142 ■ GEOLOGICAIv SURVEY OF NEW JERSEY. 



country surrounding L/ondon or upon • our own Passaic water- 

 shed does not increase the annual evaporation over what would 

 take place from fallow, tilled ground more than from 4 to 5 inches. 

 The experiments of L^awes and Gilbert were upon soil kept 

 hoed. Prof. King, in the Wisconsin experiments, shows the 

 effect of cultivation in reducing evaporation. Keeping this fact 

 and the experiments of Greaves, and Evans and Dickinson in 

 mind, it seems probable that evaporation from barren soil, not 

 cultivated, is as great as from soil with vegetation.* 



The above cited estimate of 16 to 19 inches, representing the 

 requirements of forest growth, rests upon good authority, but 

 we must not conclude that this means 16 to 19 inches more 

 evaporation from a forested than from a fallow area. Neither 

 must it be concluded that the estimates are reliable for all 

 climates, for we shall show in our discussion of stream measure- 

 ments that the evaporation from a forested area varies from 20 

 to 31 inches per annum in the eastern United States, the actual 

 amount being determined by the climate. The same is true of 

 the experiments of Professor King. While they were carefully 

 conducted, and no doubt fairly accurate for the climate of the 

 place of observation, it may be doubted if they will hold good 

 for places having a different climate. 



Now, if we adopt the conclusion, that the total effect of vege- 

 tation, such as that of the Passaic catchment, is to increase the 

 annual evaportion less than five inches over what would take 

 place from the ground, without vegetation, it is plain that a 

 variation in the amount of forest represented in this vegetation 

 cannot possibly produce an effect upon evaporation so great as 

 five inches. 



We will not add to the above, as we have quoted what we 

 regard as the highest authorities, or more satisfactory data, in 

 each case. A large number of early observations of evaporation 

 made in Europe were entirely unreliable, as may be illustrated 

 by the fact that it was for a long time believed in France that 

 the total evaporation from the land surface exceeded the amount 

 of rain-fall thereon. Experiments made on a small scale may 



* The controlling cause of evaporation being the capacity of the air to absorb moisture, it will be 

 seen that the water transpired by vegetation goes to supply this demand, consequently it is a part of the 

 water which, in the absence of vegetation, would be drawn direct from the earth into the atmosphere. 



