372 APPENDIX 



absorb more water than the local fall produces, the balance being provided by natural 

 or artificial irrigation by means of the surplus of higher regions. 



In the plains, on the contrary, surface flow does not exist, and the feeding of the 

 subterranean sheet depends upon the permeability of the soil and upon evaporation. 



The Influence of Forests on the Infiltration of Water in Mountains. The pro- 

 portion of the surface flow increases according to the declivity, and according to 

 the greater or less rapidity with which rain falls or snow melts on the slopes. It can 

 become very great. M. Imbeaux, 28 in a study on the course of the Danube, has dis- 

 covered "during the three exceptional floods of Oct. 27, 1882; Oct., 1886, and Nov., 

 1886, that the proportion of surface flow at Mirabeau was from 0.33 to 0.39 and 0.42, 

 that is to say, more than a third of the rainfall; it fell to 0.27 during lesser floods, and 

 even to 0.23 and 0.18 for average and small floods, thus demonstrating that the law 

 of its decrease is parallel with that of the intensity of the rainfall." Of the Danube at 

 Vienna the Central Hydrographic Office of Vienna has discovered, in applying the 

 same method, 42.1 per cent for the period from July 28 to Aug. 14, 1897. 29 



Other authors (D6montzey and M. Ney) have proved that the proportion of surface 

 flow can reach 40 to 50 per cent of the rainfall on wooded slopes. D6montzey even 

 quotes a case where it has reached three-quarters of the water brought by a storm of 

 rain into the bed of a torrent, extending over more than 1,977 acres. 



The action of forests on the reduction of "wild waters" is so well known, so univer- 

 sally recognized, that to insist upon it has become commonplace. We will only recall 

 that it results principally from: (a) the fact that, thanks to the obstacle caused by 

 mountain tops, water reaches the soil with hardly any celerity; (6) that the rainfall, 

 other things being equal, is more frequent and less violent in forests, and above all the 

 melting of snows is much less rapid, as this process often lasts a fortnight or even a 

 month longer in the forest than in open ground; (c) that the obstacle offered to the course 

 of the water by stalks and the roots of trees, and lastly the absorption of a considerable 

 quantity of water by mosses or dead leaves covering the soil. It has been calculated 

 that such coverings of dead leaves or moss retain by their hygroscopicity a rainfall of 

 2.91 inches of depth falling in one day before allowing anything to run off by surface 

 flow. 30 



Even when the covering is saturated, it only allows the water to escape drop by 

 drop, so that the soil is able to imbibe the whole of it, to the great benefit of the sub- 

 terranean sheet. One may say that the process of surface flow is almost entirely 

 suppressed on wooded slopes in good conditions where the covering of the soil is left 

 alone. 



We could not better sum up the subject of the action of forests on the feeding of 

 springs in mountainous districts than by quoting so great an authority as Professor 

 Henry: 31 "Wooded mountains attract rain; it is there that precipitations from the 

 atmosphere attain their maximum; it is there that great reservoirs of water are found; 



28 Essai-Programme d'Hydrologie, by Dr. Imbeaux, ing^nieur des Fonts et Chaus- 

 sees. Published in the Zeitschrtft fur Gewasserkunde, 1898 and 1899. (Quotation 

 borrowed from M. Henry.) 



29 Die Hochwasser Katastrophe des Jahres, 1897, in Oesterreich. . . . Beit- 

 rage zur Hydrographie Oesterreichs. Published by the K. K. Hydrographischer 

 Central Bureau of Vienna, II bulletin, 1898. 



30 Ebermayer ("Die gesammte Lehre der Walstreu," Berlin, 1876, pp. 177 to 181) 

 shows that moss can retain in suspension 2.8 times its weight in water. The sphagnums 

 and species of hypnums such as N. loreum can absorb up to 4.5 kilograms of water by 

 the square meter of ground where it grows. Dead beech leaves retain about 2.3 times 

 that of pine or spruce, 1.2 times their weight in water. See also M. Ney's work, "Der 

 Wald und die Quellen," p. 70 (Tubingen, 1894). 



31 Communication made to the "Congres International de Sylviculture a Paris en 

 1900," p. 327 of the " Compte-rendu Official." 



