-month dry period by dividing annual demand by four. The manual notes West Texas dry spells have exceeded three months.
Tanks should be opaque, covered and have screened vents. Water weighs about 8 pounds per gallon, so the pad must be level and stable.
The manual mentions stacked-ring concrete tanks as familiar in the Hill Country.

Pumps and pressure

Municipal pressure is typically 40 to 60 psi, while many appliances need 20 to 30 psi. Water gains 1 psi per 2.31 feet of vertical rise, so gravity rarely gives enough pressure. Options include a pump with pressure tank, pressure switch and check valve, or an on-demand pump. Put the pump close to the tank, because pumps push better than they pull.

If you drink it

The manual says composite or asphalt shingles are not advisable for potable systems, and that lead must not be used as gutter solder. A common Texas treatment train is a 5-micron sediment filter, a 3-micron charcoal filter, then UV; UV lamps are replaced after a maximum of 10,000 hours. Potable water should be tested for pathogens, at least quarterly if used for drinking. Not verified: current Texas or county rules for potable rainwater; ask your county and a licensed installer.

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General information only. Not plumbing, legal or code advice. Confirm with a licensed professional and your local authority.