By Diane C. Kennedy.
Water harvesting is a main practice in permaculture. In undisturbed habitats, plants and plant debris cover the ground. Water falling directly onto the soil can be as compacting as driving a truck over it, and nature combats compaction with leaves. Raindrops hit upper canopy tree leaves, shatter and so on down to the ground where leaves ease it into the soil. In landscapes leaf clearance creates an expanse of uncovered soil, rain and irrigation acts like a hammer. Compacted earth cannot infiltrate water, therefore runoff occurs. Very little water penetrates the soil down deeply where the roots need it, but instead carry bits of soil through erosion rills to deposit them into stream beds, silting them in. Flooding is a natural consequence of defoliating the landscape.
Roots actively seek water. Causing water to gently penetrate deeply into the soil encourages deep root growth and healthier, longer-lived plants.
There are several easy methods to prevent erosion and to encourage deep irrigation. First, sheet mulching is important to hold in moisture, protect the soil from water impact, provide habitat for soil microbes, and keep weeds at bay. Sheet mulch around trees is simply a thin layer of newspaper or cardboard placed on top of the weeds, beginning a few inches from the trunk (to prevent rotting) out to the dripline or beyond, and topped with 4-6 inches of mulch of any type. With each inch of mulch you should reduce irrigation by about 10%. Sheet mulching pathways also prevents weeds from emerging and thereby eliminating the need for herbicide or mowers. Free mulch can be had from arborists.
Another water harvesting method is to dig swales. Swales are level-bottomed ditches that run along contour lines across your property, or smaller ditches above each plant. These swales capture and pacify water, spreading it along their length to gently sink deeply into the soil. Swales can be used in conjunction with irrigation to capture 100% of water and deliver it deeply to the root ball. Where swales are impossible, rain catchment basins work in a similar way to collect, pacify and sink rainwater. One inch of rain over one acre in an hour is 27,154 gallons of free, clean water. Simple earthworks such as swales can sink that water into the soil, which will then move slowly through the landscape, hydrating roots and water tables, and eventually seep into waterways during the rainless months to keep streams alive.
Swales and sheet mulch can make a major difference to the ecology, to keeping wells and waterways viable, to the health of your plants, and to your water bill, and they cost little to nothing (depending upon their size and equipment needed) to install. You can dig a swale above an existing fruit tree with a shovel and collect a significant amount of water by doing so.
Diane and her daughter Miranda Kennedy operate Finch Frolic Garden Permaculture, a food forest in Fallbrook, CA. They consult, design and lecture on permaculture. Search for more on swales, water harvesting and sheet mulching at www.vegetariat.com