Aromatic solvents for submersed weeds In canals with capacities of 70 cubic feet per second or less, control of submersed weeds with aromatic solvents or acrylaldehyde (acrolein) is less expensive than mechanical methods. In larger canals with capacities up to 200 cubic feet per second and where sprinkler irrigation is extensively practiced, chemical control is economical, as the aquatic weeds killed by the chemicals disintegrate slowly and do not cause clogging. Mechanical devices dislodge masses and fragments of aquatic weeds, which cause frequent clogging of the sprinkler heads, valves, and pumps. Recently, in the Columbia Basin of Washington, new techniques with xylene have controlled submersed water weeds in canals with capacities as large as 300 to 500 cubic feet per second. 2,4-D for emersed weeds Cattails, tules, and other emersed weeds in channels and ponds are largely controlled either by 2,4-D in oil-water emulsion, by amitrol, or by dalapon. Timely and regular use of these herbicides decreases the cost of controlling weeds and results in less frequent need for removing silt deposits by mechanical methods. The Bureau of Reclamation has reported 22 that the average cost of chemical control of cattail ranges from $25 to $43 per mile of channels, as compared with $407 to $418 per mile for draglining. 2, 4-D, silvex, or 2,4,5-T for bank weeds and phreatophytes Bank weeds and phreatophytes along channels, around ponds, and on flood plains can be controlled by 2,4-D, silvex, or 2,4,5-T. Chemical methods usually are more convenient and effective than mechanical methods. However, mowing, burning, and bulldozing often remove the vegetation more rapidly, are sometimes less expensive, and may be less hazardous to crops on nearby farmland. Nevertheless, the development of less expensive or more selective herbicides undoubtedly will extend the replacement of mechanical methods. Mechanical methods preferred in certain areas In areas where cotton, grapes, tomatoes, and other crops highly sensitive to 2,4-D, silvex, 2,4,5-T, and other phenoxy-type herbicides are extensively grown, mechanical methods are better adapted than chemical methods. However, weedy grasses can be controlled by amitrol, dalapon, or aromatic or fortified weed oils in such areas. Change in vegetation due to chemical treatments 23 Control of broadleaved weeds and brush along channels and around farm ponds by selective herbicides like 2-4-D, silvex, or 2,4,5-T has greatly aided the establishment of desirable low-growing grasses for stabilizing banks and preventing erosion and sloughing. The dense stands of grasses also prevent or reduce the encroachment of undesirable bank weeds. A recent study in Washington indicates that regular use of copper sulfate at frequent intervals to control algae in irrigation canals encouraged the spread of a dwarf species of water plantain, which competes with and eliminates or reduces the troublesome rankgrowing pond weeds. 22 Timmons, F. L. Profits from research on control of aquatic and ditch bank weeds on irrigation systems. West. Weed Control Conf. Proc. 16, pp. 41-46. 1958. 23 Boyle, W. D., and Suggs, D. D. Pond weed control, chaining versus aromatic solvents. U.S. Bureau of Reclamation Mimeo. Pub., 18 pp. 1960. SELECTED REFERENCES Edminister, T. W., and Staff, C. E. Plastic in soil and water conservation. Agr. Engin. 42: 182-185. 1961. Haws, Frank W., and Lauritzen, C. W. Plastics prevent water loss. Utah Farm and Home Sci. 17(2): 40-41, 48. 1956. Israelsen, O. W., and Lauritzen, C. W. Linings for irrigation canals. Reclam. Era 35(8): 167-168, 176. 1949. Lauritzen, C. W. Lining irrigation canals to save water and land. Agr. Engin. 34: 407-408, 410. 1953. Lauritzen, C. W. Ways to control losses from seepage. In Water, U.S. Dept. Agr. Yearbook 1955: 311-320. 1955. Lauritzen, C. W. Soil stabilizers for seepage control in irrigation canals and reservoirs. Indus. Engin. Chem. 47: 2245-2248. 1955. Lauritzen, C. W. Seepage control with plastic film. Irrig. Engin. and Main. 7(5): 18-19, 32-33. 1957. Lauritzen, C. W. Plastic liners for reservoirs. 10(3): 19-20. 1957. What's New in Crops and Soils Lauritzen, C. W. Linings for irrigation canals. Irrig. Engin. and Main. 9(13): 10-11, 1959; and 10(1): 12-13, 21. 1960. Lauritzen, C. W. Asphalt-coated jute liner developed for canals and reservoirs. Utah Farm and Home Sci. 21: 12-13, 28-29. 1960. Lauritzen, C. W. Ground covers for collecting precipitation. Utah Farm and Lauritzen, C. W. Lining irrigation laterals and farm ditches. U.S. Dept. Agr., Lauritzen, C. W., and Haws, Frank W. Asphalt-burlap linings for canals and reservoirs. Agr. Engin. 40: 340-342, 344. 1959. Lauritzen, C. W., Haws, Frank W., and Humpherys, Allan S. Plastic film for controlling seepage losses in farm reservoirs. Utah Agr. Expt. Sta. Bul. 391: 18 pp. 1956. Lauritzen, C. W., and Israelsen, O. W. West's canal linings studies. West. Con Earth linings for irrigation canals and 13: 531-538. 