38 ELEMENTARY FORESTRY. 



This is certain that since summer and winter drouth, that is, 

 rapid evaporation due to continuous dry winds, is the bane of 

 the farmer on the plains, rationally disposed timber belts will 

 do much to increase available water supply by reducing evap- 

 oration. 



Evaporation, of course, goes on much less rapidly within 

 than without the forest. How great this difference is in this 

 section we have no exact figures to tell, but it is certain that 

 it is much more than in Bavaria, where the following result 

 was obtained: In an experiment which was carried on to 

 determine the amount evaporated from April to October it was 

 found that from a certain area without the forest 40.8 centi- 

 meters were evaporated, within pine wood 15.9 centimeters and 

 within deciduous woods 6.2 centimeters. This shows that the 

 evaporation was six and one-half times as great in the open 

 field as in deciduous woods. 



Transpiration. Another factor by which forests dissipate 

 water supplies and which has been referred to (page 17) is 

 transpiration. The quantity of water so used is as variable as 

 the amount of precipitation, and in fact within certain limits 

 depends largely upon it; that is to say, a plant will transpire 

 in proportion to the amount of water which is at its disposal. 

 Transpiration is also dependent on the stage of development of 

 the plant, on the nature of its leaves and amount of foliage, on 

 temperature, humidity and circulation of the air, on intensity 

 of the sunlight, and on temperature and structure of the soil 

 and on other meteorological conditions. Rain and dew reduce 

 transpiration; wind increases it. 



The amount of transpiration depends considerably upon the 

 thickness of the leaves; therefore the surface of the foliage is 

 not a reliable measure, but should be compared with the weight. 



In some European experiments carried on during the period 

 of vegetation, the amount of water transpired by the different 

 species per pound of dry matter in the leaves was as follows: 



