ture improves crop growth on agricultural lands and betters forage on pasture and meadow lands. Need for improved supply In many areas of the West, notably parts of southern California, central Arizona, and the Panhandle area of northern Texas, ground water levels are declining, and it is becoming increasingly evident that alternate sources of water must be developed or a limit placed on the rate of economic utilization in order to curtail use in these areas. The needs of an expanding urban population must be met, at least in part, from reduction of losses tolerated in times of plenty and supplemented from supplies made available by improved resource manage ment. REVIEW OF BACKGROUND EXPERIENCE IN PROTECTION OF WATER RESOURCES CONTROL OF SUPPLY Vegetative and tillage practices Traditional practices for the control of surface runoff have consisted largely of vegetative and tillage practices on agricultural lands, and improved management practices on forest and rangelands. Various vegetative and tillage practices have been applied to reduce rates of runoff for erosion control and to conserve moisture for the growth of crops rather than for increasing streamflow yields. Control of vegetative density In some of the mountainous areas of the West, notably in California and Arizona, there have been some attempts to increase volumes of runoff by reduction of the volume density of vegetative growth. This has been particularly significant in the high mountainous areas where snow accumulates during the winter season. Reduction in aerial density of brush and tree growth has reduced the interception and evaporation of new fallen snow, contributing to an increased snowpack. In general, manipulation of vegetative density to increase the amount of runoff is limited to some extent by an increase in erosion potential. Spreading of runoff The spreading of surface runoff over large pervious areas such as mountain outwash fans has been practiced extensively in some areas of the West for the purpose of ground water recharge, thereby increasing ground water availability. This practice is limited, of course, to areas of soil, geologic, and topographic suitability. In other areas, the application of appropriate cropping and tillage practices on agricultural lands has resulted in increased infiltration and recharge to ground water aquifers. Loss STORAGE LOSS AND PREVENTION Loss of water by seepage from storage facilities is difficult to prevent, particularly in large reservoirs. It is known that some reduction occurs because of sealing with natural sediments carried by the inflow, but as the storage depth becomes greater this effect on newly submerged land probably decreases because of early deposit at the inlet. Areas of fractured rock and open seams in canyon sidewalls are occasionally grouted to reduce losses. Suspected seep areas in small reservoirs may be scarified and compacted to prevent excessive losses. Loss prevention Aging concrete dams which show severe leakage through construction joints can be effectively sealed from the upstream side with an elastomeric system based on neoprene rubber. Spalled areas of concrete on the downstream side and elsewhere on the structure can e repaired neatly and effectively, with epoxy resins. Special epoxy esin compounds for concrete bonding and repair are applicable to all ypes of concrete repair and construction. CONVEYANCE LOSSES AND PREVENTION Means available to control losses Extensive means are available to the engineer in treatment of conreyance systems to control losses. The method chosen will depend n such factors as the value of water, soil conditions, subsurface eology, length of canal, and availability of materials. Where the rops to be raised have high value or the distance to be traversed is great, concrete lining, concrete pipe (precast or cast-in-place), may be ustified. If local materials are suitable, the compacted earth lining nay be quite satisfactory. In areas containing loessial soils, scarifyng the canal bottom and recompacting the material has been found effective. Occasionally, fine sediments carried by water will deposit n a canal, accomplishing a degree of sealing. In isolated cases, natural sediments have been flushed from reservoirs into the conveyance system to plug percolation paths. Asphalt membrane lining.