777 /-; IRRIGATION AGE. 



11 



first place, tree growth transpires as a 

 rule much less water than the annual 

 vegetation, or more than would be evap- 

 orated by sun and wind if the ground 

 was not shaded. The shade and the 

 mere wind breaking quality of a forest 

 growth the velocity of the winds in- 

 creasing their evaporative power in 

 acceleration ratio preserves a large 

 amount of moisture and especially where 

 the supply is dependent on winter snows 

 the effect is most noticeable in prevent- 

 ing the rapid' wasting by evaporation. 



In the second place, the forest floor, 

 if in good condition and even without 

 that, the deep reaching root system of 

 the trees keep the soil in a granular, 

 open condition, which allows the water 

 to penetrate, while in the open barren 

 the soil is compacted until the rain must 

 run off superficially, having no chance 

 to enter the ground. That soil con- 

 ditions and geological structure must 

 also have a potent influence in this re- 

 spect stands to reason. The loose sand, 

 no matter whether forest covered or not, 

 will remain permeable, while compact 

 clay soils, even though forest covered, 

 will still resist the passage of water to a 

 degree. The penetrability of the soil 

 under forest cover induce subsoil drain- 

 age instead of surface drainage, or at 

 least a longer time is given for the for- 

 mer, before the latter begins. 



This is especially important when the 

 snow melt begins in spring; in the un- 

 frozen penetrable forest soil the melt 

 water has to percolate, while over the 

 usually frozen and compact denuded soil 

 the water rushes off superficially and 

 makes or aggravates the spring floods. 



This same permeability of the ground 

 together with the mechanical obstruc- 

 tion which the tree trunks and the 

 shrubbery of a well kept forest offer, 

 and by which the .surface drainage is de- 

 creased in amount and changed into 

 sub-drainage, also prevents the washing 

 of the soil, the gullying and the carry- 

 ing of silt and debris into the water 

 channels. Again I recall that the degree 

 of this must depend on the character, 

 condition and location of the forest 

 cover; it may become nil if the forest 

 floor has been repeatedly burned, has 

 been trampled by cattle and sheep and 

 the trees have been mostly removed, 



thereby reducing forest conditions. 



That these things are so modified to 

 be sure according to local conditions, is 

 not any more a fancy, a guess, a suppo- 

 sition, for the experience in France dur- 

 ing the last 100 and especially during 

 the- last 30 years have proved them, if 

 we did not have sufficient proof in our 

 own country over the hill lands of Mis- 

 sissippi, and in fact more or less in all 

 parts of the country. In France the 

 deforestation or devastation of about 

 8,000,000 acres of mountain forest has 

 resulted in a devastation of 1,000,000 

 acres of agricultural soil in the plain 

 below by the sudden changes in water 

 stages and the torrential action of the 

 rivers, the silting over of grounds as far 

 as 200 miles away from the source of 

 the evil. 



Again the government having spent 

 over $12,000,000 and the impoverished 

 communities about as much to reforest 

 some parts it will take three times as 

 much to remedy the evil the experience 

 has been gained that this kind of a 

 cover, forest cover, will do what is ex- 

 pected of it and reduce the wild action 

 of the waters and prevent the silting of 

 the channels. While these experiences 

 have been had without reference to irri- 

 gation problems it is evident that they 

 have a bearing on the same, if equable 

 flow, instead of repeated floods and un- 

 obstructed channels for carrying the 

 waters to reservoirs, etc., are desirable. 



Irrigationists then should be interested 

 not in the preservation, but in the proper 

 use and conservative management of the 

 forest cover at the headwaters and along 

 the water courses, so as to secure the 

 most favorable conditions of water flow, 

 for without forest management no 

 rational water management can be main- 

 tained for any length of time. There 

 are, to be sure, irrigation districts where 

 natural forest conditions are if not absent 

 such as to assist but little in the directions 

 mentioned. Yet even there that little 

 helps and would be a matter of prudent 

 management to improve those conditions 

 rather than to leave them to further 

 deterioration. 



I have laid particular stress upon the 

 difference between forest preservation 

 and forest management. 



