14 THE BOOK OF FORESTRY 



covering a tin roof. If water is allowed to fall upon 

 a steep roof, it runs immediately to the spout, whereas 

 if this same roof were covered with a layer of spongy 

 material a very large amount of the shower would be 

 absorbed and very gradually released. 



In a light summer shower it is estimated that from 

 twenty-five to sixty per cent of the rainfall never 

 reaches the ground at all, but is caught upon twigs 

 and limbs and trunks of the trees. After the shower 

 has ceased this moisture is evaporated directly into the 

 air without reaching the soil. 



Of the rain that does reach the ground, a very 

 large amount is absorbed by the humus, that organic 

 layer of partly decomposed leaves and twigs which lies 

 on top of the mineral soil. This humus has a marvelous 

 capacity for holding water; in fact it can hold many 

 times its own weight. The trunks of the trees and 

 supporting roots form countless little basins on every 

 hillside and when the humus has become thoroughly 

 soaked so that it can no longer contain another drop, 

 these little basins gently fill and overrun one into the 

 other, allowing the water to reach the brook very 

 gradually. 



"When the forest cover is removed the humus is 

 decomposed by the air; the leaves, twigs and branches 

 no longer catch the rain and as a consequence the water 

 rushes immediately from the steep slope to the river 

 bed below. In case of heavy rains the soil may be 

 badly washed and gullied and large amounts of gravel 

 and stones may be washed from the steep slopes upon 

 the flats and fertile fields farther down the river. In 

 many of the Southern States the heavy clay soils are 

 often badly eroded during torrential rains, and it is 

 no uncommon sight to see rolling fields located just at 



