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<br />001502
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<br />rains of O. 5 inch or less seldom produce runoff except when the Lntensi-
<br />ties are very high or soils in the drainage basin are uncommonly tight:
<br />When soils are sandy, rains of an inch or greater may be required to
<br />produce any runoff. By decreasing soil permeability through various
<br />methods of treatment, increased runoff may be obtained from both larger
<br />and smaller storms (Meyers, 1961). Theoretically, if the surface is
<br />rendered completely impervious, the runoff should approach the total
<br />amount of the rainfall, but in practice this is impossible to attain.
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<br />To be successful, any treated area must yield sufficient water for
<br />the livestock needs. Assuming again that the facility must produce suf-
<br />ficient water for 100 head of cattle, the need will vary from 150,000 gal-
<br />lons for a 5-month use to 360,000 gallons for year-long use. For smaller
<br />operations where fewer than 100 head are to utilize the supply, the water
<br />need will be correspondingly less and the treated area can be reduced.
<br />On most Bureau of Land Management range lands the average annual pre-
<br />cipitation varies from about 6 to 10 inches. Using an average of 8 inches
<br />and assuming a 75 percent runoff factor for treated lands, it would require
<br />slightly less than 1 acre of runoff area to supply the requirements for 5
<br />months of grazing for 100 cattle and "!-pproximately 2.2 acres for the full,
<br />year and since there are certain to be losses from evaporation and other
<br />causes, a compensating increase in the collection area will be essential.
<br />With higher rainfall and greater runoff the runoff area can be reduced and
<br />naturally the opposite will hold for less favorable conditions.
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<br />Methods of treatment to increas e the runoff fall into two general
<br />classes: (1) treatment of the soilto decrease permeability and (2) cover-
<br />ing the ground with some type of impervious blanket or pavement. The
<br />effectiveness of each can be increased by selecting areas for treatment
<br />which have a natural high runoff potential, such as snowdrift areas or
<br />areas where orographic influences favor increased precipitation.
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<br />Reducing soil permeability
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<br />In land treatment methods, s oil permeability is deer eased either
<br />by mechanical compaction or by dis per s ion of clay particles through us e
<br />of chemicals. Compacting by sheep-foot rollers or pneumatic tampers
<br />when the soil has optimum or slightly higher moisture content can greatly
<br />reduce permeability and thus increase runoff. Specifications for this
<br />type of treatment are available in standard engineering texts and need not
<br />be repeated here. To be effective the soil must have a fairly high content
<br />of silt and clay, be relatively free of rock, and the relief must be such as
<br />to permit use of the heavy compacting equipment. Water will probably
<br />need to be applied artifically in treatment as rains seldom are sufficient
<br />to supply optimum moisture content to the soil to the depth needed.
<br />Driving bands of sheep over wet soil is another effective method of
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