94 



Rainfall. 



The water level of a region is necessarily affected by the amount of 

 precipitation which falls upon its soil. It cannot be said, however, that 

 forests have any great influence uix>n the rainfall of a country. This 

 question has long been debated but no conclusion, backed by convincing 

 proofs of sclentiflc exactness, has been reached. It is true that rainfall 

 iu most abundant wliere forests grow, but it is more reasonable to believe 

 that rainfall controls the density and distribution of the forests rather 

 than that forests are great factors in determining the amount of rainfall. 

 Precipitation talies place whenever the air is suddenly cooled below the 

 dew point. Forest air is cooler and contains a relatively greater amount 

 of moisture than air in the open, and for this reason it is fair to infer 

 that forests may have at least some effect in increasing local precipita- 

 tion. The trees also have a mechanical effect in retarding a vapor laden 

 wind, which condition may be conducive to the precipitation of moisture. 

 On the other hand, the following quotation from Blanford (3) shows the 

 opinion that meteorologists are adopting. "As a result of a long study of 

 rainfall in India, and i>erhaps no country affords greater advantages for 

 the purpose, I have become convinced that dynamic cooling, if not the 

 sole cause of rain, is at all events the only cause of any in)portance, and 

 that all the other causes so frequently appealed to in popular literature 

 on the subject, such as the intermingling of warm and cold air, contact 

 with cold mountain sloi)es, etc., are either inoperative or relatively insig- 

 nificant." 



Many experiments and observations made in Europe and elsewhere 

 show an excess of rainfall in forested areas over that of open countries. 

 Some of these excesses were so small, however, that they might have been 

 due to errors in rain gauges and other extraneous conditions which affect 

 them. In Prussia the following records have been gathere<l from the ordi- 

 nary meteorological stations showing the excess of rainfall in forest sta- 

 tions over those in the oi)en regions. 



Between sea level and 328 feet elevation, 1.25 per cent. 



Between 328 and 556 feet elevation, 14.2 per cent. 



Between 1.9'.)7 and 2,297 feet elevation, 19 per cent. 



Between 2,297 and 2,025 feet elevation, 43 per cent. 



These figures seem to show that forests have very little effect on rain- 

 fall in the plains, but that their influence becomes greater with increasing 



