New study finds the pooling effect of rain garden basins essential to drought-proofing water-conserving and water-harvesting landscapes
Check out the newly published paper “Green stormwater infrastructure in a semi-arid climate: The influence of rain gardens on soil moisture over seven years” by Aaron T. Kauffman of Southwest Urban Hydrology and Cody L. Stropki in The Western Planner. November 7, 2022.
Below are some highlights…
This seven-year study in Santa Fe, New Mexico (elevation 7,000’ [2,133 m], average annual rainfall of 14 inches [355 mm]) compared soil moisture up to 30-inches (76-cm) deep within two types of side-by-side landscapes with clay loam soils, each receiving about 24,750 gallons (93,688 liters) of average annual stormwater runoff via curb cuts from a 3,500ft2 (325m2) section of parking lot.
The control landscape had no rain garden topography/basin, and was planted with native grasses.
The rain garden landscape had a roughly 15ft long by 10ft wide by 1ft (4.5m x 3m x 0.3m) deep rain garden basin, and was planted with native grasses, wildflowers, and woody perennials (shrubs and tree). The only irrigation water the landscapes received during the study was direct rainfall and parking lot runoff.
The study’s results found moisture in rain garden soils were significantly higher across the 30-inch (76-cm) depth profile compared to sites without rain gardens.
Rain gardens regularly replenished and retained soil moisture above the irrigation threshold (meaning there was sufficient moisture to maximize plant growth and avoid permanent wilting point plant stress—the point at which common annual agricultural crops, such as sunflowers, begin to struggle to extract water from drying soils) across seasons and during periods of drought.
While soil moisture in the control landscapes without rain garden basins consistently fell below the irrigation threshold.
Differences in water holding capacity across the greater soil profile is believed to be a result of increased residence time (temporary ponding of water in basins before infiltrating deeper into the soil) of stormwater in the rain garden basins compared to the sites with no basin where stormwater runs off the landscape.
Enhanced potential for groundwater recharge
Soil moisture in the 24-inch (60-cm) and 30-inch (76-cm) depths of the study’s rain gardens was much greater than that in the landscapes with no rain garden basins, suggesting that a reservoir of soil moisture in rain gardens could be accessible for deeper rooted vegetation to utilize as plants mature and/or experience periods of dry conditions. Whereas in landscapes with no rain garden basin, soil moisture fell below the permanent wilting point at the 30-inch (76-cm) depth.
This is encouraging in terms of groundwater recharge, in that by maintaining higher moisture in the soil profile of rain gardens, gravitational movement of water to deeper parts of the soil profile could more easily occur during storm events.
Making the most of little rain
Soil moisture replenishment in rain gardens was remarkably better than sites without stormwater catchment basins. There were 224 days, during the study, in which rain garden soil moisture exceeded field capacity (maximized their harvest of water) across the 30-inch (76-cm) soil profile versus only four days at the control sites without basins.
Thanks to both the rain garden basins and the paved catchment area draining stormwater runoff to them, an average rainfall event of only 0.19 inches (4.8 mm) was needed to harvest enough stormwater to exceed field capacity (maximize the basins harvest of water).
But in the control landscapes with no basin, the average rainfall event needed to cause field capacity (maximize the harvest of water) was 0.40 inches (10.1 mm).
So, less rain is needed to get the greatest benefit when you have stormwater-harvesting basins within the landscape.
Deeper rooted shrubs and trees might be tolerant of periods when soil moisture drops below the irrigation threshold, but soil moisture depletion near permanent wilting point could compromise plant health and survival. There were ten occurrences during which there was no measurable precipitation for at least 28 consecutive days at the study site.
But the rain gardens’ average soil moisture across the 30-inch profile never dipped below the permanent wilting point, whereas the averaged soil moisture across 30-inch profile of the control landscapes with no basin always fell below the wilting point four out of the end of the ten extended dry periods.
Big potential savings of water and money
As the study states, “In Santa Fe, treated water is often used to irrigate trees along streets and parking lots at a cost of $0.02/gallon even though lower quality water (e.g. stormwater runoff) would suffice in many situations. The city irrigates trees in street medians with two 5-gallon/hour emitters twice per week for four hours during establishment and four hours every two weeks as they become older (personal communication). This would amount to $6.40/tree/month or $76.80/tree/year and $1.60/tree/month or $19.20/tree/year respectively. Once trees are established, they are irrigated manually if soil moisture drops below 23%; a value that might occur with regularity given the general absence of curb cuts along streets or medians to provide added passive irrigation.
In contrast, each curb cut at the study site would drain roughly $495/yr of equivalent water volume into a rain garden (i.e. 24,750 gallons/year x $0.02/gallon).”
Not only do the curb cuts provide abundant free irrigation water, but the associated rain gardens also double as flood control strategies. And there is no plastic irrigation pipe that springs leaks and needs repair.
Rain gardens also reduce weeds and weeding.
Gary Wittwer, the landscape architect with the City of Tucson department of transportation (now retired) told me there were far less weeds in Tucson’s rain gardens only receiving passively harvested rain and stormwater than there were in landscapes irrigated with drip irrigation systems because the weeds grew better with regular watering, while the desired perennial plants in rain gardens were better adapted to, and out competed, the annual weeds in rain gardens only getting irrigation in the form of occasional stormwater pulses.
