96 



FOREST INFLUENCES. 



RELATIO?^ OF EVAPORATION TO FORESTS. 



The ainoimt of evaporation (Icpouds especially on the teiupeiature, 

 the Aviud, and the amount of vapor already present iirthe air. Tlie first 

 two are much changed by the presence of the forest, and it is to be ex- 

 pected that the evaporation in and about woods would show some 

 peculiarities. The evaporation varies very much also with the char- 

 acter of the surfoce from which it proceeds. Some surface must, there- 

 fore, be taken as a staniUird for evai^orations, and the measurements be 

 made with reference to this. There is considerable variation in the 

 standard surface selected, but in the German service it is that of water. 

 The reductions have been made, in this section, to terms of evapora- 

 tion from a water surface. At the same time, since the evaporation 

 is chiefly of meteorological interest because of its relations to precii)i- 

 tation, the amount of the latter is also included in the comparisons. 

 The observations were carried on in the open fields and in the forest. 

 The instrument emi)Ioyed is described on page 39. 



Fig. 49 exhibits the comparison of the annual evaporation in fields 



n Evaporation In Flel(te (E.O.) 

 and Woods (E.W.) compared 

 with Precipitation. (Pr.) 



li Precipitation anJ 



Evaporation. 



Deciduous 



C Precipitation and 



Evaporation. 



Evergreen. 



d Precfpltation and 

 Evaporation, 

 Young Trees. 



Fig. 49. — Evaporation and preriiiitation. 



{E 0) and woods {E W) with the i)recipitation. It will be noted that 

 the evaporation uiuler trees is about one half of that in open fields. 

 The precipitaticni is that of the open fields and, for the German stations 

 and years reduced (the ten years, 1M79 to 18S8), its annual value was 

 34.0 inches (871 mm.). The evaporation in the fields for the same sta- 

 tions and time was 1-J.7 inches (3U2.5 mm.) anuually, and that in the 

 woods 5.4 inches (137 mm.). The corresponding percentages of evapo- 

 ration were 37 and 16, showing a saving of 21 per cent, of the precipi- 



