APPENDIX C-REPORT OF AN AUSTRALIAN RESERVOIR EVAPORATION FIELD TEST The following quotation from the May 1957 issue of Research (see reference No. 12 in the list of references on pp. 25 and 26) reports a recent Australian field-scale test: As we go to press, notification has just been received of the successful completion of a program of tests of the Mansfield process on a large reservoir at Broken Hill. Over the 14 weeks ending April 8, evaporation from the Stephen's Creek Reservoir, Broken Hill's main water supply, has been reduced by 37 percent, it is claimed. More than 200 million gallons of water, equivalent to 6 weeks summer consumption in Broken Hill, are stated to have been saved. From this experiment the estimated cost of the water saved is 1 penny 1 per 1,000 gallons. It is expected that under less favorable conditions the cost of saving water will be greater but will not exceed sixpence per 1,000 gallons. In these tests, it is stated that a satisfactory film of cetylalcohol was maintained for more than 3 months in spite of exceptionally high wind velocities. About 1 foot of evaporation has been saved over an average area of 930 acres. It is suggested that these results indicate the way to apply this process to the largest water storages, although additional research may be required to deal with new difficulties encountered. 1 Approximately 1 cent United States (1£Australian=240 pennies- US$2,223.). 41 APPENDIX D-NOTE REGARDING THE EFFECT OF RESERVOIR EVAPORATION ON CLIMATE Reservoir evaporation control would not intensify the dryness of arid areas in the West because reservoir evaporation does not materially increase the humidity of the air a short distance away. This is strikingly demonstrated in the Southwest. Large reservoirs on the lower Colorado and Salt Rivers for several decades have annually dissipated hundreds of thousands of acre-feet of water into the air without causing a perceptible change in the surrounding desert vegetation. It may be initially confusing to reconcile the insignificance of reservoir evaporation in terms of its contribution to atmospheric moisture with its major effect on water-supply requirements. This apparent paradox is clarified by considering that, even in the drier parts of the country, atmospheric moisture represents an enormous amount of water whereas in most of the West the margin between the usable water supply and water requirements is precariously small. Moisture for precipitation and for atmospheric humidity even in inland and intermountain regions is derived mainly from the ocean surfaces. This moisture, traveling in great air masses, is so enormous in total amount that any contribution from reservoir evaporation is comparatively negligible. For example, the average annual water content of the air above the Western States is in excess of 14 billion acre-feet. This is about 100 times as much as the evaporation from water surfaces. Another factor that discounts any effect of evaporation control on atmospheric humidity is that such control merely defers, rather than diminishes, evaporation. That is, reservoir evaporation control conserves the water for uses at downstream locations, where it then is evaporated substantially as would have occurred at the reservoir. Thus, although reservoir evaporation control will materially increase beneficial use of water, it will not materially reduce the amount of the contribution to atmospheric moisture in a river basin. The relationships of runoff, water loss, and atmospheric water vapor are indicated by the following data for major river basins, prepared by Mr. A. L. Shands of the Weather Bureau: 43 TABLE IV.-Approximate water losses, by river basins 1 1 Regions, drainage areas, and runoff from U. S. Geological Survey Circular No. 398. 2 Estimated by eye from enlarged copy of United States precipitation map published in Crop Bulletin Jan. 10, 1957. Prepared by A. L. Shands, Weather Bureau, Washington, D. C. О READING ROOM SYNOPSIS OF TESTIMONY GIVEN MARCH 24-27, 1958, IN RESPECT TO BEFORE THE SUBCOMMITTEE ON MINERALS, MATERIALS, OF THE COMMITTEE ON INTERIOR AND INSULAR AFFAIRS UNITED STATES SENATE EIGHTY-FIFTH CONGRESS SECOND SESSION APRIL 18, 1958 Printed for the use of the Committee on Interior and Insular Affairs 24348 UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON: 1958 PURCHASED THROUGH DOC. EX. PROJECT |