Microcatchment Water Harvesting
The construction of microcatchments, alterations in the topography of a site to direct precipitation runoff to plants, offers a low cost, "passive" means of increasing the amount of water available to plantings (Fidelibus and Bainbridge, 1995).
Microcatchment systems provide many advantages over alternative irrigation schemes. They are simple and inexpensive to construct and can be built rapidly using local materials and manpower. The runoff water has a low salt content and because it does not have to be transported or pumped it is relatively inexpensive. The hydrological data needed for an efficient design can be collected through observations over two to five years even in areas with limited rainfall. And finally, the systems are easy to operate and maintain and relatively safe from failure.
Microcatchments are simple to construct and can be built rapidly using local materials and manpower. Once constructed, minimal maintenance is required. The rainfall runoff collected by microcatchments has low salt content and within catchments leaching is enhanced and soil salinity is often reduced (Shanan and Tadmor, 1979). Microcatchments have been used successfully with domesticated crops for centuries, but have only recently been used to supplement rainfall for native vegetation (Ehrler et al. 1978).
There are four types of microcatchment systems: micro-watersheds, runoff strips, contour bench terraces, and catchment basins. Of these, runoff strips and contour bench terraces are best suited for agriculture as they require expensive mechanical re-shaping of the surrounding terrain and create regular patterns which are inconsistent with a natural appearance. Microwatersheds and catchment basins can be built using hand labor and are thus less expensive and more adaptable to revegetation projects.
Microwatershed systems include mound and strip collectors. Both can be built with mechanical equipment or by hand on gently sloping plains (ideally with slopes less than 5%). The strips are bordered on each side by ridges 8-20 inches (20-50 cm) high and 6.5-16.5 feet (2-5 meters) apart. The result is a series of linear strips well suited for crops such as corn.
In mound systems the soil surface is shaped by hand into 4-20 inch (10-50 cm) tall mounds spaced 2-5 meters apart. When organized into a regular pattern, this system is suitable for many types of farm crops, including melons and squash. The mounds are also effective when arranged in a more random manner suitable for revegetation and restoration efforts. The gradients of the microcatchments should be between 1-7% . Since square or rectangular plots are the easiest shapes to stake out, they are the most commonly used, but basin shapes can also be tailored to suit the geography of the site, causing less disturbance, and maintaining a more natural appearance.
To determine expected yields from microcatchments three rainfall characteristics must be evaluated: the average annual rainfall, peak rainfall intensity, and the minimum expected annual precipitation. Consideration of a site for microcatchment construction must also include four main physiographic factors: the runoff producing potential; the soil surface condition (cover, vegetation, crust, stoniness); the gradient and evenness of slope; and the water retaining capacity of the soil in the root zone profile. These all contribute to the runoff threshold coefficient which is a key factor in determining the optimum size for a catchment. Other factors affecting the infiltration capacity of a particular area include: the moisture content of the soil; macro-pores in the soil as a result of decaying roots or burrowing animals; and the compaction of the soil.
The optimal size of the microcatchment for each species depends upon many factors including normal precipitation for the area where the catchment is planned, the soil quality, and the slope (Evanari et al., 1982). Also important are the size and depth of the planting basin in relation to the size of the catchment area. These factors determine the size of the surface area wetted by runoff and the volume and depth of the water column in the soil. If the infiltration rate of the soil and the water demands of the plant are known, the size of a catchment basin can be estimated. Microcatchments are of course only effective when rainfall is sufficiently intense to generate surface flow. To improve catchment efficiency under limited precipitation, catchments are often covered with a water impermeable barrier. These barriers can dramatically increase runoff. In Anza-Borrego Desert State Park a butyl rubber catchment apron was found to move water to storage with rainfall as low as 0.25 millimeters (Bainbridge, unpublished).
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By David A. Bainbridge
Associate Professor
United States International College of