Harvesting Water

The various ways to capture water run off.

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  • Topic: Watering
  • Keywords: watering, water catchment, water harvest, Watering
  • Date: July 2002

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As gardeners, we are all familiar with the process of harvesting fruits and vegetables to use for our meals. When we have more than we can use at the moment, many of us preserve the bounty from our gardens for later use. So it must be with water, our most precious resource that is in very short supply in much of the West this summer. Harvesting water becomes not only an environmentally appropriate thing to do but is also a matter of necessity. With the outdoor watering restrictions many communities have in place, we must seek ways to harvest as much water as possible to keep our landscapes alive.

Any impervious surface such as roofs, driveways, or parking lots, will generate large quantities of water during rainfalls. If not harvested, it will become runoff and will not contribute much benefit to your property and may cause erosion problems, either on your property or downhill parcels. This water needs to be harvested and put to beneficial use on your property. There are many simple and inexpensive ways to do this.

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.

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 rainbarrels can be connected together with piping for a larger storage capacity or there are portable cisterns that can be used in conjunction with a rainbarrel. To capture the maximum amount of water, there should be a rainbarrel under each downspout.

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

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 possibility 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.

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 of 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.

Your landscape will benefit from whatever you do to harvest water and you will be able to appreciate the beauty of flourishing plants.