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<br />46 <br /> <br /> <br />001837 <br /> <br />LOSSES OF WATER <br /> <br />. <br /> <br />Evapotation <br /> <br />Evaporation constitutes one of thEl major losses of water from <br /> <br />stock ponds. The drop in t)J.e level of the reservoir surface due to <br /> <br />this cause is equivalent to the annual evaporation less the precipita- <br /> <br />tion on the water surface, and the total loss in aC.J"'efeet from the <br /> <br />pond can readily be calculated by applying ,the annual evaporatio.n <br /> <br />rate to the avez:age area of the water surface. The average annual <br /> <br />evaporation in the PacificS,outhwest area, as measured in class A <br /> <br />evaporation pan, and as determined from lake surface, has been <br /> <br />compiled by the Weather Bureau as previously noted. Expe.rience <br /> <br />has shown that because of the s)J.allow depths, evapor,ation from <br /> <br />stock ponds may exceed that in most lakes and at times it probably <br /> <br />approach~s that in th.e standard Weather Bureau pan. Measurements <br /> <br />. of evaporabon losses from stock ponds by the GeologicalSurvey <br /> <br />(Langbein, 1951; Culler, 1961) suggest that a coefficient of 0.85 <br /> <br />applied to pan evapot,l/rtIon approximates t)J.e corrElct figut,e in mos,t <br /> <br />localities, as' generally the ,figure will probably faU somewhere' <br /> <br />between 0.70 andO. 95, with. 0.85 being.a fair average. <br /> <br />Using.a coefficient of 0.85, evaporation from stock ponds in the <br /> <br />Pacific Southwest area will vary from a minimum of about 42. inches <br /> <br />in the higher elevation northE:crn:areas to a maximum of about 85 inches <br /> <br />. <br />