A Primer on Water Harvesting
Relish our recent rains. Celebrate our seminal snows. Verily cherish our verdant vistas. Just don’t be fooled.
Fluke weather is no excuse for ignoring the truth. Ours is a parched, windswept land of desiccated soils and sparse vegetation. Ironically, on those rare days when storm clouds form and our incessant sun eases up, monsoons of mass destruction can wreak havoc. It’s often even unclear as to whether certain storms help heal the land or if they ultimately injure her with their ensuing erosion.
Yes, enjoy wet weather, but remember: we choose to live in a desert. If we are to bestow a livable community to our children, an entirely new approach to water is necessary. Water harvesting — collection of precipitation — is the fountainhead of this step forward.
Assuming 12 inches of rain, a 2,000-square-foot pitched-roof house plus 1,000 square feet of garage, portals and sheds, over 22,500 gallons of water can be harvested in an average year. (Note that the evaporation associated with our flat roofs makes our potential per capita average harvested gallons slightly less). Since, according to the city, an average Santa Fean uses 55 gallons of water daily, this amount of surface area would provide all the water needed for an average resident. If we decide to treat and recycle effluent, it’s possible to imagine a Santa Fe household consuming only harvested precipitation.
Certainly mainstream culture wanders far from this ideal. However, if our community continues to grow, our conventional water supplies will continue to decline. Logic dictates inevitable, revolutionary changes. At this historical moment our role is to be part of a smooth transition toward commonplace water harvesting. When this transition is complete, conventional water supplies will be used only in times of drought. This will allow our aquifers, rivers, lakes and reservoirs to recharge during soggier times.
Two types of water-harvesting systems are easily distinguishable: active systems and passive systems. Active systems include moving parts that need regular maintenance. These systems often feature a storage tank (called a cistern), which allows for harvested water to be used on demand long after a given precipitation event.
Cisterns can be above ground or below ground. Above-ground tanks usually cost less than below-ground tanks. Not only do the materials cost less because they do not have to withstand the weight of the surrounding soil, but labor costs are also less compared to the costs of a below-ground system (namely, the excavation, installation, backfill and appropriate use, or disposal, of excess excavated material).
Active systems also need delivery systems (often called conveyance systems) that capture runoff from canales and downspouts. Collecting all the water from a pitched roof with gutters and downspouts is a relative breeze. The canales that project from our flat roofs complicate matters because runoff can shoot several feet away from a building or be blown with the breeze sideways or sometimes back against the stucco. The best solution to this spastic situation is to direct runoff to a box drain via a below-grade “funnel,” made on site with a 5-foot by 5-foot sheet of shower liner (found at hardware stores). Runoff flows directly into the drain or slides easily into the drain after first hitting the shower liner. Cover this vortex with a few inches of medium-sized gravel or river rock. In places where significant debris collects on a given roof, delivery systems also include sediment traps that prevent particulate from polluting the tank.
Once the water is in the tank, getting it out can be as simple as using a sump pump and hose. It can also be as complex as building an underground pump house and installing a pump, pressure tank, water-level reader, various electrical connections, an air vent and an incandescent light and/or insulation (to prevent freezing in winter). At this point in your efforts, it’s often worth connecting your cistern to a system for use inside the house or to a drip irrigation system. This means adding float switches, filtration, a pressure regulator, a pump start relay, irrigation valves, a computerized irrigation clock and a connection to a supplemental water system such as a well, private utility or municipality. Certainly active systems are the way to go when it comes to providing efficiency and productivity, but they do not fit every budget and do have appreciable potential for things to go wrong.
Fortunately, one of permaculture’s most helpful principles is “Start small.” If resources are spent unwisely, the transition to a sustainable approach to anything is sluggish, so it’s best to build constantly on one’s successes.
Passive water-harvesting systems require little or no maintenance and often cost much less money to install. On the flip side, passive systems do not allow for future use on demand. Water in passive systems is stored at or near the root zones of plants.
Other than mulching, the easiest way to harvest water locally (and recharge the aquifer regionally) is to contour your land appropriately. Even seemingly flat land can be subtly sculpted to slow the flow of water during a monsoon. On-contour swales are ditches dug perpendicular to any slope with the dirt from the ditch placed on the downhill side in the form of a berm.
If we install appropriate plants to take advantage of our swaling, supplemental water is needed only to establish a landscape and, sometimes, to keep certain plants alive during drought. As opposed to a being a constant drain on our aquifer, such a landscape ultimately helps restore regional water supplies by allowing excess water to percolate through the earth and not slide sediment into supplies downstream.
Check-dams in Arroyos
Surface runoff should also be harvested in arroyos. Depending on the expected peak volume and velocity of runoff during an extreme storm event, check-dams of various materials and sizes can be installed perpendicular to flow. In low-flow arroyos and head cuts (the beginnings of arroyos), branches and other organic matter or even fences (called weirs) can be effective. In medium-flow arroyos, properly sized and stacked rocks work well. In high-flow channels, 4- to 8-inch river rock wrapped in wire cages (called gabions) are often prescribed.
Upstream from such structures, relatively level terraces form naturally with sediment brought in by large storm events. Water collects around the perimeter of such terraces and at the base of these structures, and can be used both by volunteer and planted species.
Although swales and check-dams can use water that comes directly off of a canale or downspout, the most efficient passive system for harvesting roof runoff is the pumice wick. In this case, runoff flows into a perforated pipe surrounded by a trench filled with pumice, which is extremely hygroscopic and absorbs several times its own weight in water. The pumice is covered with a thick layer of newspaper to prevent the wick from clogging. Then a few inches of native soil are spread and lightly tamped on top of the trench. Trees, shrubs, perennials and carefully placed annuals are planted on either side of the wick. Since their roots suckle up to this underground sponge, plants establish much more quickly with much less supplemental water.
There are many more techniques for harvesting precipitation. If they should ever work in conjunction with serious treatment and recycling, the only forces preventing us from being aquifer-independent are our own motivation and lack of money. These forces, of course, are real, so this transition will take time. But if we start now, maybe our children will have a chance to take water harvesting to the level that may one day prove necessary for their survival.