Rainwater Harvesting From Your Gutters

Why harvest rain here

Texas A&M AgriLife Extension describes rainwater harvesting as capturing, diverting and storing rainwater for later use, and notes it reduces demand on existing water supplies and cuts runoff, erosion and contamination of surface water (Texas A&M AgriLife Extension, Rainwater Basics). On the Coastal Bend, rainfall is uneven: NOAA normals for the Corpus Christi airport list about 31.7 inches a year (NOAA NCEI 1991–2020 annual normals, Corpus Christi International Airport), with February averaging about 1.3 inches and September about 5.4 inches (NOAA NCEI 1991–2020 monthly normals, Corpus Christi International Airport). Storing some of the wet-season water for drier stretches is a natural fit.

The system in five parts

AgriLife breaks every system into catchment, conveyance, storage, treatment and distribution (Texas A&M AgriLife Extension, Rainwater Basics). Your roof is the catchment. Your gutters and downspouts are the conveyance. A barrel or tank is storage. Screens and first-flush devices are basic treatment. A spigot, hose or drip line is distribution.

Catchment: your roof

AgriLife explains that the catchment area is the roof footprint, calculated from the building area plus overhang, and that roof slope does not change it. It also notes that metal roofs shed contaminants easily, common asphalt shingles work fine for landscape use, and that wood shingles and metal flashing containing lead should be avoided (Texas A&M AgriLife Extension, Rainwater Harvesting: Catchment Area).

Conveyance: gutters and downspouts

AgriLife's conveyance guidance is essentially a gutter installation checklist (Texas A&M AgriLife Extension, Rainwater Harvesting: Conveyance (gutters and downspouts)):

• Size gutters to carry a 100-year storm; AgriLife names Corpus Christi as a place with intense storms that needs wider gutters.
• Use gutters at least 5 inches wide.
• Provide one square inch of downspout per 100 square feet of roof.
• Rounded-bottom gutters limit debris buildup.
• Slope gutters about 1/16 inch per foot; keep the front half an inch lower than the back.
• Place hangers every three feet.
• Paint PVC piping to reduce UV breakdown.

If your gutters are undersized or poorly pitched, fix that before adding storage. A tank cannot collect what overflows the front of the gutter.

Storage and overflow

Barrels and tanks fill fast in a Gulf downpour. NOAA Atlas 14 estimates for the Corpus Christi airport show one-hour rainfall rates of about 3 inches per hour in a 10-year storm (NOAA Atlas 14 Precipitation Frequency Data Server (Corpus Christi International Airport point)). Every container needs an overflow outlet at least as large as the inlet, discharging away from the slab, ideally to an extension, splash block or bed set well away from the house. Without it, water backs up into the downspout or spills at the foundation.

A simple example

Picture a single-story home with a 1,600-square-foot roof footprint, split so that about 500 square feet drains to a back corner downspout near a vegetable bed. Using AgriLife's downspout rule, that corner needs at least 5 square inches of opening, so a standard 2x3 downspout works and a 3x4 gives extra room (Texas A&M AgriLife Extension, Rainwater Harvesting: Conveyance (gutters and downspouts)). A barrel or small tank at that corner, with a screened lid, a leaf diverter above it and an overflow hose leading to the far side of the bed, captures a useful share of every rain without disturbing the rest of the gutter system. In a heavy September storm, the barrel fills quickly and the overflow carries the rest away from the slab.

Start with one corner, see how fast it fills and how much you use, and expand from there. AgriLife's site has calculators for more precise sizing.

Keeping debris out

• Gutter guards reduce leaves reaching the downspout, especially during the spring live oak drop.
• Leaf diverters at the downspout shed debris before water enters the pipe.
• First-flush devices divert the first, dirtiest water from each rain away from the tank.
• Screened inlets keep mosquitoes and debris out of storage.

Mosquitoes

Any standing water in a warm climate can breed mosquitoes. Seal tank lids, screen inlets and overflows, and make sure gutters drain fully so water does not sit in troughs. Pitch is the key; see gutter repair if water stands in clean gutters.

Using the water

For most homes, harvested rain goes to landscape beds, potted plants and gardens. AgriLife's rainwater program covers more advanced uses, including in-home systems that require treatment, in its publications (Texas A&M AgriLife Extension, Rainwater Basics). Keep it simple at first: a barrel at the downspout nearest your garden is a good start.

Gutters built for harvesting

If you are planning new gutters with harvesting in mind, tell the estimator. Downspout locations can be chosen to land near storage, outlets can be sized for capacity, and guards can be matched to your trees. See gutter installation, downspouts and drainage and the estimate factors page.

Barrels need well-sized gutters and outlets above them; see downspouts and drainage.

Sources: Texas A&M AgriLife Extension, Rainwater Basics; Texas A&M AgriLife Extension, Rainwater Harvesting: Catchment Area; Texas A&M AgriLife Extension, Rainwater Harvesting: Conveyance (gutters and downspouts); NOAA NCEI 1991–2020 annual normals, Corpus Christi International Airport; NOAA NCEI 1991–2020 monthly normals, Corpus Christi International Airport; NOAA Atlas 14 Precipitation Frequency Data Server (Corpus Christi International Airport point).