I have been delighted with many of the early rainstorms that delivered steady and gentle rains. The gentle rain allows water to percolate into the soil. This is where we want it, to feed the roots of our landscapes and forests, ensuring their health and vigor, and on down to recharge our aquifers. Water deep in the soil is like money in the bank. When it is offered to us we need to accept it readily and conserve it, especially in times of drought.
The rain is obviously not always gentle. So, to accept water into our soil we need to have good water infiltration, meaning that when the water hits the ground it enters the soil. When raindrops fall onto bare soil they splash up tiny particles of clay which return to the soil and eventually coat the surface. This ends up making the soil less permeable to water. We don’t want this.
Soil with good structure and organic matter has the porosity to soak up and hold onto water well. A layer of mulch on the soil has a porous surface to accept the water and hold it until it soaks into the soil. The mulch keeps the layer between the mulch and the soil porous because of the shelter it provides for the worms, insects, and small mammals that tunnel about and aerate the soil. These animals take mulch particles into the soil and increase the organic matter content of the soil. The mulch then keeps the moisture in the soil by preventing it from evaporating, keeping the money in the bank.
Vegetation also increases water infiltration by softening the impact of raindrops. Forests have the highest water infiltration rates of all ecosystems due to tree cover, understory vegetation, and leaf litter on the forest floor. We can emulate this in our landscapes to maximize the use of the rainwater that falls, while reducing erosion.
As water keeps soaking into the soil it ultimately percolates down to a water table or aquifer, thereby recharging it. We want that!
When water input exceeds the infiltration rate of a soil surface, as in a heavy, driving rain, as is predicted for us as I write, there is runoff. Also, where there are rooftops, paving, and bare soil areas there is nothing but runoff. Usually this runoff is escorted quickly off the property through storm drains, drainage ditches, or seasonal creeks where it ends up loading natural waterways. When the water comes off roads and parking areas there is some car waste and other human detritus along with it, polluting water bodies downstream.
There are some things we can do to prevent water from making such a quick exit off our properties. Swales can be dug to allow some of the runoff to collect and slowly soak into the soil. Swales are a depression in the soil that usually run along a contour to slow and capture some runoff and allow it to sink into the soil instead of being whisked away on a fast course to the ocean. This helps recharge our aquifers and puts money in the bank for our downhill neighbors.
Swales and bioswales are being used all over the country to mitigate pollution from storm water runoff. Bioswales are planted swales that serve to filter pollutants and particulates from water and allowing it to soak back into the ground. We have a demonstration bioswale here in town at the Rood Center with one channel of it at the library. Please, if you haven’t noticed it, go check it out. It has good interpretive signage.
Rain gardens are a variation on this theme of soaking rainwater into the ground. They have applications on a smaller scale, useful for smaller home landscape situations. Rain gardens need to be built (dug) at least 10 feet from a structure. They should be planted to keep the soil rich and porous. And the plants planted in them can use the stored moisture long into the warm season, needing less irrigation.
Rain gardens and bioswales have different levels to consider for planting. The bottom level will be inundated with water at times and plants need to be selected to accommodate this. The driest level is at the top and may either need summer irrigation or preferably be planted with more drought tolerant plants. And the middle level can offer range of planting options, including vegetables, flowers, and herbs depending upon conditions.
Two plants that come to mind for the sometimes flooded bottom is Juncus, a native rush, which has a nice structural form and can tolerate summer dry as well as flooded winter conditions. Another is the native Douglas Iris. It can handle the seasonal flooding in a rain garden and has beautiful flowers.
A Google search of swales, bioswales, and rain gardens will yield oodles of photos and information.
We live in a climate with a long, warm, dry season. When we maximize our water infiltration, with both the rain and irrigation techniques, and conserve with mulch, we are depositing and maintaining our water in our soil bank. With prudent selection of drought tolerant plants we can minimize our withdrawals from the soil and increase our landscape resilience to droughts.