DISPOSAL OF RAINFALL 



47 



Tnches 



vestigation it may, however, often 

 be found necessary to make indi- 

 vidual rainfall measurements. 



Diagrams, Figs. 12 and 13, repre- 

 sent a 75-year rainfall record at 

 St. Paul, as reported by the Minne- 

 sota Board of Water Commis- 

 sioners. 



3. DISPOSAL OF RAINFALL 



Of the rainfall a portion evap- 

 orates, a portion enters the soil and 

 is either absorbed by plant growth 

 or by ground flow reaches the rivers 

 or lakes, while the third portion 

 finds its way into streams as surface 

 flow or run-off. 



Evaporation. Of the tremendous 

 losses due to evaporation from the 

 ground surface comparatively little 

 is known. It is impossible to arrive 

 at such losses by taking the differ- 

 ence between rainfall and run-off, 

 as in this there would also be in- 

 cluded the losses due to absorption 

 by the soil and by vegetation, and 

 again the rate of run-off does not 

 altogether depend upon the rainfall. 



The rate of evaporation or the 

 proportion of the rainfall to the air 

 varies greatly under different con- 

 ditions and is affected by atmos- 

 pheric conditions as well as by the 

 character of the soil. The capacity 

 of the atmosphere to take up and 

 dissipate the moisture depends in 

 turn on the temperature, the wind, 

 and how saturated it already is. 

 Wind increases the evaporation to 

 a great extent, especially from ex- 



18X0 

 1881 

 1332 

 1883 

 1884 

 1885 

 1888 

 1887 

 1888 

 1889 

 1890 

 1891 

 1892 

 1893 

 1894 

 1895 

 1898 

 1S97 

 1898 

 1899 

 1900 

 1901 

 1902 

 1903 

 190t 

 1905 

 1908 

 1907 

 1908 

 1909 

 1910 

 1911 

 1912 



FIG. 12. Annual Precipitation at 

 St. Paul, Minn., 1837-1912. 



