76 



BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



in which E is the evaporation in cubic centimeters, W the wind 

 movement in total miles for the same period, VD the sum of the 

 daily saturation deficits, in inches, tunes 1,000; and SS the average 

 minutes of sunshine per day, divided by 10,000. 



Conversely, to calculate the rate of evaporation when C has been 

 determined — x 



E=CXSS(W+VD). 



As these data are lacking in precise value, it is necessary only to 

 state that the coefficients vary widely for corresponding periods of 

 the year, and that on the average they vary, according to the sun's 

 elevation and other factors, from 0.477 in January to 1.336 in July. 



The evaporation computed by decades for all periods of record 

 from March, 1910, to December, 1916, is summarized in Tables 21 

 and 22, with the actual record since January, 1917. (See fig. 4.) 



52 



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Fig. !. Variations in the relation between precipitation and evaporation for growing seasons. Control 



station. 



The principal value of Table 21 is that it shows the maximum 

 evaporation that may occur during the winter, the maximum rates at 

 this time being those which may cause injury to trees. On the 

 whole however, this has little advantage over Table 20, for it is seen 

 that t he maxima during the winter do not go to such great extremes, 

 relative to average rates, as, for example, they do during June. 



On the other hand, Table 22 brings out certain facts which can not 

 be reached from a consideration of any single climatic condition. In 

 the evaporation data alone it is seen that the variations as between 

 years and growing seasons are much greater than might be expected 

 from a consideration of air temperatures or wind movements, and 

 that saturation deficits (Table l ( >) are not always fully reflected in 

 the evaporation quantities. In a general way, when precipitation 

 beric humidity are high, evaporation is Low, and vice 

 1; l»i it as there are some exceptions to this rule, to measure the 

 moisl ure conditions it is really necessary to establish the ratio between 

 water supplj (precipitation) and water loss (evaporation . 



