III. EVAPORATION IN THE WESTERN STATES EXTENT AND SIGNIFICANCE In the Western States, losses due to evaporation from water surfaces average 11.5 million acre-feet a year. In most of the West, between 4 acre-feet and 8 acre-feet are lost every year from each acre of water surface. On a per acre basis, this is much more than consumptive use of irrigated crops. In the Great Basin and the Colorado River regions, where the needs for water exceed the amounts available, evaporation takes almost one-sixth of the entire supply-equivalent to more than onethird of the total amount of water consumptively used by irrigated crops. In many areas of the West, water-supply limitations retard needed developments and curtail economic and population growth. Such limitation of agricultural development has long been recognized. Now, there is concern also that inadequate amounts of water of acceptable quality may be a deterrent to needed industrial enterprises and even to urban expansion. In the Northern Great Plains States this situation is so acute that in some localities consideration is being given to treatment of brackish ground water in order to make it suitable for community water supplies (39). On the Pacific coast, the urgency is so great that water-supply projects are being planned with the anticipation that their construction costs will finally total several billion dollars. Representatives of these areas contemplate that municipal and industrial water costs up to $100 per acre-foot may be in prospect (39). An example of the economic effect of evaporation from a water supply system is a calculation of the loss from Lake Hefner, the supply reservoir for Oklahoma City. This shows that the average loss due to evaporation is in the order of $660,000 per year (24). Areas troubled by inferior quality of water are especially damaged by evaporation because it aggravates these problems. Evaporation removes pure water it is, in effect, distillation that removes pure water and leaves behind the salts. This significantly intensifies the quality problems of water that is stored in stock ponds and municipal water supply systems in the Great Plains (15). In some areas, the tight water supply situation generates costly litigation, yet in these same areas water lost by reservoir evaporation generally exceeds the amount of water that is in contest. For example, evaporation from Lake Mead exceeds 700,000 acre-feet annually in the water year 1952-53 it was 875,000 acre-feet (36). Similarly, net annual evaporation from the storage units now under construction in the upper Colorado River Basin is expected to average 691,000 acrefeet, or more than one-third of the replacement storage that they will provide (4). 23674-58- --3 The relationships of evaporation losses with water supply and beneficial uses in five major western basins are shown in table I. TABLE I.-Surface water, 5 western basins, supplies, uses, and losses 1 Based on data adapted from U. S. Geological Survey Circular 398, 1957. Data from special study by U. S. Geological Survey, December, 1957. See also table II. NOTE.-Except for about 570,000 acre-feet diverted into the South Pacific region for public supplies, the basic data do not identify transregion diversions. Losses from reservoirs High evaporation loss in relation to usable yield is characteristic of western river systems. This is the result of the climate and the character of the streamflow. The great seasonal variations that are the common pattern of runoff in the West result in large water surface exposure. This is because, through much of these watercourses, spring and summer floods spread over wide valley areas, and low flows meander. Storage reservoirs are a principal means of improving streamflow regimen so as to increase usable water yield. Reservoirs, however, expose wide surfaces to evaporation, and thus they are a major source of water loss. This is true even though reservoirs may lessen natural evaporation by confining floods in deep pools instead of their being spread over wide flood plains. However, a significant part of most reservoir capacity is relatively shallow, and this is true expecially with respect to carryover storage reservoirs. This, together with the extended periods of holdover, increases the evaporation losses of river systems. The net result of reservoir operation is, of course, advantageous in conserving floods that would otherwise be unusable. The high proportion of evaporation loss from water in storage in relation to other sources of evaporation is, however, shown by the Geological Survey data summarized in table II. TABLE II.-Evaporation from water surfaces in the 11 Western States, by major basins (preliminary) 1 Data in this table are for only the portion of each river basin that lies within the 11 Western States. Prepared by J. S. Meyers, U. S. Geological Survey, Dec. 27, 1957. Losses from stock ponds In the Western States, evaporation from small impoundments is important because of its effect on land use. This is true particularly with respect to livestock watering places-stock tanks and ponds. Conservative and efficient use of many millions of acres of range land depends on whether the stock tanks have water in them at the right season. A large part of the range livestock water is provided by natural or constructed reservoirs that impound the runoff of small drainages. Generally, because of topography, these tanks are small and shallow, and this, of course, maximizes evaporation loss. Because of the seasonal and uncertain character of runoff in much of the rangeland area, carryover storage from good runoff years would greatly aid efficient use of forage, but this is rarely possible because of the high evaporation loss. These two factors-irregular runoff and high evaporation from the tanks severely