One of the founders of the permaculture movement, Bill Mollison, sets a high bar for property owners. “Every drop of water that leaves our system is a failure in our system,” he once told me. It is difficult to live up to this standard with active water-harvesting alone. Tanks will inevitably fill up, and most precipitation in many sites will not make it into a given cistern. Passive water harvesting is the act of storing precipitation, roof runoff, or stormwater directly in the soil. In addition to roofs, precipitation from roads, parking lots, sidewalks, pathways, patios, and earthen slopes — be they steep or gentle, vegetated or denuded — are all fair game for wise landowners in the desert.
In this age of inexpensive apps, strong plastics, and easily overnighted technology, passive water harvesting would seem almost anachronistic if it were not for its relatively low cost and high yield. Here is a mini-glossary of passive water-harvesting strategies to apply before our next monsoon season arrives:
Check dam – an installation used in gullies, ditches, and arroyos that slows the flow of stormwater, holds back sediment, and retains moisture. Often made of rocks ranging in size from 6” to 18” in diameter, check dams are typically placed in the narrowest sections of ephemeral waterways.
Curb cut – an opening or diversion point from a roadway that diverts runoff to an on-contour swale.
Mulch – any material placed on bare earth for one or more of the following purposes in addition to retaining moisture: creating shade, insulating the soil from extreme temperatures, and preventing erosion from wind, forceful forms of precipitation, and stormwater. Thick mulches can prevent some weeds. All mulches create habitat for beneficial microbes.
On-contour swale (a.k.a. “rain garden,” “ponding”) – a ditch with the dirt from the ditch placed on the downhill side in the form of a berm. The primary purpose of this type of swale is to allow stormwater to soak into the swale. The ditch and berm are typically level except where overflow spillways are located. Swales function best when they are long, skinny, and shallow rather than wide and deep.
One-rock dam – a check dam or gabion-like structure that is only one rock high. Built by hand with 4” to 8” rocks placed in a tessellating pattern, these dams can be of any width. In some cases, they resemble a rock-mulched area rather than a typical dam. Seeds sown under these dams germinate and mature due to the moisture retained under the rocks.
Wattle (a.k.a. “fascine”) – a large sausage-like product that can be purchased or locally fabricated. Often filled with straw, jute, coconut or other fibers, wattles can serve the same function as check dams, gabions, and weirs, or they can be tacked along contours in order to function like on-contour swales.
Wick – an installation that stores runoff at the root zones of plant material. Typically made of pumice, Growstone (now made in Albuquerque with Santa Fe’s recycled glass!), scoria, or other porous aggregates, wicks act like subsurface sponges.
Nate Downey is the author of Harvest the Rain (Sunstone Press, 2010) and the president of Santa Fe Permaculture, Inc. You can contact him through his new company website, www.permadesign.com.