Six water harvesting techniques
With climate change, Southern California is predicted to grow warmer and drier. Climate change also threatens the reliability of our supplies of water from Northern California and the Colorado River.
While wiser use of our lands and potable reuse of wastewater provides promising solutions to drier times ahead, better harvesting of rainwater will need to be part of future water use strategy. New technology offers hope, but many rain-wise methods have been around for a long time.
Here are six rain harvesting techniques San Diegans will see more of in times to come.
Rainwater can be harvested from the roof of a house or garage with a rain barrel. Installation is pretty much a no-brainer: Place the barrel on a level surface and divert a downspout into it. Yields can be great. Half an inch of rain falling on 400 square feet of roof will produce 125 gallons of water.
The same amount of rain falling on parking lots and roadways can result in tens of thousands of gallons flowing into local streams, increasing flooding. Brad Lancaster, author of Rainwater Harvesting for Drylands and Beyond, suggests harvesting stormwater runoff, which diverts rainwater to areas where the water can infiltrate and support water-reliant trees and shrubs.
Swales collect rainwater during storms, greatly increasing infiltration. This method consists of creating a lowered basin with a berm that lies perpendicular to the slope of the land. Water-reliant plants such as fruit trees or native sycamores and cottonwoods can be planted around the periphery of a swale.
Once water infiltrates into the ground, it can be kept there. One technique is using the shading of structures. In my work as a horticulturist, I’ve found that shade can make a big difference in how much supplemental watering is needed. By planting highly deciduous plants, such as coastal sage and California sunflower, under a south-facing overhang, they receive a full day’s sun during the winter, yet are fully shaded during the summer. The plants, particularly the sage, stay green all year long.
Mulch is any substance—including leaves, wood chips, and even rocks—placed on top of the ground that reduces evaporation. If you have or know of anyone with a big tree, leaves are free. Just as easily, winds can blow them away. Wood chips are more substantial, but they can weather badly and look unattractive. Rocks are initially hard to move but remain attractive. I prefer using the round river rocks ubiquitous to coastal San Diego. Mulch can keep plants greener during long, dry summers and increase the length of time the plant spends in the flowering stage.
We can collect and store rainwater, but what about the times when San Diego doesn’t get rain? Researchers at MIT have found a way to harvest water directly from the air. Much like the way our native Torrey pines gather dew on their needles to produce their own “rain,” MIT’s apparatus draws in moisture during the night and uses the power of the sun to release it the next day.
New and Old Ways to Make More of Our Rain originally published in the summer 2023 issue.