18 Effect of Woodlands : Water Supply. 



73. It has been found, many times, that where the trees and 

 bushes are cleared away from large areas of rocky suriace, the re- 

 gion begins to suffer from drouth. The rocks being heated by the 

 sun, remain warm in the night, and the rain-clouds, which, in pass- 

 ing over the wooded surface, formerly condensed in gentle showers, 

 now dry up, upon coming to the heated air, and perhaps yield a co- 

 pious rainfall, over a better- wooded district beyond. These effects 

 are more apparent where the clearings have been extended over 

 considerable areas, and they could with certainty be overcome by 

 allowing the rocks to be again clothed with a growth of trees. 



74. The effect of Woodlands upon the Rain that falls upon them may 

 next be considered. Admitting that the amount of rain that falls 

 upon the woods is the same as that in adjacent fields, it is evident 

 that a part would be intercepted by the foliage, and in transient 

 showers be evaporated from it, without reaching the ground. But 

 the air within the woods is always more humid in the growing 

 season, and the surface is always shaded from the sun and sheltered 

 from the winds. It is also generally covered with a layer of dead 

 leaves and litter, so that the rain that does actually reach the earth, 

 although it may be rather less in amount, is more in effect, because 

 it is not readily evaporated. It sinks into the ground, instead of 

 running off on the surface. It can not wear away the soil upon 

 steep slopes, nor form sudden and destructive floods, as in a naked 

 and treeless region. The streams rising in w r oodlauds may swell 

 after a rain, but more gradually, and they will subside again more 

 slowly. If they rise in woodland swamps, they are scarcely liable 

 to floods at any season, and tend to an even flow throughout the 

 year. 



75. Springs and wells in a wooded region have a much more 

 uniform supply of water than in the same region when cleared. 

 Instances have often been observed where these become dry upon 

 clearing, and again well supplied with water as before when a forest 

 growth was restored. 



76. These principles become important when applied to the supply 

 of water for cities and towns, and for the maintenance of water in 

 canals, or for hydraulic power. The basins of supply should, if pos- 

 sible, be kept wooded, and the rivulets kept shaded, if we would 

 avoid failure. 



77. In Illinois, and some other prairie states, there has been ob- 