-The Bureau has performed extensive investigative work on use of asphaltic materials to control seepage from unlined canals. Approximately 6 million square yards of buried asphalt membrane lining has been installed on Bureau projects. Construction of the buried asphalt membrane lining involves spraying special high-softening-point asphalts at high temperature to a 4- to -inch thickness directly on prepared subgrades. The membrane is then covered with layers of earth or gravel to protect it against weathering, displacement, and physical damage. A high degree of durability is required of asphalt membrane lining during aging under conditions of wetting and drying, varying pressures and temperatures, soil stress, and others. Studies were completed of aging characteristics of asphalt membranes in service from 2 to 12 years in canals on different projects. Generally, it was shown that the membranes at all ages were in good condition. Asphaltic concrete lining. More than 270,000 square yards of asphaltic concrete lining have been placed since 1946. The design of an asphaltic concrete lining involves many factors which can only be adequately evaluated through laboratory testing. Asphaltic mixes are designed for an optimum degree of plasticity in the completed structure, a high degree of workability (while hot) to permit placement and good compaction, and aggregate gradings to obtain maximum benefits from locally available materials. Such mixes are higher in asphalt content (7 to 10 percent) than mixes used in highway construction. Certain phases of the design are a compromise between high plasticity to minimize cracking from subgrade movements as by settlement or frost heave, and hardness to obtain good resistance to erosion from high velocity or turbulent water. Asphaltic prefabricated lining.—Asphaltic prefabricated linings of lightweight construction for buried membrane installation and heavy 2-inch thick materials for exposed installation have been investigated in the laboratory and in the field. The lightweight prefab lining is furnished in rolls similar to roll roofing material. The exposed prefabricated lining is essentially a complete lining, performing the combined functions of seepage control and erosion resistance without FIGURE 1.-Thick compacted earth canal lining developed by the Bureau of Reclamation is one of several low-cost methods devised to minimize seepage losses. equiring a protective cover. Observations of installations have hown that the prefab liner performs satisfactorily provided the joints emain watertight and hydrostatic pressure is not permitted to act gainst the lining. Asphaltic macadam.-Experimental work with asphalt macadam ndicates that this construction may have a number of important pplications, both as a protective cover over various types of memranes and as an effective erosion prevention surface in various types f linings. With development of construction techniques, lower costs nay be expected which would make more practical the uses of this ype of construction. Other work accomplished on use of asphaltic materials include pneumatically applied asphalt emulsion mortar linng, and repairing portland cement canal linings. Plastic and butyl rubber lining. Recently, the Bureau has performed extensive laboratory and field investigations of plastic and butyl ubber linings. One advantage in the use of these materials is the elative light weight per unit area, permitting shipment of the material ong distances without excessive transportation charges. Another advantage is the fabrication of these materials into long and wide sheets, making it possible to provide a one-piece covering for relatively large laterals. It is necessary to cover the plastic lining with a minimum depth of 1 foot of a free draining earth or sandy material for protection from weathering and physical damage by animal traffic and other forces. Although costlier, vinyl plastic lining is believed to be better than the polyethylene material for permanent canal lining installations. The vinyl film is more uniform, defect-free material, more pliable to conform to irregular subgrades, and capable of being readily fieldpatched or spliced by the use of liquid cement. Tests show that butyl rubber is highly resistant to weathering and therefore can be used as exposed canal lining. However, the thinner butyl (less than 6-inch thick) must be earth covered for protection from physical damage by animal traffic. Concrete pipe.-Concrete pipe has been used in conjunction with canals to distribute water within an irrigation district. Rubbergasket joints have been developed to reduce water losses. Joint losses are small, and seepage losses are nonexistent within the pipe. Concrete feeder pipes of small diameter carry the water directly to the furrows to be irrigated, thereby resulting in a maximum conveni ence. Vegetation control.-A high percentage of conveyance channels are infested with aquatic weeds. With an increase in water level in a canal to overcome resistance caused by unwanted vegetation, water losses are brought about through increased evaporation, transpiration, and seepage. Mechanical devices and chemical methods are employed by the Bureau to control vegetation and to reduce these water losses to a minimum. Phreatophytes are a group of plants that use excessive quantities of water through transpiration. Growth of these plants in southwestern United States is very dense along waterways. The plants are vigorous, aggressive, and spread rapidly. The desert climate results in high rates of transpiration. Water salvage has been accomplished |