Water insecurity. It’s in the news nearly every day. From Guadalajara, Mexico, where up to 50% of the municipal water supply is lost to leaks, to Cape Town, South Africa, where the reservoirs nearly dried up, bringing the city to the brink of water bankruptcy, to places closer to home like Corpus Christi, where overallocation and a shrinking supply could leave over 700,000 people without water. But in all the research I have done for this column, I have rarely, if ever, seen any reference to rainwater harvesting, one of the most economical, readily available solutions to water insecurity.
According to Ricardo Briones and Robert Mace of the Meadows Center for Water and the Environment at Texas State University, rainwater harvesting systems are similar to surface-water reservoirs because they “both collect and store rainwater for use.” In their report titled “Reliable Rainwater Is Only a Roof Away: The Firm Yield of Rainwater Harvesting in Texas,” the authors explain that rainwater collection systems also function much like underground aquifers because “most storage for rainwater harvesting is closed,” meaning “there is little to no evaporative loss.”
Residents in some parts of the state have long used rainwater catchment in the absence of other water sources. Elizabeth Thompson has lived for 29 years in Terlingua in the Big Bend, where annual precipitation averages about 10 inches, but can dip to as low as 1.3 inches, as it did in 2011, according to the National Weather Service. Aside from doing laundry at the local laundromat, she relies exclusively on rainwater for domestic needs like “showers, dishes, mopping, drinking water.” Thompson told Our Water Matters that the lowest level her 7,000 gallons of storage capacity has ever reached is 50%, and she likes that “you always have an idea of how much [water] you have available.” Similar anecdotal evidence of effective rainwater harvesting can be found elsewhere in Texas. So, why is it barely mentioned as a way to combat water insecurity?
Briones and Mace assert that “rainwater harvesting remains underutilized in modern water planning, primarily due to the perception that it is unreliable—particularly during droughts.” In order to address this perception and “advance rainwater harvesting in the state,” Briones and Mace created “a firm-yield framework to quantify the reliability of rainwater harvesting systems across Texas.” The authors describe “firm yield” as the “term of art” for “how much water can be consistently produced” from rivers, streams and reservoirs throughout the state’s “drought of record.” Determining the firm yield of rainwater harvesting under such conditions is critical to establishing it as a recognized water source because “[w]ater planning in Texas is based on how much water existing and planned supplies can provide” under worst-drought conditions.
Toward this end, Briones and Mace have developed “a governing equation” for rainwater harvesting which they then incorporated into an Excel workbook called “Rainwater Assessment and Interactive eNumator for Firm-yield Analysis Limits,” or “RAINFAL.” According to the authors, “There are three primary variables when calculating the firm yield of a rainwater harvesting system: (1) rainfall, (2) catchment area, and (3) storage.” For rainfall, Briones and Mace used data from Climate Data Online because it is “the only online database that has continuously collected precipitation data throughout the state.” The authors arrived at a 3,000 square-foot catchment area based on a “median square footage” of 2,609 square-feet according to the most recent data available from the U.S. Census Bureau, along with a certain allowance for eaves and a two-car garage. A storage volume of 30,000 gallons was applied as “the ‘base’ volume for our analyses,” explain the San Marcos-based authors, “because this is the commonly installed volume in the Texas Hill Country for whole-house systems and is cost-competitive to drilling a well.”
The authors then fed precipitation data into RAINFAL for 19 cities in Texas covering at least one city in each of the state’s 16 regional water planning areas. “Not surprisingly, smaller catchment areas are needed in the wetter, eastern parts of the state than the drier, western parts,” write Briones and Mace. But “in all cases, firm yields for the entire state can be attained for all the cities investigated” with drier climates requiring “lower levels of daily use.” Based on the assertion “that 25 gallons per person per day is achievable in a water-conserving home,” the authors found that “all cities could achieve that firm yield with catchment [areas of] less than 4,000 square-feet.”
RAINFAL is a tool that homeowners, landlords, and businesses can use to retrofit existing buildings or design new structures. The tool’s robustness proves that “rainwater harvesting is not merely a conservation strategy— it is a legitimate, quantifiable water-supply option,” write the authors. The conservation benefits of rainwater harvesting should not be downplayed, however, because “[p]eople who rely on rainwater to meet their water needs tend to be efficient users,” write Briones and Mace. In addition to conservation, harvesting practitioners like Thompson in Terlingua recommend installing as many tanks as possible because, in her experience, “[I]t’s painful when it’s raining and your tanks are overflowing because you don’t have enough storage capacity.”
Rainwater harvesting offers Texans an opportunity to declare their water independence. It could also contribute to a broader resilience strategy encompassing the decentralization of our food and energy supplies. As Briones and Mace assert: “With thoughtful planning and robust design, rainwater harvesting can play a meaningful role in Texas’s water future.”
Visit https://www.meadowscenter.txst.edu/research/water-conservation/rainwater-firm-yield.html to access the full report and download the RAINFAL tool.