42 



BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 



tions seem to indicate, however, that in the Red River valley open 

 drains do not materially increase run-off caused by melting snows in 

 the spring, because a large majority of drains are completely filled 

 or badly obstructed by snow and ice and do not begin to function 

 until several days after the snow on the watershed has melted. Un- 

 der such conditions run-off collects and flows in natural depressions 

 or channels without being affected to any considerable extent by 

 artificial drainage. 



EVAPORATION AND TRANSPIRATION. 



By far the greater part (between 85 and 98 per cent) of precipi- 

 tation on the Red River watershed is disposed of by evaporation and 

 plant transpiration. As run-off comprises only the remainder of the 

 precipitation those processes largely govern total run-off. Large 

 quantities of water evaporate from the soil and from water and snow 

 surfaces. Annual evaporation from the soil has not been determined 

 definitely but under ordinary conditions this alone is probably as 

 great as annual run-off. The process, however, goes on at a much 

 slower — although more uniform — rate than does run-off, so its rela- 

 tive effect is small when the run-off rate is high. 



The rate of evaporation from shallow water near Grand Forks 

 has been measured and recorded by Prof. E. F. Chandler for the 

 period 1905 to 1914. The results have been used in computing the 

 probable evaporation from lakes and reservoirs. Table 10 shows the 

 mean monthly evaporation and the mean monthly temperature for 

 the period 1905-1914. 



Table 10. Mean monthly temperature and evaporation at University {near 



Or and Forks), N. Dak. 



Month. 



Billions of 



cu. ft. per 



sq. mi. 



0. 0009 



.0014 



.0028 



. 0075 



. 0095 



.0114 



. 0134 



.0107 



. 0084 



.0049 



. 0015 



.0009 



January . . . 

 February. . 



March 



April 



May 



June 



July 



August 



September. 

 October . . . 

 November. 

 December . 



Mean 





tempera- 



Mean eva 



ture. 





° F. 



Inches. 



4.0 



0.40 



7.3 



.60 



22.8 



1.20 



44.5 



3.22 



52.1 



4.12 



65.0 



4.94 



67.5. 



5.78 



65.5 



4.60 



57.7 



3.63 



45.9 



2.11 



27.6 



.63 



11.3 



.40 



The evaporation figures for April to November, inclusive, are results of observations by Prof. E. F. 

 Chandler, 1905 to 1914; those for December to March are estimates made from available data relating to 

 evaporation during winter months. 



The figures deduced by Meyer, 5 as to evaporation from snow and 

 ice, agree very closely with the results of observations made by 



6 The Elements of Hydrology, Adolph F. Meyer. 



