48 



HYDROLOGY 



posed water surfaces, as the saturated air in contact with such 

 surfaces is rapidly removed and continually replaced by fresh. 

 In cool climates with light breezes the evaporation is considerably 

 lower than in warm climates with strong winds. 



The nature of the earth's surface, on the other hand, deter- 

 mines the rate at which moisture is supplied. Thus, a very 

 large evaporation takes place from exposed water surfaces such 

 as lakes, swamp lands, etc., and the amount may, in certain 

 instances, equal the actual rainfall itself. They tend, however, 



<=>' ^ o' o' el o' o 1 ^ r4 o' us" *i o- o' so* o'-j -* o' rf o' o M o' t- co o- t- ci o' us -H o' ei o 

 a* > * s* G% . d ^ -cd c ^ c ^ -cd a ^ .^>< -s>< . 



SoS>'Sc3>.Sc3>-Sra>.5e4>..5cSt.5cS>..5c3t.ac3t.SoS>-.ac3f <S> 



Jan. Feb. JUar. 



31ay June July Aug. Sept. Oct. Nov. Dec. 



FIG. 13. Monthly Variation in Precipitation at St. Paul. Minn. From 

 Records 1837-1912. 



as a storage of flood waters and add, therefore, materially to the 

 regulation of the stream flow. 



The depth to the water in the soil and its capillary action in 

 bringing the water to the surface also naturally affect the evapo- 

 ration. A light rain falling on an impervious rock surface may 

 simply wet the surface and quickly disappear as vapor, while 

 saturated surface layers of the soil, such as after heavy rains, will 

 also cause considerable evaporation. 



A large amount of water is necessarily taken up by the vege- 

 tation and evaporated, while the effects of forests are to greatly 

 reduce the evaporation as compared to open fields. 



A more complete study has been made of the evaporation from 

 the water surface of lakes and rivers, the greatest use of such 

 studies being in the investigation of storage and the losses which 



