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A new formula and coefficients for climatological factors was developed for estimating pan evaporation based on an analysis of data from 23 states in the U. S. and from five other countries. This formula may be written: Ev=K R Ct Cw Ch Cs Ce Cm in which K is a dimensionless constant R is the theoretical radiation reaching the outer atmosphere, expressed as equivalent evaporation in the same units~ , as Ev. Ct, Cw, Ch,and Cs are dimensionless sub-coefficients for temperature , wind, humidity, and sunshine. Ce is a sub-coefficient for elevation, and Cm is a monthly coefficient Equations were developed for all of the coefficients and table s were prepared to facilitate the application of the formula. The study was a continuation of a thesis study by Patil (1962) who analyzed mE}-re than 3,200 months of records from 40 stations in the western states to develop a formula and equations for the coefficients of climatological factors . Later, a separate analysis was made of the same data by Mathison (1963) using similar, but different procedures. The present study differs from those of Patil and Mathison in two major respects. The data includes, in addition to those used by Patil and Mathison. 507 months of records from the stations in the Midwest and southern states, Puerto Rico, Panama, Hawaii, Canada, Alaska, Nigeria, and Peru. These data extended the range of the weather factors. The second difference was i.n the analytical procedure. A technique suggested by Grassi (1964) was used to develop coefficients for temperature, hum[dity, and sunshine. These coefficients are more independent of other climatic factors than those developed by Patil and Mathison.


This work made publicly available electronically on May 17, 2012.