WATER CIRCULATION AND ITS CONTROL 



415 



statement to be discussed under "Ero- 

 sion," we may compare rainfall on 

 wooded mountains with that on barren 

 ranges on the basis of extended human 

 experience. The mountains of south- 

 ern Spain, the Pyrenees, the Basse- 

 Alps of southeastern France, the north- 

 ern Apennines, the Dalmatian and Gre- 

 cian hills, have all been stripped and 

 eroded. The change from green-cov- 

 ered slopes to brown, bare steeps has 

 been accompanies by a change from 

 beneficent rains to destructive down- 

 pours. Statistics of precipitation in 

 earlier times are not available for com- 

 parison with those of the present, but 

 historical evidence is entirely adequate 

 to prove that deforestation of a moun- 

 tain range has modified the conditions 

 of precipitation in such a manner that 

 occasional heavy, destructive rains re- 

 place gentle and more frequent show- 

 ers, at least in the countries mentioned. 

 These countries differ from northern 

 Europe in a measure as the semiarid 

 region west of the looth meridian in 

 North America differs from the Mis- 

 sissippi Valley. To this fact is due in 

 large part the difference of opinion 

 which exists among European writers 

 as to the influence of forests on precipi- 

 tation, springs, and run-off. Those who 

 have observed in northern Europe the 

 effects of deforestation of flat lands un- 

 der humid climates minimize or deny 

 entirely any influence exerted by for- 

 ests ; while those whose experience is 

 with warmer, more arid, and conse- 

 quently more critical conditions in 

 mountainous southern Europe are con- 

 vinced that forests exert an important 

 beneficial influence upon rainfall, 

 ground storage, and surface waters. 

 To cite the latest conclusions we may 

 quote from Cipolletti.- 



It is generally admitted that regions cov- 

 ered with forests have a lower temperature 

 than the open country, and the reason as- 

 signed for this is the evaporation from the 

 surface of the leaves moistened by the rain, 

 the area of which surface is estimated by 

 Mr. Schleiden at fifty times that of the 

 ground. Another cause is the constant evap- 



oration during the vegetation season by ex- 

 halation of the same leaves, which Mr. La- 

 fosse estimates at 150 cubic meters per hec- 

 tare and day; and, finally, the large quan- 

 tity of heat lost through the fixation of 

 carbon, which the same Mr. Lafosse, upon 

 the data furnished by Messrs. Lecart and 

 Parisel, estimates, per hectare and day, at 

 so many calories as will melt a block of ice 

 316 cubic meters. On the other hand, these 

 same quantities of water which the forests 

 continually emit into the atmosphere must 

 help to increase the degree of humidity of 

 the parts which immediately surround them. 

 Thus, while on the one hand the tempera- 

 ture is lowered, the humidity is, on the other 

 hand, increased ; and hence it appears to be 

 quite a logical and natural conclusion that 

 within the forests the point of saturation 

 of the air must be reached more quickly and 

 more frequently than in the open, and con- 

 sequently rain will also fall more frequently 

 and in greater quantities. 



This theoretical conclusion is borne out 

 by abundant facts and experimental data. 



Mr. Riedel quotes the island of St. Helena 

 and Lower Egypt. On the former, owing 

 to reforestations effected on a large scale, 

 the rainfall has actually been doubled since 

 the times of Napoleon I ; and in Lower 

 Egypt, where in the eighteenth century rain 

 only fell on from ten to twelve days in the 

 year, the number of rainy days nowadays 

 reaches from thirty to forty, since some 

 20,000,000 of trees have been planted. On 

 the other hand, on the shores of the Mediter- 

 ranean, in Syria and Palestine, there are 

 numerous regions which were formely in a 

 flourising condition, but have become to- 

 day arid and waste in consequence of the 

 destruction of forests. 



Mr. Ponti refers to the observations made 

 at the Nancy school of forestry, which were 

 confirmed in Russia by Professor Volsky, 

 and by which it has been found that the 

 rainfall in the interior of the forests was 

 greater than outside of them. He adduces 

 also the facts that in England, although it 

 has not been possible to prove that the de- 

 forestation has caused here any reduction 

 in the amount of rainfall or any appreciable 

 change in the climate generally, it has been 

 found nevertheless that the temperature is 

 lower in forest-covered regions, the differ- 

 ence being as much as three degrees on the 

 hottest days. 



In regions where a considerable part 

 of the precipitation is in the form of 

 snow the forest catches it in the 

 branches, protects that which falls to 

 the ground from the sun, and retards 

 its melting. These effects bear upon 

 the proportions that evaporate or run 



"Permanent Intern. Assn. of Navigation Congresses. X Congress, Milan, 1905. Report 

 by Cipolletti, C, on Deforestation, etc. 



4 



