Rainwater Harvesting in Central Texas

Rick Scadden, local resident and environmental engineer
LMUD Open House presentation, May 21, 2024

Treated Rainwater Quality & Benefits

• Excellent quality for human consumption
• No spots on glassware from dishwasher
• Less soap needed for dishes and laundry
• No scaling on plumbing fixtures which can happen with well water
• Water quality studies

Rainwater harvesting is beneficial for the following reasons:

• Water Conservation: Harvesting rainwater conserves water by capturing and storing it. This is particularly important in areas that experience frequent droughts or where water is scarce.
• Cost Savings: Harvesting rainwater saves money on water bills by reducing the amount of water that needs to be purchased from a utility company.
• Environmental benefits: By reducing the demand for municipal water supplies, harvesting rainwater reduces the strain on local water resources and the environment.
• Watering and Irrigation: Rainwater is ideal for watering plants.
• Drinking Water: Properly treated rainwater is delicious to drink.
• Emergency preparedness: Harvesting rainwater provides a backup supply of water in case of emergencies, such as natural disasters or power outages.

Rain Barrels for Landscape Watering

Rainwater harvesting can be very simple through the installation of a rain barrel: They come in all different sizes; just reroute the downspout from your rain gutters into the barrel. Installing a screen on the barrel’s inlet is important to keep out leaves and debris. Keep the barrel raised off the ground enough so that a spicket installed at bottom of the barrel allows for enough room to put a watering can under it to access the water for use in gardens and raised beds.

On a larger scale, go check out the Lady Bird Johnson Wildflower Center’s beautiful example of a rainwater collection system. The Center’s 6,000-gallon collection tank is reminiscent of the stone-and-mortar cisterns used by Hill Country settlers. This and its arching stone aqueduct form the distinctive entry to the research center. It is used to harvest an average of 300,000 gallons of rainwater annually from the almost 19,000 square feet of irrigation roof collection area for the Center’s native plant landscapes.

Rainwater Collection Systems for Whole-House Water Needs

Basic Components

Regardless of the complexity of the system, the harvesting of rainwater for whole-house uses, including drinking and bathing, comprises six basic components:

1. Catchment surface: the collection surface from which rainfall runs off.  A metal roof is recommended. Clay or concrete would be another good option, but these porous tiles will absorb some of the water. Composite or shingle roofs are not advised since the tar and asphalt can add dissolved chemicals into the water, however this roof style is sufficient if you are using rainwater just for landscape irrigation (not for drinking/whole-house use).
2. Gutters and downspouts: channels water from the roof to the tank. Consider buying options that use materials that resist corrosion and wear and do not leach harmful substances into the water.
3. Components which remove debris and dust from the captured rainwater before it goes to the tank:
   • Leaf Screens: Installing high-quality screens on gutters is very important.
   • First-Flush Diverters: the first water that comes off the roof after the downspout is the less quality water so you need to discharge it before it enters your tank. Note this is not removing any dissolved materials. A flush valve and/or box filter installed before your storage tank is used for this purpose.
4. One or more storage tanks, also called cisterns. We have an 8,500 gallon food-grade fiberglass tank which we installed in 1995.
   • Tank Protection: Protect your tank from sun damage as well as from traffic.We have a metal shed over our tank as well as have painted it with UV-protective paint.
   • External Water-level Indicator: I recommend installing an external water level indicator, such as a bobber system, to be aware of how much water you have in the tank. These systems, however, are not fool proof so you will still need to visually inspect the inside of the tank since indicators can malfunction and you could end up with less water than you think you have.
   • Tank Overflow Outlet:  A drain at the top of the tank to expel any excess water that may occur during heavy rain events. It’s a bittersweet moment when this happens because it means your tank is full, but it means this precious resource is being wasted. We typically use this as an opportunity to do extra laundry or run the dishwasher or other things that will help balance our consumption with the supply of water.
   • Disinfectant (optional): I don’t add any chemicals to my harvested rainwater at any point throughout my delivery system. It is not very common for residential use, but some people do add chlorine in their tank as an extra precaution. This may be used more for people who don’t have regular use of the tank and are worried about the water sitting stagnant for periods of time.
5. Delivery system: typically gravity-flow into the tank and then pumped to the end use. To ensure there is enough hydraulic head (force of gravity flow) from the gutter to the tank, ensure your tank is lower than your gutters. You can dig under the tank to bury part of it to get it to the right level.
   • Pressure Management: Your pressure pump should be able to produce about 45 to 60 PSI to help force the water throughout your home’s plumbing system. Adding a pressure gauge will help you monitor your system’s performance. A pressure tank will help regulate the pressure.
6. Treatment/purification: for potable systems, a series of filters and other methods are used to make the water safe to drink.
   • Filters: I use a course filter and carbon filter (reusable and disposable versions are available) as well as an ultraviolet disinfection unit. If you get bigger filters, you don’t have to change them out as often. We regularly monitor them – if there is a lot of debris collected in the filters or notice the water pressure in the house goes down we know it is time to change them, which ends up being about every month to six weeks. With our ultraviolet unit, there is a sight glass that allows us to see if the bulb is functioning; we typically replace our bulb annually.

