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<br />?1 <br />oon..-" <br /> <br />Expl"essed mathematically <br /> u ~ kf <br />or U ~ KF <br />in which u, U <br /> k, K ~ <br /> f, F <br /> <br />for an individual month <br />~kf, for a term of months <br />evapotranspiration <br />an empirical coefficient for the particular <br />species, and cover density, for an area. <br />a climatic factor, the sum of the products <br />of mean monthly temperatures and monthly <br />percentages of the annual daytime hours, <br />for a term of month--the growing season. <br /> <br />Values of K WE,re derived by correlating evapotranspiration use data <br />with values of F for the growing season. For different species of <br />phreatophytes, K ranges from 0.80 to 1.30. Also, in order to c()mpen- <br />sate for different growth conditions the authors have developed K <br />values for cover densities of "light," IImedium," Ildense," and "very <br />dense." None have been developed for different depths to the water <br />table although recently S. E. Rantz, G~ological Survey, Menlo Park, <br />California, proposed a graphical method for estimating K for different <br />depths to the water table. <br /> <br />The formula was developed for extrapolating measured values of evapo- <br />transpiration in one area to other areas, where growth conditions were <br />similar and where temperature records are available. <br /> <br />As the coefficient K depends on experimental measurements of evapotrans- <br />piration, results can only be as accurate as these measurements. <br />Assuming the experimental value to be representative, then all uncer- <br />tainties are transposed into the climatic factor F. Thus, where <br />there are marked differences in climatic conditions, the Blaney-Criddle <br />values of evapotranspiration become uncertain. However, in extrapolating <br />to areas where the climatic conditions are similar, the Blaney-Criddle <br />formula is at present the simplest and most usable empirical method <br />available to the hydrologist. <br /> <br />The Thornthwaite formula (Thornthwaite, 1948, p. 90) also includes <br />the parameters of tempeature and daytime hours, but has no consumptive <br />use factor. Based on catchment area data and controlled experiments, <br />it is expressed as: <br /> <br />where <br /> <br />Et ~ 1.6 (10 T/r)a <br /> <br />Et potential evapotranspiration for a 30 day month <br />T = mean air temperature (oC) <br />r ~ heat index, the sum of 12 monthly indexes <br />a a cubic function of I <br /> <br />13 <br /> <br />" <br />