handicap efficient forage utilization and economical livestock operations. This important aspect of the evaporation problem is understood to be receiving special attention at the University of Arizona. A recent discussion of evaporation pointed out that in the Southern Great Plains the 75 inches average annual evaporation rate results in an annual loss of over 1.4 million gallons per acre of water surface. This loss from a stock pond is as much water as would be consumed in a year by about 500 head of range cattle. In another reference, the Oklahoma A. and M. College is reported to have observed that, during a 6-month period, water taken from a farm pond by evaporation was 10 times as much as the amount taken for use (8). Rates of evaporation in the Western States are shown on figure 1, prepared by the Geological Survey. The rates shown are in terms of the measurements of standard weather station evaporation pans; reservoir losses generally will be about 70 percent of these deter minations. FIGURE 1.-Average Annual Evaporation in Inches, From Weather Bureau Class A Pans. (Preliminary.) Prepared by U. S. Geological Survey, Denver, Colo., December 1957. Reservoir evaporation rates generally are about 70 percent of these pan evaporation rates. REDUCTION OF EVAPORATION LOSSES Evaporation losses are held to a minimum by exposing the least possible water surface area. This means that streams and reservoirs are kept deep instead of wide. Familiar examples are stream channel, improvement so that the water moves at relatively high velocity through a confined channel instead of being permitted to meander slowly over a wide stream bed. An even more general practice is to use deep, narrow canyons for reservoir sites. In a recent review of reservoir evaporation control, Mr. S. W. Freese discusses the economic benefits of constructing reservoirs to maximum average depth (14). He refers to a west Texas situation where storage of 40,000 acre-feet in a favorable site avoided the excess evaporation that would have occurred at a less favorable site. Freese states that the water thus saved was worth $164,000 a year. In the same paper, Freese shows comparable benefits from reservoir evaporation control by other methods, including concentrating water into single reservoirs, elimination of shallow areas, elimination of water growths, use of ground-water reservoirs, roofs, floating covers, and windbreaks. Use of such methods where physically feasible often may be advantageous, and undoubtedly they will be employed increasingly as water supply situations become tighter. There are, for example, many locations requiring phreatophyte control to reduce nonbeneficial consumption of water by plant growths of little economic value. Salt cedar, willows, and cottonwoods not only consume large amounts of water, but also they aggravate channel meandering, thus causing further water loss. Techniques of phreatophyte control are being perfected, and continuing research promises further improvements. A major difficulty, however, is that effective control generally requires repeated treatment with resulting high costs. Possibly, this may be relieved by management techniques that will result in grasses becoming established in place of the phreatophytes. Another method of evaporation reduction is use of underground storage. Of prime significance in this connection is maintenance of the great natural ground-water reservoirs such as the one underlying the Sacramento Valley of California (25). Many of the underground reservoirs are recharged by percolation from flows in stream channels, and this natural process often is augmented by releases into such channels specifically for the purpose of recharging the ground water. However, the extent to which underground storage can be artificially induced by present methods seems to be limited. Although improvements are being made in recharge techniques, the rate at which water can be introduced into the ground does not permit direct underground storage of large flows. In addition to the physical difficulties, there are also the legal problems of preserving the water so stored from adverse withdrawal. At the present time, it appears that the use of underground storage for surplus surface flows may be suitable mainly for relatively small flows or controlled releases from surface reservoirs, and for situations where underground storage would not be accessible to unauthorized withdrawals. There are limitations also on the opportunities for management of surface reservoirs to hold storage at maximum depths, although this method is widely and successfully practiced. There are only relatively few deep, narrow reservoir sites at suitable locations, with acceptable geology, and not preempted by railways, highways, and other essential developments. Another limitation of the canyontype sites is that they do not provide adequate capacity in the canyon section. In practice, therefore, reservoir operations must contemplate that a substantial amount of water will be stored at relatively shallow depths with correspondingly large water surface exposure. Up to the present, this has meant that high evaporation losses have been unavoidable. New methods for chemical protection of water surfaces, however, now give promise of effective evaporation control. In field tests as well as in the laboratory, monomolecular films of hexadecanol significantly reduce evaporation losses. This method, while still in the stage of test and development, appears to be susceptible of practical application at costs within economic limits. Investigations by a number of governmental and private organizations support the expectation that hexadecanol, along with conventional techniques, can significantly increase the usable water supply by reducing evaporation losses. |