Rainwater Collection Potential in Central Texas

Your roof’s collection surface footprint is the primary deciding factor on how much rainwater you are going to collect. The shape of your roof is not as important as the footprint, except during a hard rain.

The Texas Water Development Board has developed a series of maps showing average inches of rain per year that falls in various areas across Texas. Central Texas currently has an average of 35 inches per year or around 32 inches in the Hill Country. Future projections indicate less yearly average rain fall or less frequent rain fall with greater intensity. For rainwater collection system, this means you’ll most likely need to plan to store more water that carries you over through times of drought. This can be achieved by having a bigger storage tank, several storage tanks, and/or increasing your collection square footage (add the roof of a storage shed or garage).

Let’s do the math:

• 1 inch of rain on 1 square foot of collection equals 0.62 gallons (7.48 gal/ft3 at 100%)
• Assume 80% efficient at capturing rainfall
• Therefore 1 inch of rain on 1 square foot of collection area is approximately 0.5 gallons
• Austin has about 35 inches of rainfall per year on average or 17.5 gallons of water captured per square foot of collection surface
• 2,000 sq ft house (footprint, not total sq ft of structure) is estimated to collect 35,000 gallons per year (There is no 35,000 gallon tank, so you have to manage collection and use over time)

Our Rainwater Collection System

• Our home (collection surface) is approximately 36.5 feet x 39 feet (no current collection from garage): 1423.5 square feet
• 1423.5 x 17.5 gallons = 24,911 gallons per year
• Our storage capacity is an 8,500 gallon tank
• My wife and I estimate our consumption is about 50 gallons per day (1,500 gallons per month). This is much lower than the average American, but we do not do any outdoor irrigation and use water very consciously, so you will have to consider your households needs.
• We project that our storage should last us about five months with a full tank and no additional rainfall

Backup Plan

Drought (extended periods without rainfall), tank or system failure (general wear and tear or accidents that cause breaks in the tank or system components), and power outages (which leads to pump failure) are the main concerns when you are fully reliant on a rainwater harvesting system for all of your household water needs. We decided to take one of those concerns out of the equation by installing solar panels so that our house is also fully reliant on solar power. If you do have a power failure, you can still pull water from the tank for flushing toilets, etc., but it won’t be treated so it cannot be used for drinking.

When failures do occur, after doing what you can to prevent your tank from draining or being contaminated, your first effort should be to dramatically reduce consumption/conserve water. Next, you’ll probably need to arrange for water delivery. We have only had water delivered six times since installing our system in 1995 – twice in 2011 (there was not much rain in spring and in summer there was none). The last time was last year, when we had a break in the plumbing and all of the water ran out of our tank. We expect that about 1,500 to 2,000 gallons can be delivered at a time, but that’s only when it’s available. During severe droughts, it can be hard to get on their delivery list.

Key Takeaways

Stay tuned to your system. Being on a rainwater harvesting system for your whole house is not like being a customer of the MUD where you just turn your faucet on anytime you want to use water. You have to know what’s going on. The MUD has trained professionals to do all of those things to look out for you. If you are being self-sufficient with your own system, you have things you constantly have to stay on top of to maintain an adequate supply of quality water.

• Rainwater systems require owner involvement.
• Know your system and maintain it faithfully. This is your drinking water.
• Have a good service provider/contractor since you can’t do everything yourself.
• Use high quality gutter screens to avoid material getting washed into the water supply.
• I’ve heard that many people wish they had installed more storage capacity (more or larger tanks). I’m not sure if this is because they run out often or want to be better prepared in case of emergency. I’d suggest having multiple tanks (the second being used as a backup tank). I’m currently looking into this as an option for strengthening our system to become more self-sufficient going into the future.

For more information, I suggest the Texas Manual on Rainwater Harvesting from the Texas Water Development Board: https://www.twdb.texas.gov/publications/brochures/conservation/doc/RainwaterHarvestingManual_3rdedition.pdf