By Diane Kennedy for Let's Talk Plants! October 2021.
One inch of rain falling on one acre in an hour is 27,154 gallons of pure, free, slightly acidic water. The best place to capture that water is in the ground. Now, before the rains begin, is the time to prepare your ground for harvesting water.
Another dry and warm winter is on the books for this year. Unfortunately, very little rain actually sinks into the ground but rather is channeled out into the ocean. Compacted soil under grass or unmulched ground, hardscape, gutters leading to drains, and artificial turf all prevent rainwater from sinking into the soil and doing plants and the water table any good. Rain barrels and cisterns are fantastic ways of holding some water for use, but they are quickly filled in our inundations. Instead of, or along with, rain barrels, work to sink water into the ground away from the foundation of your house.
Water harvesting can be done in many ways. The first and easiest method is to thickly mulch the ground. Rain hits the mulch, shatters and sinks into the ground rather than rolling off. The mulch prevents it from evaporating right away. Three or more inches of mulch is best; too little mulch and the pieces of bark heat up and wick moisture out of the ground. Another important way to harvest water is to plant trees and shrubs. Tree leaves and branches break the fall of rain drops and guide them to the ground gently so that the force of the rain won’t compact the soil. Tree roots take up and hold a lot of the water, gradually releasing it to moisten and cool the air.
Another method of rainwater capture is to dig level-bottomed swales on contour across your property. Water rolls into the swales, pacifies, spreads and percolates into the soil instead of eroding. A dedicated overflow prevents the swale from being compromised by heavy rain. You can dig small swales above existing trees so that the water sinks down to the root ball of the tree. Any depressions or dimpling of the ground will help capture and sink water. Percolated water will move through the landscape and eventually emerge in streambeds, giving them water during the dry months and keeping them alive.
You can create a dry streambed which is actually a rock-lined swale that is lovely most of the year and also functions during the rains. These can include a dry pond, which also fills with hundreds of gallons of water and sinks it gently into the soil. The trick is to create the streambed feature so that it doesn’t channel water away but harvests it.
If you have a rain chain or downspout, consider digging a rain garden where that water can be directed. Dig down several feet, fill with mulch and top with soil. You can plant on top and around the edges of the rain garden. When water sinks into it, the bark mulch, and any buried logs or branches, will become sponges and soak up some of the water. As the surrounding ground dries the water is released into the soil long after the rain event occurred. Any plant or tree nearby will have a water source for awhile as well as long-term fertilizer as the mulch decomposes.
There are many more ways to harvest rainwater, but no matter what you do it is imperative that you do it. Safely harvesting rainwater into the ground is the only way we can keep trees alive and rehydrate our water table.