Domestic water supply is augmented by this roof-top catchment on a home near Mountain View, Hawaii. -f- Harvesting water for agricultural, wildlife, and domestic uses Reprinted from the Journal ol Soil and Water Conservation May-June 1380, Volume 35. Number 3 © 1980 Soil Conservation Society of America By Gary W. Frasier WATER harvesting can be a source of American Indians used similar systems 700 water for a variety of purposes in to 900 years ago in the southwestern arid and semiarid regions when common United States (11). sources, such as streams, springs, or wells, In some parts of the world, precipitation Collection and storage of fail. In addition to supplying drinking collected from the roofs of houses provides precipitation from treated water for people, livestock, and wildlife, a household water supply. However, de catchments provide an important water-harvesting systems can provide sup velopment of central water supply systems supplementary water supply for plemental water for growing food and for homes and large irrigation projects for livestock, wildlife, and humans fiber crops. Often the necessary water can agriculture have resulted in many water- when normal sources of supply be obtained without large expenditures of harvesting techniques being forgotten. Fu fail in arid and semiarid regions energy. ture demands for water may well necessi The principles of water harvesting are tate the rediscovery of some of these old not new. These techniques were used as practices. early as 4500 B.C. by the people of Ur and Designing a water-harvesting system others in the Middle East (9). In fact, re searchers have reconstructed water har Regardless of the ultimate water use, all vesting systems used for runoff farming in water-harvesting systems have two basic Israel's Negev Desert 4,000 years ago (4). components, a catchment apron for col lecting precipitation and some type of wa Gary W. Frasier is a research hydraulic ter-storage facility. The collected water engineer at the Southwest Rangeland Watershed may be stored in a tank or reservoir or in Research Center, Science and Education Ad ministration, U.S. Department oj Agriculture, the soil profile for runoff farming. Tucson, Arizona 85705. Most water-harvesting systems also re- May-June 1980 125 quire peripheral equipment, such as con treatments, such as asphalt-fiberglass, facility is not controlled, the sizes of the veyance pipes, drinking troughs, evapora have runoff efficiencies greater than 95 catchment and storage must be increased tion controls, and fencing. This equipment percent. They will collect water from rains to compensate for the quantity of water is important to the system's performance. of less than 0.1 inch. Treatments with low that will be lost by evaporation. Evapora A $5 float valve can cause a $20,000 water- er runoff efficiencies (less than 50 percent), tion losses from a storage facility are often harvesting system to fail at a critical time. such as land smoothing or compacted greater than the water quantities needed No water-harvesting system is suitable earth, require relatively high rainfalls to for the intended use. for all applications. There are a wide initiate runoff. These less efficient treat Harvesting water for wildlife, livestock range of construction materials, site condi ments usually require larger catchment tions, methods of installation, and uses for areas and storage facilities to provide The Forest Service, Bureau of Land the collected water. Each system must be water during periods when precipitation is Management, and other agencies use wa individually designed to fit the needs and less than necessary to produce runoff. ter-harvesting systems where more com conditions of a specific site and use. The site selected for a water-harvesting mon types of water development are not Some design factors are easily estimated, system should have a topography that min feasible. Systems are being installed in for example, the time distribution of the imizes the work needed to prepare the areas accessible only by four-wheel-drive required quantities of water. Other fac area. The catchment treatment dictates vehicles. Some relatively small systems fur tors, such as rainfall quantity and frequen the required site conditions and the nish water for wildlife. Other units feature cy or probability, require careful consider amount of surface preparation necessary to large catchments and storages capable of ation and judicious use. Other important ensure satisfactory performance. Treat furnishing water to several hundred head factors include the availability of labor and ments based on soil modifications, such as of livestock. The systems are dependable equipment, a site's soil and topography "roaded" catchments and soils treated with water sources for both wildlife and live characteristics, the site's accessibility, water repellents, require careful attention stock when other water supplies fail (2). catchment runoff efficiency, and the rela to maintaining slope angles and lengths to tive unit costs of suitable catchment treat minimize soil erosion on the catchment Catchment treatments. Two treatments ments and water storages. apron. Site conditions and surface prepa of catchment aprons used extensively in the Usually, certain key factors are selected, rations are not as restrictive for membrane southwestern United States are asphalt-fi and the system is designed to satisfy these treatments, such as sheet metal and as berglass membranes and paraffin wax soil items. For example, catchment area and phalt-fiberglass. These treatments have treatments (5, 10). The asphalt-fiberglass storage volume can be overdesigned to en been used on extremely rough surfaces treatment is commonly used on catchment sure that water is available during periods with slopes up to 20 percent. aprons up to or exceeding 3,000 square of below-average precipitation. This same For many water-harvesting systems, the yards. The membrane is assembled in system would be oversized for periods of water storage facility is the single most ex place by laying strips of chopped fiberglass average or above-average precipitation, pensive item. By minimizing the size of the matting across the catchment surface and however. The user must decide if the cost storage, a system's cost can be reduced. saturating the matting with an asphalt of the larger installation is justified by the However, minimizing storage size may re emulsion. Once the asphalt has partially assurance of having some water during sult in water being lost as overflow with cured (2-10 days), a second coat of the critical periods. Smaller systems that cost the storage facility during parts of the emulsion is applied to seal the membrane's less can be used if limited quantities of water year. surface. water in periods of below-normal precipi The presence or absence of subsurface Some installations are coated with a pig- tation are not critical. rocks or rock layers within the soil profile mented paint to reduce the rate of asphalt Catchment treatments affect the re may restrict storage alternatives to some oxidation and/or deterioration, which in quired sizes of both the catchment and form of above-ground storage facility. creases the life of the treatment. Installa storage facility. Sheet metal or membrane If evaporation from the water storage tions without a protective coating usually require a new seal coat every 3 to 6 years. Catchments with protective coatings last 10 years or more. Unfortunately, protec tive coatings suitable for asphaltic surfaces are relatively expensive, more than $1 per square yard. When initially installed, the asphalt-fi berglass membrane is flexible and con forms to irregularities in the ground sur face. After curing, the membrane becomes semirigid, capable of withstanding minor mechanical damage. On several unfenced catchments, cattle and wild burros have walked across the surface without damag ing the membrane. The asphalt-fiberglass membrane has a runoff efficiency exceed ing 95 percent. Rainfalls of less than 0.05 inch produce runoff (6). The paraffin wax soil treatment is being An asphalt-fiberglass membrane covers this wildlife water catchment near Phoenix, Ari used successfully on catchment areas up to zona. 4,000 square years. This treatment consists 126 Journal of Soil and Water Conservation of spraying molten paraffin wax (average -,E8t'mate.d ins»al|a"on cost, longevity, and runoff efficiency for various water-har- melting point 125-130cF) from a roofing trostnionts. tar kettle or asphalt distributor truck on a Runoff Estimated Installation Catchment smoothed catchment surface at a rate of 3 Efficiency Longevity Cost Treatment (%) to 4 pounds per square yard. The sprayed <vr) ($/sq. yd.) Land clearing 20-30 5-10 0.01-0.02 wax usually solidifies upon contact with Soil smoothing 25-35 5-10 0.04-0.06 the soil surface. Within a few days, the Silicone water repellents 50-80 5-8 0.15-0.30 Paraffin wax sun's heat will warm the soil surface 60-90 5-8 0.30-0.50 Concrete ■ 60-80 enough to remelt the wax, which then 10-20 2.00-5.00 Gravel-covered sheeting 70-80 10-20 0.50-0.75 Asphalt fiberglass soaks into the soil to a depth of about 0.5 85-95 5-10 1.00-2.00 Artificial rubber inch. The wax coats each soil particle, 90-100 10-15 3.00-5.00 Sheet metal 90-100 20 creating a water-repellent layer that resists 3.00-5.00 infiltration. The wax does not fill or plug the soil pores. Table 2. Summary of analysis of elements In parts per million (ppm) from samples collected from various water harvesting catchment surfaces. This treatment is particularly applicable Catchment Surface to lighter, coarser textured soils (sandy, Silicone sandy loam) in areas where the tempera Public Paraffin Water Galvanized Health ture of the soil surface will exceed 130 °F Constituent Asphalt Wax Butyl Repellent Steel Standard for a few hours during the day.