10 



THE IRRIGATION AGE. 



THE INFLUENCE OF FORESTS ON IRRIGATION. 



BY B. E. FERNOW.* 



'T'here has been considerable wild dis- 

 cussion on the influence which for- 

 ests are supposed to exercise on climate, 

 waterflow and other conditions. On the 

 one hand enthusiastic forestry advocates, 

 who clamor for forest preservation in 

 itself a misleading term have claimed 

 extravagantly and unconditionally such 

 influences, on the other hand, men' w r ho 

 ought to have had enough scientific 

 training to know better, have as uncon- 

 ditionally and extravagantly denied such 

 influences. The one position is as un- 

 philosophical as the other. Every 

 student of nature, be he only an observer 

 of it in the field, or be he only a reader 

 of what has been written by observers, 

 knows that all things are in relation, 

 that, therefore, we cannot take aw r ay 

 anything from the complex conditions 

 of nature, without affecting more of less 

 all other conditions. The question 

 then can only be as to character, degree 

 and extent of the influence of one condi- 

 tion, or its change and removal on other 

 conditions, and that this depends neces- 

 sarily also on the character, degree and 

 extent of the changeable condition is 

 self-evident. 



In other words, the question as to 

 whether the removal or devastation, or 

 the continuance and proper treatment of 

 a forest has a practical influence on cli- 

 mate, soil or waterflow cannot be discus- 

 sed in general terms, but its solution 

 must be dependent upon the character, 



extent, location and specific conditions 

 of the forest cover in each case. 



It stands to reason that the dense tall 

 forest growth of evergreens of the 

 northern Sierra Nevada must exercise a 

 very different degree of influence on its 

 surroundings, than the open, stunted, 

 shrubby growth of acacia and mesquite 

 around Phoenix or the beautiful. park- 

 like, pine forest of the Colorado plateau, 

 or the scanty growth of juniper and 

 pinion that covers so much of our drier 

 mountain slopes. Each of them has its 

 influence on surroundings, but the degree 

 must differ according to their constitu- 

 tion and location and finally it may be 

 practically of no value. 



The requisites of satisfactory water 

 supply in irrigation, I take it, besides 

 the quality of the water, its availability 

 for irrigating purposes are: sufficient 

 quantity, equable flow, absence of debris 

 and silting in the channels. 



In all these directions a forest cover 

 may exercise beneficial influence. 



Whether a forest area may increase 

 the total amount of rainfall over its own 

 or an adjoining area is still an open 

 question, which will not be settled until 

 we have better means of measuring rain : 

 for as is w T ell known our rain guages are 

 but poor means for the purpose for 

 which they are intended. 



Nevertheless the forest may have an 

 influence upon the amounts that remain 

 available of what has fallen. In the 



*Chief of Forrestry Division, Department of Agriculture. Address delivered at Irrigation Congress. 



