WATER HARVESTING
Mary Ann Walz

Water harvesting is not a new science. Evidence of water harvesting by prehistoric cultures is abundant in this area as well as other parts of the arid world. Waffle gardens created with a grid of depressions and ridges to hold water were often filled with stones to act as mulch and to capture additional heat from the sun. Natives used these gardens in the Ojo Caliente area to grow cotton.

With our arid climate and precipitation patterns where we have little or no rain for weeks or months at a time, it makes good sense to harvest whatever water we can. Harvesting techniques range from simple swales to sophisticated cisterns. My personal preference is toward those that are simple and inexpensive to install by a homeowner.

Modification of topography can take many forms and can be one of the least costly ways to retain water in your landscape. The key is to observe where water goes during a rainfall and determine the best way to slow it down so it can soak into the soil. Since most home sites have some degree of slope, whether small or steep, it’s necessary to slow the flow of water down the slope. A series of swales and berms or terracing are two ways to harvest water on slopes.

Swales and berms are inexpensive and easy to install. A swale is a shallow ditch excavated along the contour of a slope. The excavated soil is placed on the downslope side of the swale to create a mound of earth called a berm. Swales and berms can take many forms and can be designed to retain a little or a lot of water. Plants placed in the swale or along the lower part of the berm will benefit from added moisture. Swales and berms can be built so that any excess water will flow into another swale farther down the slope. A wide, shallow swale combined with a low berm will be barely noticeable once covered with vegetation. A similar concept should be used for trees or shrubs planted on slopes. Tree wells made of soil should be “U” or “V” shaped and open on the high side so water flowing down a hill can be captured in the well

Terraces are the more traditional method to retain water on slopes. A terrace requires the use of stone, landscape timbers, or other material to create a retaining wall where the slope is cut to make a flat planting area. A terrace usually requires the importation of topsoil to backfill behind the retaining wall. Small terraces are a potential do-it-yourself project but any large terrace may need to be done by professionals to make sure the retaining wall won’t collapse from the weight of the soil behind it.

Hard surfaces at ground level can be designed so rainwater flows onto landscaped areas instead of into the street. Most large parking lots that have landscaping place the plant materials a curb’s height above the lot. If the landscaping were below the grade of the parking lot then the rain would run into the landscaped area. Next time you go shopping, look at the design of parking lots to see where the runoff goes.

French drains, covered trenches used to spread out excess water, can be used to direct rainwater from hard surfaces to areas of the landscape that need extra water. Trees are one of the best candidates for this supplemental water because of their deep roots. There are several ways to make a French drain. One way is to dig a trench from the water source (roof, driveway, and parking lot) to the area where water is needed. Make sure the trench has a slight slope away from the water source. Place a perforated PVC pipe in the bottom of the trench, line the trench with gravel, and backfill with soil.

Gray Water is used household water from bathroom sinks, showers, tubs or laundry. Any residence has a large potential supply of gray water but there are regulatory requirements governing its use due to the potential for pathogens in the water. If used, gray water is more suitable for ornamental landscapes than for food crops.

Rain barrels can be used under each downspout to catch rainfall as it runs off the roof. The barrels have spouts at the bottom to drain the water when needed for irrigation. Most rain barrels can be connected together with piping for a larger storage capacity or there are portable cisterns that can be used in conjunction with a rain barrel. To capture the maximum amount of water, there should be a rain barrel under each downspout.

Cisterns are large capacity containers, either above or below ground, typically used to store rainwater. Rainwater is directed off the roof through gutters and downspouts into the cistern. The roof of a typical house will generate thousands of gallons of water that can be used for irrigation later. A submersible pump can be used to gain access to the water in underground cisterns while many above ground cisterns operate on gravity flow. Cisterns can hold large amounts of water but can be fairly expensive to install, particularly those that are underground.

Talk to your friends and other gardeners and they’ll let you know about other ways to harvest water. Your plants will be appreciative and reward you with a beautiful garden.