CITY WATER SUPPLY 235 



and their beds become filled. The levees along the Ohio 

 and Mississippi must be continually raised in order to 

 keep the water within the river banks. The bed of the 

 Mississippi is in many places higher than the country on 

 either side; hence when the water breaks through its levee 

 much damage is done. 



If our rural population would learn how to keep the 

 cleared fields covered with vegetation and the soil supplied 

 with the proper amount of humus, -this cause of floods 

 would be removed, and there would be less need of the 

 forests so far as the control of water supply is concerned. 

 There are, however, many good reasons why there should 

 be forest regions and wooded slopes. 



The soil in the woods during a long dry season is much 

 dryer than the soil in a properly cultivated field. A 

 grass field is also much dryer than a field under cultiva- 

 tion. The reason is that the roots of the trees absorb 

 the water from the soil rapidly and it is transported to 

 the leaves, which give it to the air by evaporation. Water 

 from the deeper parts of the earth gradually works its 

 way to the surface during a dry season, but the roots of 

 the trees absorb it as fast as it comes by capillarity 

 toward the surface. In a cultivated field where crops 

 are growing, the leaf surface of the plants is not nearly 

 so great as the area of all the leaves on the trees in the 

 woods, and for this reason there is not so much water 

 given off by evaporation by the cultivated crop as there 

 is by the forest trees. Evaporation from the bare soil is 

 also prevented by stirring the surface of the soil and pro- 

 ducing what is known as dust mulch. Farmers who have 

 not learned how to conserve soil moisture during dry 

 seasons do not have much success in agriculture. 



From this comparison it can easily be seen that during 



