394 



Woods and Water Supplies. 



When a catchment area is covered with trees, and with the 

 vegetable matter that accumulates on the surface of the ground,, 

 the water that reaches the soil as rain is impeded in its flow, and 

 its evaporation is hindered so that the general effect is equiva- 

 lent to an increase in the size of the reservoir. It is also 

 important to note that snow melts more slowly underneath 

 trees than in the open country, so that at a time of thaw the 

 snow-water is yielded up more gradually. 



Forests not only affect the degree of moisture in soil, but 

 they also exert considerable influence on the soil-temperature. 

 Although this influence is greatest at the surface of the ground> 

 it is also perceptible to a depth of several feet. On the average 

 of a large number of Continental stations it was found that 

 woods of various species and ages depressed the mean annual 

 temperature at the surface of the ground by about 2*6 deg. F., 

 while even at the depth of 4 ft. the reduction of temperature 

 was 2 deg. 



This general cooling influence is due to a variety of causes. 

 The foliage of the trees excludes the sun's rays, the decaying 

 vegetable matter that covers the ground prevents the free 

 exchange of air between the soil and the atmosphere, while the 

 water in the soil absorbs much heat without its temperature 

 being much affected. 



While woods have a depressing influence on the mean annual 

 temperature, it is found that this effect is much greater in sum- 

 mer than in winter. On the average of eleven German stations, 

 the July temperature of the surface soil in the forest was found 

 to be 7 deg. F. lower than that in the open field, whereas in 

 December the former was rather warmer than the latter. 

 Forests, therefore, tend to equalise the temperature of water 

 collected in them, the temperature being slightly raised in 

 winter and markedly reduced in summer. This result would 

 appear to be of considerable practical and hygienic importance 

 where a supply of water for domestic purposes is concerned. 



To the credit of forests is also to be placed the fact that they 

 exercise a purifying influence both on the air and on the soil» 

 germs of all kinds being markedly scarcer in a well-wooded 

 district than in a similar extent of treeless country. 



