166 



FORESTRY AND IRRIGATION 



June 



assigned being similarity in prevailing 

 temperatures. 1 Any rise in tempera- 

 ture quickens evaporation by lessening 

 the humidity of the atmosphere and 

 thus making possible the absorption of 

 much, larger quantities of moisture. 



Whatever influence forest cover 

 may exert in this direction is undoubt- 

 edly secondary to that of temperature, 

 the primary factor in controlling 

 evaporation. In the present uncer- 

 tainty on this subject, no absolute con- 

 clusion as to the influence of forest 

 cover can be drawn. 



The second influence attributed to 

 forest cover upon a drainage basin is 

 the equalization of stream flow by 

 transforming surface run-off into un- 

 derground seepage. This is due in 

 part to the mechanical obstruction of 

 surface water by the leaves, branches, 

 litter, roots and humus of the forest. 

 Any such obstruction lengthens the 

 time required by precipitated water to 

 reach drainage channels and tends to 

 convert a larger proportion into un- 

 derground seepage. Underground 

 seepage is increased, in the second 

 place, by the marked capacity of forest 

 litter and humus for absorbing and 

 holding rain water. The porous, 

 spongy character of humus not only 

 enables it to absorb and hold two or 

 three times as much water as the most 

 absorbent soils, but it greatly increases 

 the porosity and absorptive power of 

 the mother soil with whih it becomes 

 mixed by gradual decomposition. 



The equalizing influence of forest 

 cover is exerted in another important 

 respect by delaying the melting of 

 snow and precenting, by its thick 

 mulch of humus and " litter, the 

 freezing of the soil beneath dur- 

 ing the winter months. It is 

 claimed that the melting of snow 

 upon open, exposed mountain slopes 

 is often ten times as fast as upon for- 

 , ested slopes in the same region. 2 

 Moreover as the snow melts it stands 



a much better chance of absorption 

 and filtration to underground sources 

 in the porous, unfrozen ground under 

 the forest than on the hard, frozen 

 soil of the exposed slopes. This 

 protective influence is seen most clear- 

 ly in the Rocky Mountain region 

 which is subject to the warm Chinook 

 winds of early spring. 



The exact extent of the equalizing- 

 influence exerted by forest cover in 

 each of these ways it is of course im- 

 possible to determine. Of their com- 

 bined effect in equalizing stream flow 

 much substantial evidence is cited. 

 Such evidence is furnished by the les- 

 sened number and severity of floods 

 upon forested as compared with de- 

 n-ued basins, by the greater preman- 

 ency of flow 7 from springs and other 

 underground sources, and by the bet- 

 ter sustained stream supply during 

 months of little or no precipitation. 

 As illustrating the usual difference be- 

 tween forested and deforested 

 streams, Mr. Yermeule cites 3 figures 

 from the forested Passaic and the de- 

 forested Raritan of New Jersey, show- 

 ing for the former a marked superior- 

 ity in evenness of flow, less water pro- 

 portionately being discharged immed- 

 iately after heavy rains and more pro- 

 portionately during dry months which 

 follow. A forested basin thus shows 

 a marked tendency to store flood 

 water and discharge is more evenly 

 and uniformly into the stream chan- 

 nel than a denuded basin in the same 

 region. An interesting case from Cal- 

 ifornia is cited by Mr. Manson. 4 Of 

 two small tributaries of the Yuba 

 River, of approximately the same 

 drainage area, one is well covered 

 with timber and brush, the other be- 

 ing largely denuded. Both streams 

 were gaged in September, 10,00 after 

 three successive seasons of deficient 

 rain. The forested stream showed a 

 minimum run-off for the 120 days of 

 low water of 1 13 second feet, while on 





^cf. Reports State Geologist of New Jersey. 1899, page 145; 1894, page 333. 

 cf. Water Supply and Irrigation Papers, "Division of Hydrography, U. S. G. S., No. 

 46, page 46. 



3 cf. Report State Geologist of New Jersey, 1899, page 162. 

 4 cf. Bulletin 7. U. S. Bureau of Forestry, page 136. 



