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<br />IlU016~1 <br /> <br />. <br /> <br />Canal and Reservoir System Efficiency <br /> <br />Canal and reservoir system efficiency can be defined as the percentage <br /> <br /> <br />of water diverted from the river that is delivered to farm headgates or <br /> <br /> <br />turnouts on the system. The principal factors that influence canal and <br /> <br />reservoir system efficiency are (1) evaporation, (2) transpiration, (3) <br /> <br /> <br />seepage and (4) operational losses, <br /> <br />. <br /> <br />Evaporation <br /> <br /> <br />Evaporation is defined as the process by which water is changed from <br /> <br /> <br />the liquid into the gaseous state through the transfer of heat energy. <br /> <br />At every free water surface, whether in a reservoir or a canal, there <br /> <br /> <br />is a continuous interchange of water molecules across the free water <br /> <br /> <br />surface. When the net sum of the interchange of the water molecules <br /> <br /> <br />represents a loss from the water, there is evaporation. The evaporation <br /> <br /> <br />rate is expressed in depth of water measured as liquid water removed <br /> <br /> <br />from the free water surface per unit of time. The average annual evapor- <br /> <br /> <br />ation rate from open water surfaces for the Balzac-to-Julesburg reach <br /> <br />has been estimated to be about 50 inches per year. <br /> <br /> <br />Evaporation rates from free water surfaces have been established <br /> <br /> <br />for specific areas using a "standard" circular pan, which is installed <br /> <br /> <br />on the ground as a land pan or in the water as a floating pan, The U. S. <br /> <br /> <br />Weather Bureau Class A pan is 4 feet in diameter and 10 inches deep. <br /> <br /> <br />Theoretical approaches to the prediction of evaporation from free water <br /> <br /> <br />surfaces involve equations representing mass transfer processes and energy <br /> <br /> <br />transfer. Evaporation rate has been directly related to air and water <br /> <br /> <br />temperature and wind speed. <br /> <br /> <br />Evaporation from open water surfaces is extremely high in the warmer <br /> <br /> <br />regions of the United States. Values on the order of 90 inches per year <br /> <br />and 80 inches per year have been recorded for southern California and <br /> <br /> <br />southwestern Texas, respectively, A great deal of research has been <br /> <br /> <br />conducted during the past several years on different methods for retard- <br /> <br /> <br />ing evaporation from free water surfaces. Some reduction in evaporation <br /> <br />has been accomplished by using thin films of chemicals spread over the <br /> <br /> <br />water surface. Evaporation retardant processes are fairly expensive and <br /> <br />. <br /> <br />I <br /> <br />-12- <br />