Cost of ground covers Cost would appear to be about the only limiting factor involved in developing water by this means, when we consider that two-thirds of all the precipitation is lost through evaporation and transpiration without producing any economic gain.20 The cost of developing water with ground covers depends on two things-cost of the ground cover, including installation and replacement, and the precipitation in the area. Based on the assumption that a ground cover in place will cost $1 per square yard and precipitation is from 8 to 18 inches, the cost per 1,000 gallons will vary from $0.86 to $3.10 per 1,000 gallons (table 4). Potential use of covers Not only do ground covers offer an opportunity to develop water for livestock but culinary water as well. Culinary water developed by this method will be essentially free of salts and organic materials frequently found in water from other sources. Except for cost, it even offers a potential source for irrigation: If the precipitation falling on a section of land were collected and stored it would be sufficient to irrigate 213 acres, allowing 4 acre-feet per acre. "Hendricks, E. L. Hydrology. Science 135(3505): 699–705. 1962. FIGURE 4.-Layout for collection of water from a ground cover installation. The bag has not been filled. TABLE 4.-Cost of developing water by means of ground covers 1 Cost per 1,000 gallons At two locations (North Dakota and Texas) ground covers are being ested for use on contributing areas; in other words, the covers conentrate the runoff on cropped benches. At the Mandan field station North Dakota) black polyethylene plastic sheeting (0.006-inch thickess) is being used to cover an area that will contribute runoff which vill collect in a conservation bench terrace below. The contributing rea and the bench are each 30 feet wide. The average (1959 through 1961) soil moisture storage and yields of continuous wheat on the bench with a plastic-covered contributing area as compared with contributing areas covered by wheat or grass re shown in table 5. The plastic sheeting was laid in the fall of 1958 and is still in good condition. Patching was necessary in a few instances to repair damage by rodents. TABLE 5.— Yields of wheat on benches with various contributing areas, 1959–61 Similar results were obtained at the Southwestern Great Plains Field Station, Bushland, Tex., where plastic watersheds were installed above level basins 54 by 95 feet. The first rain of any consequence was accompanied by hail that damaged the plastic sheets. However, in spite of hail damage, a watershed equal in size to the plot area produced runoff to increase the stored soil moisture 3.1 times the soil moisture check plot. When evapotranspiration was accounted for it was apparent that almost 100 percent of the rain that fell in a 2-week period ran off the plastic watershed. Another use of ground covers is indicated by experiments in Ohio. It was found that where surface water is in short supply, runoff can be substantially increased by covering the land surface with thin (0.004-0.006 inch) pliable plastic sheets. In one season, May through September, runoff from a plastic-covered cornland area in Ohio totaled 14.7 inches as compared with 3.3 inches from similar cornland without the plastic cover. The plastic was destroyed at corn harvest, but the possibility of developing mechanical means of reclaiming and reusing such materials appears likely. Experience in application and maintenance of plastic strips over field-size areas is very limited; provision must be made for handling large rates and amounts of runoff water from major storms; durability of materials exposed to different climates are not known; and effect of plastic cover on reduced ground-water recharge is not yet evaluated. Plastic material is available in sheets up to 32 by 1,000 feet. Materials that cost 1.6 cents per square foot would total about $700 per acre. The cost of plastic film varies, but the prices usually quoted range from 2 to 6 cents per square foot, depending on the thickness required. Cutback asphalt emulsified in water and sprayed on soil to increase runoff from rainfall was not successful for the one experiment tried in Texas. The asphalt provided a good ground cover for less than 1 month. The thickness of the film seemed to have little influence on the durability. The soil of the test site cracks very badly upon drying. The particle size distribution of the topsoil at the site is sand, 39.16 percent; silt, 31.59 percent; and clay, 29.25 percent. Failure of the asphalt film appeared to be due to failure of the soil under the film rather than to failure of the asphalt. Soil adhered to the underside of the film after movement by the wind. More research is needed to determine proper methods of preparing land and installing the film. Cost ranges from $150 to $300 per acre covered and depends on the thickness of the film laid down. SELECTED REFERENCES Anonymous. More water for stock on ranges. The Utah Farmer, October 20, 1960. Farm Jour., April Anonymous. New ways to get stock water on a dry range. 1961. Edminster, T. W., and Staff, C. E. Plastics in soil and water conservation. Engin. 42(4): 182-185 and 42(5): 248-250. 1961. Agr. Harrold, L. L., Peters, D. B., Dreibelbis, F. R., and McGuinness, J. L. Transpiration evaluation of corn grown on a plastic-covered lysimeter. Soil Sci. Soc. Amer. Proc. 23: 174-178. 1959. Lauritzen, C. W. Ground covers for collecting precipitation. Utah Farm and Lauritzen, C. W. 1961. Lauritzen, C. W. Collecting desert rainfall. Crops and Soils 13(9): 7-8. 1961. RECHARGE UNDERGROUND RESERVOIRS Recharge of ground water supplies is a growing practice. A major problem occurring in many areas is the desilting and clarification of water-by flocculants, algicides, and bacteriocides-that is to be put underground either by surface spreading or through injection wells. Injection wells are particularly vulnerable to sealing by silty waters and bacterial slimes. Flocculants WATER TREATMENT Results from experiments in Texas and California indicate that flocculants, described as nontoxic, synthetic, organic materials, will reduce silt plus clay content of the water. On one lake in Texas, a flocculant was applied by a crop-dusting airplane. The silt plus clay content of the lake water was reduced 93 percent after 1 day. The effect lasted for about 3 days. In other tests on other lakes the flocculant removed 40 to 90 percent of the silt plus clay content. Desilting tests by the Los Angeles Department of Water and Power were accomplished at a cost of $1.50 per acre-foot of water treated. Algicides and bacteriocides Algicides and bacteriocides are at present used only slightly in recharge operation, primarily because of the costs. In some instances where the costs are justified, water is chlorinated as it goes into injection wells. Chlorination has proved feasible and has aided in maintaining injection rates. SOIL TREATMENT There is no great use at present of flocculants and soil stabilizers, although greater uses can be visualized in recharge operations. Practically all the use so far has been on experimental ponds. Flocculants The chemicals tested in laboratory and field were Krilium, Orzan, ferric sulfate, sulfuric acid, and gypsum. No cost-benefit ratios were established. These treatments can vary from below $100 to a few hundred dollars in cost per acre. The materials tested have been only slightly to appreciably effective for water infiltration, depending on chemical material, soil characteristics, and methods of application. Organic residues Organic residues have been effective in promoting soil structure and stability of structure and permeability. Although an organic residue like cotton gin trash has been available at no cost to the farmer, the cost of picking up, trucking, spreading, and disking the trash has amounted to $100 per acre treated. Other organic materials may be available as the result of vegetation grown; costs for use as a soil treatment would largely be associated with the incorporation of the material in the soil and wetting and drying. 86694 0-62- -5 |