How this, and other studies, can inform more effective and more regenerative policy and practice
Aaron Kauffman’s study makes it clear that we need water-harvesting topography in our landscapes to make the most of free on-site water for irrigation and flood control.
And we need to evolve our landscape ordinances and practices to reflect this.
• Hardscapes/roofs/paved areas must drain their stormwater to the site’s vegetated landscapes, rather than storm drains, to provide free irrigation and flood control
• Vegetated landscapes must have a water-harvesting topography/rain gardens to make the most of free, on-site waters
In Tucson, Arizona we have an ordinance requiring commercial property landscapes to harvest at least 50% of their irrigation demand with passively harvested rainfall and stormwater. But when, in 2018, I was hired by the city to assess the effectiveness of the 12-year-old ordinance, I found that sites typically drained stormwater to landscapes with no rain garden basins, so the potential for the site’s stormwater to provide the site most or all of its irrigation needs often was drained away along with the stormwater.
• Rain gardens must be adequately sized
Additionally, I found that when there was a rain garden present in a commercial landscape, the average rain garden basin depth was just 4 to 6 inches—inadequate to maximize the volume of stormwater draining from the abundant hardscape (paved surfaces). Thus, the average depth of the rain gardens must at least be doubled. Then plant the right plant in the right rain garden planting zone.
• Organic-matter mulch must be used within rain garden basins instead of rock or gravel whenever possible
(eddy basins are great for this)
The study “Nematode Community Response to Green Infrastructure Design in a Semi-Arid City,” in the Journal of Environmental Quality 46(3):687-694 (2017) by Mitch Pavao Zuckerman and Christine Sookhdeo found that in Tucson, Arizona vegetated street-side rain garden basins harvesting street runoff mulched with organic material (wood chips, leaf drop, etc.) had twice as much soil moisture and twice as much soil organic matter (SOM) as such basins mulched with rock or gravel.
Soil organic matter (SOM) consists of fresh and partially decomposed plant and animal residues, the living biomass of soil microorganisms, and humus (the well-decomposed organic matter). As SOM levels increase, so does the soil’s ability to sequester more climate-changing carbon from the atmosphere, as the SOM is typically estimated to contain 58% carbon. Additionally, SOM helps bioremediate, or naturally filter, pollutants.
Organic matter mulch turns on-site green “waste” into a free resource, while rock and gravel is mined and transported, resulting in more pollution and waste. Leaves are called leaves, because we are supposed to leave them—not rake or blow them away.
• Native vegetation—plants indigenous to the area—should be prioritized as they are the best adapted to the local climate, soils, and wildlife; while also connecting us to the unique ethnobotanical history/legacy of the place and its people.
Look to the local ethnobotanical record to select native plants that also produce food, medicine, craft materials, wildlife/livestock fodder, and more. See NeighborhoodForesters.org for how this is being done on a neighborhood scale.
• Growing naturally shaded islands of shelter, greater fertility and moisture (below trees, large shrubs [bush trees], and other vegetation) must be prioritized, mimicking what you can freely observe in our sun-baked Sonoran Desert ecosystem where perennial native edible plants grow to create, or grow within, shaded nutrient islands where a much greater diversity of life can flourish (compared to out in the open), and enable more rainwater to be available longer into the dry season (“Islands of diversity: Ironwood ecology and richness of perennials in a Sonoran Desert biological preserve” by Alberto Burquez. Conservation International, Occasional Paper No. 1, April 1994).
Research has found that native bean trees such as the desert ironwood (Olneya tesota) and velvet mesquite (Prosopis velutina) form symbiotic relationships with nitrogen-fixing soil bacteria to convert atmospheric nitrogen (which plants can’t use) into ammonia (a form of nitrogen plants can use), so they are living fertilizer producers. The trees also shade and cool the soil and other life below, while protecting it from winds and winter cold. This reduces water loss from understory life to evaporation, thereby creating a more humid microclimate within the trees’ shelter, where twice as many understory plant species can live compared to areas lacking such protection (Burquez, 1994). Furthermore, the mesquite trees’ deep roots are capable of “hydraulic redistribution,” a process by which in wet times they pump moisture via their roots to store it in deep soil layers, then later pump it back up into the topsoil and their canopy in dry times, benefiting both the mesquite and plants under its canopy (“Impact of Hydraulic Redistribution on Multispecies Vegetation Water Use in a Semiarid Savanna Ecosystem: An Experimental and Modeling Synthesis” by Esther Lee, Praveen Kumar, Greg A. Barron-Gafford, Sean M. Hendryx, Enrique P. Sanchez-Cañete, Rebecca L. Minor, Tonny Colella, and Russell L. Scott. Water Resources Research, 54, 4009-4027). These trees are living pumps, air conditioners, and nurseries.
For information on how to design, build, and grow many different types of rain gardens for many different types of contexts
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Great book for assessing the potential harvests of all free, on-site waters. Then creating an integrated design to effectively maximize the harvest those waters.
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