1948. Irrig. Engin. and Main. Mag. 1(3): 8. Lauritzen, C. W., Israelsen, O. W., and reservoirs to reduce conveyance losses. 1952. Lauritzen, C. W., and Peterson, W. H. Lining of canals and reservoirs saves land Materials for canal and reservoir linings. Butyl fabrics as canal lining materials. Utah Agr. Expt. Sta. Bul. 363: 16 pp. WEED CONTROL AND ITS EFFECT ON FLOW AND WATER MANAGEMENT Most irrigation and drainage channels are constructed to carry specific volumes of water, usually without adequate allowances for the effects of weeds. Weed growth and the resulting silt deposits reduce the volumes of flow in many channels much below the designed capacities. Chemical control of the weeds helps to restore the flow to designed capacities and to eliminate enlargement of channels to allow for expected weed growth. ECONOMICS OF VARIOUS METHODS OF CONTROL Hand and mechanical methods of controlling such weeds usually require much more manpower than chemical methods. In areas and seasons of labor shortage, the saving in man-hours and the more efficient and rapid weed control by herbicides are important considerations aside from the relative costs. Considerable manpower and machinery are released for other work. Effective herbicides now available that control submersed aquatic weeds are usually too expensive to use in canals larger than 200 cubic eet per second. In recent laboratory and field experiments, several chemicals show romise of giving satisfactory control of submersed aquatic weeds for or more years when applied to the bottom of the canals. Such hemicals might prove economical for use in large canals where subnersed aquatic weeds are causing a threat to adequate and timely lelivery of irrigation water. As a result of extensive and intensive use of herbicides, the time for nspection and for cleaning clogged structures, removing weed jams, and repairing ditch breaks and similar maintenance work was reduced n one area in California.24 Even more important is the more adequate and timely delivery of irrigation water to crops and the more prompt drainage of excess water from farmland. With such chemicals as xylene and acrolein, properly applied, the treated water may be used for furrow irrigation without injury to crops. CHEMICAL CONTROL MAY CHANGE CROPLAND MANAGEMENT Dependable weed control with herbicides in irrigation systems may eventually result in the acceptance of devices for automatic measuring and delivering of water, with a considerable saving in time now required for operating the turnout gates. The principal deterrent to this manpower-saving practice at present is the risk of clogging the measuring devices and automatic gates by floating weeds and the irregular flow of water caused by weed growth. The general use of herbicides to control weeds has had important effects on the management of croplands. The more ample and regular supplies of irrigation water permits greater latitude in the choice of crops and cropping practices. In other areas, the more efficient drainage of excess water through weed-free drainage ditches also permits a wide choice of crops and cropping practices. On the other hand, the differential tolerance of crops to herbicides used to control ditchbank weeds sometimes determines which crops may be safely grown adjacent to irrigation or drainage channels. Consideration must also be given to crops that may be safely irrigated with water from canals or ponds treated with herbicides. The persistence and behavior of herbicides in irrigation, drainage, and impounded waters must be more thoroughly understood to insure safety for use by man and animals. More complete and timely control of bank weeds along channels and around ponds by herbicides has, in some areas, eliminated or greatly reduced the production of weed seeds that may be carried to cropland by wind or irrigation water. 24 Timmons, F. L. Profits from rsearch on control of aquatic and ditchbank weeds on irrigation systems. West. Weed Control Conf. Proc. 16, pp. 41-46. 1958. IRRIGATION WATER APPLICATION MANAGEMENT PRACTICES Improving the efficiency of irrigation has been the aim for many years in the U.S. Department of Agriculture. Water application efficiencies now range from 30 to 80 percent, with the average less than 50 percent. Improvements can come about by better management practices proper land preparation, length of run, proper scheduling of irrigations according to time and quantity of water to satisfy the crop requirements, proper distribution systems to fit the farm layout and to fit the individual irrigated fields so as to supply adequate water in the time required and as indicated by the infiltration rate. Losses from improper management practices are excessive after irrigation if water percolates below the root zone or if it runs at the lower end of the field. FACTORS