308 FORESTRY INVESTIGATIONS U. S. DEPARTMENT OP AGRICULTURE. 



means the most satisfactory, not only on account of its irregularity in time and quantity, but also 

 on account of its detrimental mechanical action in falling; for in its fall it compacts the ground, 

 impeding percolation. A large amount of what would be carried off by underground drainage is 

 thus changed into surface drainage waters. At the same time, by this compacting of the soil, 

 capillary action is increased and evaporation thereby accelerated. These surface waters also loosen 

 rocks and soil, carrying these in their descent into the river courses and valleys, thus increasing 

 dangers of high iioods and destroying favorable cultural conditions. 



Here it is that water management and, in connection with it or as a part of it, forest manage- 

 ment should be studied; for without forest management no rational water management is possible. 

 The forest floor reduces or prevents the injurious mechanical action of the rain and acts as a 

 regulator of water flow. Hitherto water management in rainy districts has mainly concerned 

 itself with getting rid of the water as fast as possible, instead of making it do service during its 

 temporary availability by means of proper soil management, horizontal ditches and reservoirs — 

 drainage and irrigation systems combined. It seems to have been entirely overlooked that irriga- 

 tion, which has been considered only for arid and subarid regions, is to be applied for plant 

 production in well-watered regions with equal benefit and profit, if combined with proper drainage 

 systems and forest management. 



The experimental demonstration of this influence of forests and water flow is also still in doubt, 

 and the problem is as difficult and complex as that regarding the influence on temperature and 

 rainfall. Nevertheless, sufficient experience exists to sustain the general philosophy, to which a 

 close student of nature is forced, long before he can demonstrate the qualitative and quantitative 

 character of an important influence of forests on water conditions. 



Summary of Conclusions. 



For those who wish to know only what the present state of the question of forest influences 

 is, we have summarized what conclusions may be drawn from the facts presented in Bulletin 7, 

 referring them to that report for the basis of these conclusions and the discussion in extenso. For 

 easy reference the pages of the bulletin containing the data upon which each conclusion is reached 

 are given in parentheses at the end of each paragraph, and the diagrams which show in graphic 

 manner the result of the observations upon which the conclusions are mainly based are repro- 

 duced. There are also added to this summary some references to later developments in this subject. 



GENERAL CONSIDERATIONS. 



(1) We must keep separate two main questions, namely, What is the difference of conditions 

 within and without the forest? and How far is the difference of conditions within the forest com- 

 municated to the outside, i. e., how far does the forest influence the conditions outside? (Pp. 

 23-40, Bui. 7.) 



(2) The general climatic conditions in which the forest is situated as well as its situation with 

 reference to elevation and exposure, furthermore its composition, whether evergreen or deciduous, 

 its density, its height and extent, the character of the forest floor, and other features which 

 determine its quality, must all combine in producing variety, at least quantitatively, both as to 

 difference of conditions within and without the forest and as to possible exchange of the same, 

 and hence the question of forest influences can be properly discussed only with reference to these 

 other conditions. We must refrain from generalizing too readily from one set of conditions to 

 another set of conditions. (Pp. 40-121, Bui. 7.) 



(3) In the matter of forest influence upon water flow, besides the above mentioned, other 

 conditions, the topography and geology or stratification of soil, must also be taken into account 

 and generalizations without regard to these must be avoided. (Pp. 123-157, Bui. 7.) 



(4) No influence upon the general climate which depends upon, cosmic causes can in reason be 

 expected from a forest cover. Only local modifications of climatic conditions may be anticipated; 

 but these modifications, if they exist, are of great practical value, for upon them rest success or 

 failure in agricultural pursuits and comfort or discomfort of life within the given cosmic climate. 

 The same condition must be insisted upon with reference to forest influences upon water flow, 

 which can exist only as local modifications of general water conditions, which are due in the first 

 place to climatic, geologic, and topographic conditions. (Pp. 157-170, Bui. 7.) 



