Microcatchment Water Harvesting
By David A. Bainbridge
Associate Professor
United States International College of Business
Alliant International University
San Diego, CA 92131
Limited water availability is the primary factor controlling plant
establishment and growth in the dry lands and deserts of the world. Water
shortages are accentuated in disturbed areas where vegetation removal and
alteration of the surface soil further limit surface water retention and
infiltration into the soil. 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).
Further Reading:
Ehrler, W.L., D. H.Fink, and S.T. Mitchell. 1978. Growth and yield of
jojoba plants
in native stands using runoff-collecting
microcatchments. Agronomy Journal
70:1005-1009.
Evenari, M.L., L. Shanan and N. Tadmor. 1982. The Negev: The Challenge of
a
Desert. Harvard University Press, Cambridge,
Massachusetts.
Fidelibus, M. and D.A. Bainbridge. 1995. Microcatchment water harvesting
for
desert revegetation. SDSU Biology for CalTrans,
12 p.
Fink, D.H., K.R. Cooley, and G.W. Frasier. 1973. Wax-treated soils for
harvesting
water. Journal of Range Management 26:396-398.
Shanan, L. and N.H. Tadmor. 1979. Microcatchment System for Arid Zone
Development. Hebrew University, Jerusalem. 99 p.
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