WATER 103 



RUN'OFF 



To be of the most use to man and other organic life, rain 

 when it strikes the earth should as far as possible be absorbed 

 by the topsoil and then infiltered to the ground water reservoir. 

 Whether or not it is absorbed depends first on the amount of 

 rain that falls at one time. Different soils vary widely in their 

 power of absorption, but each has its maximum capacity, and 

 if rainfall exceeds this, the soil becomes saturated and the sur' 

 plus runs off. But under this maximum capacity the amount 

 of absorption of rainfall depends on three major things. First, 

 the cover of the earth. The litter of leaves and twigs that 

 cover the floor of a hardwood forest has been known to absorb 

 95 per cent of an average rainfall. A good stand of blue grass 

 will absorb as much as 99 per cent of the water that falls in an 

 average rain. Close'planted crops like wheat and rye absorb 

 a much smaller per cent of rain water. Open row crops like 

 corn and beets absorb even less. Land that is left bare without 

 either grass or a crop, in other words, fallow land, absorbs 

 very little of the water that falls on it. 



The main reason for such wide differences of absorptive 

 power lies in the close partnership between plant and soil. In 

 a virgin forest or natural prairie, fallen leaves and dead plants 

 decay and become a part of the mineral soil. The rich, black, 

 porous, spongy soil of an old forest is a typical example. Such 

 soils helps easy infiltration and percolation. 



Intensive land use that results in a reduction of plant cover 

 increases the amount of surface run-off. The destruction of 

 forest cover by clear cutting and fire, the plowing of grasslands, 

 the planting of unbroken fields of grain or row crops, and the 

 overgrazing of range lands are examples of bad land use that 

 cause water to run off the surface instead of sinking into the 

 ground reservoir. By poor methods of cultivating land man has 



