THE INFLUENCE OF FORESTS ON CLIMATE AND FLOODS 229 



may proceed in this matter, is the effect 

 of forests on evaporation. By cooling 

 the air and obstructing- the wind, for- 

 ests reduce the evaporation from a 

 water surface, but the trees themselves 

 evaporate considerable quantities of 

 moisture, so that the total evaporation 

 over a wooded area is generally consid- 

 ered to be greater than over a cleared 

 area. Those who oppose the preserva- 

 tion of the forests sometimes make a 

 great point of this. Now, the precipi- 

 tation of rain depends fundamentally 

 upon the amount of vapor in the atmos- 

 phere ; most of this vapor comes from 

 the great oceans and is carried by the 

 winds over the lands ; but if, as it pro- 

 ceeds over the land it is joined by more 

 vapor evaporated from the land itself, 

 there will be all the more vapor in the 

 atmosphere to be condensed when the 

 conditions which produce condensation 

 occur. Any cause, therefore, which 

 increases the evaporation must increase 

 the rainfall ; and if — as there is good 

 reason to believe — the growth of for- 

 ests increases the total evaporation from 

 an area, it would seem they must in- 

 crease the rainfall. 



This does not mean that if I planted 

 trees in my back yard I will increase the 

 rainfall in my backyard ; neither does it 

 mean that if forests are planted on a 

 spit of land projecting into the sea like 

 Cape Cod, the rainfall on Cape Cod will 

 be increased. The vapor is not precipi- 

 tated where it is evaporated, but is car- 

 ried long distances ; some of it may be 

 precipitated on the land and some on the 

 sea. Generally speaking, however, the 

 argument would seem to be sound ; that 

 is, if forests increase evaporation they 

 must increase the rainfall somewhere. 



All this, however, is of little conse- 

 quence as compared with the larger 

 question, and the main question, as 

 to the effect of forests upon floods 

 and the navigability of streams. It 

 must also be remembered that those 

 who are in favor of the Weeks bill do 

 not argue, as Professor Moore would 

 seem to make it appear, that the forests 

 must be preserved on areas suitable for 

 cultivation and needed for cultivation. 

 "The pleading of the poor man's chil- 

 4 



dren for bread and meat" referred to 

 by Professor Moore, is a sentimental 

 obscuring of the question. What is 

 claimed is that on the steep slopes and 

 mountain sides which are not suited for 

 agriculture, the forests should be pre- 

 served, not only as a source of lumber 

 supply but as a protection against the 

 fires which are most apt to start on the 

 cleared lands, and as a protection to 

 navigation by preventing erosion and 

 the silting up of streams, and also as a 

 source of beauty and health ; but the 

 legal justification must, as already 

 stated, rest solely on the effect upon 

 navigation. 



EFFECT OF FORESTS UPON FLOODS 



In Studying the effect of forests upon 

 floods by the statistical or inductive 

 method, we meet with even greater 

 difficulties than in studying the effect 

 upon rainfall. One of the most impor- 

 tant of these difficulties arises from the 

 fact that zve cannot vary only one ele- 

 ment a\t a time. Not only is the rainfall 

 different in different years on a given 

 area, but the proportion of that rainfall 

 which flows in the streams, even if the 

 rainfall is precisely the same, may vary 

 in different years by 100 per cent. In 

 1 87 1 the rainfall on the Cochituate 

 watershed was forty-five and thirty-nine 

 one-hundredths inches, of which thirty- 

 three per cent flowed from the surface, 

 the remainder being either evaporated 

 or sinking into the deep strata ; while 

 in 1891 the rainfall was almost exactly 

 the same, namely forty-six and forty- 

 two one-hundredths inches, of which 

 sixty-nine per cent flowed from the sur- 

 face ; in 1874 the rainfall was thirty-five 

 and ninety-three one-hundredths inches, 

 of which fifty-four per cent was col- 

 lected; in 1880 the rainfall was thirty- 

 five and eighty-three one-hundredths 

 inches, of which only twenty-nine per 

 cent was collected. Equal variation oc- 

 curs in the manner in which this water 

 flows off ; a large amount may flow off 

 without causing a flood, or a smaller 

 amount may flow off causing a great 

 flood. Now, in order to demonstrate by 

 induction the effect of forests on floods, 



