1905 



FORESTRY A^B IRRIGATI 





immediate shedding of precipitate! 

 water or its absorption and storage 

 underground seepage. Hard surfa 

 rocks of densr texture, like the gran- 

 ites and gne:s--.-s. tend to shed rainfall 

 at once as surface run-off. r- ::-. r 

 rocks of porous t_:.ture. like lime- 

 stone, favor the absorption of rainfall 

 and its gradual transfer t the stream 

 as underground seepage. The char- 

 acter of the mother rock, moreover. 

 determines very largely the nature : t" 

 the soil, its depth, per - : : ~ 7 rr 

 of absorbing and retaining water, all 

 factors of prime importance in their 

 bearing upon the disposal of precipi- 

 tated water as rapid surface discharge 

 or gradual absorption to underground 

 sources. 



The fourth of the important 

 factors governing stream now is the 

 character of the vegetation upon the 

 drainage basin, with special reference 

 to the influence of forest cover. For- 

 ests have always been regarded as 

 connected in some way with the pr;- 

 tection and preservation of water sup- 

 ply. As this vague belief has been 

 superceded by more exact investiga- 

 tion, certain definite influences have 

 been attributed to forest cover in its 

 bearing upon stream flow. Few of 

 these influences can be said as yet to 

 be clearly established, and many are 

 combated by able authority. It will 

 be attempted here simply to summa- 

 rize the more important facts relating 

 to each. 



In the first place, it has been clai:- 

 that forest cover on a drainage basin 

 actually increases the amount of wate- 

 available for stream supply b; retard- 

 ing evaporation from the ground. This 

 influence is attributed to the s: me what 

 lower temperatures prevailing under 

 forest cover and to die pi -e in- 



fluence of the tree canopy and litter in 

 insulating the soil from the sun's ra - 

 and thus checking the si gth and 

 directness of the evaporation force. 



ther reas us advanced are the high 



t humidity maintained in the 

 atmosphere b : through their 



shade and active transpiration from 

 the leaf sari ind their influence in 

 breaking up air currents and protect- 

 :rom the drying effect 

 of wind. A series :i German ex: 

 ments. representing conditions both in 

 the '.- : : is ir.i in t:te s - that 



the total evapcr under forest 



: 7- averages bat _i per cent of the 

 evaporation from unpr i sofl. 1 



Mr. :.irig the amount of 



evaporation as the difference 

 the total annual precipitation npon a 

 ism .and the estimated annual 

 stream lischarge from the same area, 

 concludes that a difference of five or 

 six inches in annual evaporation 

 about Jf ?e~ .T: t. can be listmgmshed 

 between a forested ". ^tershed like the 

 upper Hudson and a def reste: 

 like the upper Genes* 



On the other hand, the opposing 

 claim is made that actually 

 is avail.'.:. e for stream supply on a 

 sted than a c. rest ism. This 

 is due : the interception ::' j: - 

 per cent of the rainfall by the fc ig 

 of a forest and its fired evapor.t 

 into the air. and to the large amounts 

 of water consumed in the pr: :. I 

 tree crrwth and transpiration. In this 

 connection it is urc^i that variations 

 in the amount of evaporation :r 

 drainage basins .ire due primarily to 

 differences in temperature rather than 

 to dirt ere:-. :es in s . conditi as - i r- 

 est Dover Mr. Yerr.teule cites a large 

 number :t eastern - - ch have 



practically the same 

 but which show a ran... even 



inches in t: : fcal annual e 

 In nearlv every insl 

 evav 5 higbei 



annual and mean summer t. 

 tures. In like manner, a nun/ . - : 



- - streams ng a r.. g the 

 rcentagf E I 



[41 



the sanie e\"ap--"..: m< re - 



'of. Bulletin 7. U. S. Bureau of Forestry. : Kg 



I gs . - oiation. vol. x _ 



