juiyso, 1917 Evaporation from Water and River-Bed Materials 



239 



of one week have been used in this investigation and are recommended. 

 This statement is made with reference to the practical application of 

 temperature data to evaporation estimates. For the much-sought 

 evaporation law it is quite probable that the unit of time will be shorter. 

 Means will probably not be used in the final solution of the problem, and 

 the time interval will be reduced to minutes. However, then to apply 

 this to field conditions and make use of the existing data the unit will be 

 lengthened and the exact evaporation law (when that is finally discovered) 

 may have to be approximated for practical application. 



Registering maximum and minimum thermometer records are used in 

 arriving at the weekly means. Their agreement with the mean as found 

 by the use of the thermograph proves their reliability. A thermograph 

 record sheet for one week is shown as figure 7. From it the maxinmm 

 and minimum temperatures for each day have been taken and their 

 means found. Table XIII, showing these figures and daily means found 

 by integrating the thermograph cun/es, gives from the curve a weekly 

 mean of 51° F.; by the other method 53°. 



Table XIII.- 



' Analysis of temperature from thermograph record for the week of September 

 Ji-iS, igi6. {See Jig. 7) 



Day of week. 



Temperature (°F.). 



Maximum 



for 24 

 hours end- 

 ing 7.00 

 a. m. 



Minimum. 



for 24 

 hours end- 

 ing 7.00 



I Mean ob- 

 tained by 

 Mean maxi-! planimeter 



mum + 

 minimum 

 -i- 2 tor 24 

 hours end- 

 ing 7.00 

 a. m. 



from graph 



for 24 

 hours end- 

 ing 7.00 

 a. m. 



Tuesday. . . 

 Wednesday , 

 Thursday. . 

 Friday .... 

 Saturday. . 

 Sunday. . . , 

 Monday. . . 



Mean for the week. 



50 



72 

 80 



55 

 68 

 70 

 73 



39 

 44 

 40 



32 

 39 

 40 



38 



44 

 58 

 60 



44 

 54 

 55 

 56 



43 



57 

 58 

 42 



50 

 51 

 56 



51 



WIND MOVEMENT 



Practically all of the proposed evaporation formulas have included the 

 wind factor. It is evident, that to be applicable directly, the wind 

 movement figures used should represent that movement quite close to 

 the water surface. The Weather Bureau records represent standard con- 

 ditions as near as possible, but with the anemometer 1 8 to 30 feet above 

 the building on which it is located, and that building of no standard 

 height (26). Ordinarily these records will have to be reduced to apply 

 them directly to evaporation estimation. At the Denver laboratory the 

 records show for the period April 17 to October 31 an average velocity 



