I\Ki,ri:\<K UK I'oi.'KsTs ON Naiukal Watkh Si l•l'l,^. 129 



Mr. TouiiK-y srates tlwil. in humid i-egimis wliere tlie pieeipitatioii 

 is fairly esciily distiilaited over tlu^ year, ami where the catchment 

 area is sutMcieiitly large to permit the greater part of the seepage to 

 enter the stream above the point where it is gauged, the evidence 

 accumulated to date indicates that stream How is materially increased 

 hy the presence of foiests. 



In regions characterised by a short wet season and a long drj' one, 

 as in southern California and in many other portions of the West, pre- 

 sent evidence indicates, at least on small mountainous catchineut areas, 

 that the forest \ery materially decreases the total amount of run-oif.* 

 This important ([uestion is now being investigated by the Swiss expeii- 

 mental station in the Emmen Valley. Observations were commenced 

 in 1900. but the authorities have not yet issued any report. Gaugings 

 in connection with new forest plantations have been instituted in the 

 Transvaal. 



Many factors, such as the geological fortnation, pliysical condition 

 of the soil, topogra[)hy, incidence of rainfall and even the variety of the 

 trees, largely conti'ibute to the lesult. The action of various trees is 

 referred to on p. Il'O in connection with their effect on springs and 

 swamps. 



HoAu/ull . —T\\id eti'ect of forests o\\ rainfall has long been a dis- 

 puted point, and observations are still being continued ; but the most 

 modern results are in favour of the forests. The lowering oi tempera- 

 ture in wc>ofled regions is purely a matter of physics, and a surface 

 which keeps the air moist and coul, and frf)m which there is as much 

 evaporation as takes place from extended forests, must have a tendency 

 to increase the amoiint and frequency of precipitation, as contrasted 

 with an open, dry country, but one in wiiich the conditions ai'e other- 

 wise similar. This is the opinion of Dr. Hann, who admits that forests 

 frequently pioduce clouds, and that some sort of clouds are constantly 

 .seen to dis.solve over open country. He says that, as far at least as 

 the tropics are concerned, it may be concluded, with a good deal of 

 certainty, that forests may actually increase the amount of rainfall. 

 He then, in supi)ort of this view, refers to the writings of Blanforcl, 

 llibbentrop, Muttricli and Hettner, who deal with regions as far apart 

 as India. Germany and 8outh America, and adds, " Extended forests. 

 even in middle and higher latitudes, certainly ha\e some intluence in 

 increasing the fre(juency of rainfalls, but it is almost impossible to 

 determine the extent of their intluence by measurement and obser 

 vation.f HuiFel maintains that the effect of forests is to increase the 

 quantity and frequency of the rainfall, and that as this has been 

 proved by numerous lainfall returns it should be considered as a well- 

 established fact. The mean of thirty-three years' observation at the 

 experiment station at Nancy proves that the increase of rainfall pro- 

 duced by the forest amounts to 23 per cent. The tops of branches 



'■' But increases the regularity of the <lisehar"e. — AuTHOi;. 

 i HfiiiilhfinJ: nf <'f'nyirtf(>li>fi)i. Prof. J. Hanu. University of Vienna, pfi. I'.t'i 

 et sei{. 



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