RESULTS OF MEASUREMENTS. 45 



The loss of moisture by evaporation is the crucial feature of the 

 effect of winds upon crops. A fair estimate could be made of the 

 effect of the windbreak in modifying those atmospheric factors 

 which cause transpiration, by the use of evaporometers, exposed to 

 the same conditions as were the plants in the fields. 1 



The distance to which a windbreak may protect objects on its lee- 

 ward side has been variously estimated at from ten to fifty times its 

 height. While some experiments have been carried on to determine 

 this point (see diagram 18), it has in general been assumed that the 

 influence, if any, was not of great importance beyond ten times the 

 height of the trees. 



WINDBREAK EFFICIENCY DETERMINED BY EVAPORATION. 



The object of the evaporometer readings was to determine the effect 

 of a windbreak upon the amount of evaporation in the area which it 

 protects. If this evaporation is expressed as a percentage of the evapo- 

 ration in the open during the same period, the figure may be used as a 

 measure of the degree of efficiency of the windbreak at the particular 

 point where the instrument was exposed, and the simultaneous 

 measurements of evaporation at several points in the affected zone 

 will form a basis for a curve which may be called "the efficiency 

 curve" for the windbreak, under the given conditions of wind. 



Tests have been made on a large number of windbreaks of various 

 kinds and of different degrees of density or thickness, to determine 

 the comparative efficiency of windbreaks of the several classes and to 

 discover, if possible, how large a mass of trees is needed to form a 

 barrier against the most powerful winds. 



The results obtained for windbreaks of the same class have been 

 averaged to show the efficiency of the windbreak with all wind veloci- 

 ties which occurred during the period of observation. In order to 

 simplify the demonstration only the efficiency figures with wind 

 velocities of 5 miles per hour or a multiple thereof have been used. 

 These are given as "efficiency curves" for the various classes of wind- 

 breaks, on diagrams 16 to 24, inclusive. 



The conclusions to be derived from the curves representing the 

 protection afforded by a single windbreak against winds of various 

 velocities, and from comparison of the curves which show the efficiency 

 of different kinds of windbreaks, are as follows : 



( 1 ) The efficiency of a windbreak in checking evaporation is pro- 

 portional to its density. It may save at a single point in extreme 

 cases 70 per cent of the moisture ordinarily lost by evaporation. 



1 No claim is made for the accuracy of the comparison between the quantity of water evaporated from 

 this instrument and that transpired from a leaf under similar conditions, but that the two are subjected 

 to the same conditions and governed by much the same laws of evaporation can not be doubted. Admit- 

 ting this, the quality of the evaporation under different conditions will be indicated quite as accurately 

 by this instrument as by any potometer or similar device. 