AFFECTING IRRIGATION EFFICIENCIES When water is scarce and crop products are unusually valuable, as in some irrigated sections of the Southwest, the irrigator uses the available water with care and relatively high irrigation efficiencies are obtained. In such sections, large expenditures are justified in order to save water. In farming regions where crop returns do not warrant large expenditures in order to reduce losses and waste, irrigation water is often applied with low efficiencies. This is especially true in regions where large supplies of water are available. Another factor entering into efficiency of water application is the cost of labor. In some cases this cost outweighs the value of water saved by more efficient operations. Future design of irrigation systems and methods of irrigation must take this item into consideration if irrigation applications are to be improved. Efficiency of irrigation water application may be affected by the infiltration rate of the soils. On sandy soils the infiltration rate may be excessive for good irrigation and on tight soils the infiltrationmay be too low to permit sufficient water to enter the root zone. The problem of infiltration, therefore, resolves itself into a reduction of infiltration rate or an improvement of infiltration rate, depending on the site conditions. Much research is going into the control of infiltration rates on irrigated lands, but at present there seems to be no economical materials available or method known. INFILTRATION REDUCTION Most data on reduction of infiltration rates have been developed for control of canal seepage and for the increase of precipitation runoff from water harvest areas. A need exists, however, for the reduction of infiltration on some irrigated soils. Furrow irrigation on sandy soils is not practical under present conditions, because of the very high intake rate of water into the soil. Controlled reduction of the intake rate so that the soil profile under reasonably long furrows will be wetted uniformly may be desirable. Water repellants of different kinds may be used to control intake rate. These would include asphalt emulsions, bituminous compounds, salts of fatty acids, and resinous compounds. The difficulty in the use of such materials would be in the control of the desired effect. Natural waterproofing materials often accumulate in sandy soils, so as to prove detrimental in reducing the water storage capacity. It should be added that such water-repellent sands have become noticeably erosive. INFILTRATION IMPROVEMENT Infiltration into tight soils may be improved by the use of soil stabilizers such as the polyacrylonitriles and other polymers. Some of these materials are quite effective in the soil layer treated. In deep, plastic clay soils, the mechanical difficulty of treating deep soil layers would pose problems. The cost of soil stabilizers at present makes their use on field soils of doubtful economic value. CONTROL OF EVAPORATION FROM SOIL Large amounts of water are lost to the atmosphere by evaporation from soils. This is a nationwide problem and research is being done at several locations. In the West the prime concern is with evaporation during and after irrigation. In the Midwest and East the concern is with evaporation after rains. Polyethylene film and plastics as mulches-Humid areas Four States in the Northeast Branch have used polyethylene film for moisture control and for mulch, with good success. A thin polyethylene film only 1.5 mil thick does an adequate job of preventing surface evaporation (fig. 11). Studies in New York showed that covering the soil with translucent plastic, which was slit to facilitate infiltration, increased available water as well as soil temperature early in the spring. Yields of corn. in 1961 were increased by 16 bushels per acre for a normal-season hybrid, and by 35 bushels for a long-season hybrid. On one plot the plastic was sealed to the cornstalk to prevent rain entering the soil. This plot, located on the top of a hill, produced 113 bushels of corn on the estimated 7 to 8 inches of water that was stored in the soil at planting time. In New Jersey black polyethylene film has been used to control surface evaporation in growing cantaloups. Tests have shown that there was enough moisture in the soil at field capacity to produce a crop of cantaloups. With black plastic film cover, the yields were as good as or better than where the melons were clean cultivated. Soil fertility measurements indicated less nutrient loss by leaching under the plastic. In Virginia a study of moisture conservation measures showed that evaporation suppression by a plastic mulch was of greater benefit to tobacco than transpiration suppression by fog nozzles. There were no evaluations of relative humidities or soil moisture conditions to determine just how greatly conditions were altered by the different treatments. |