78 ENGINEERING ON THE FARM 



of water that falls in a given time and on the quantity 

 disposed of by the other four agencies in the same length 

 of time. 



Absorption. The soil takes up and holds a certain quan- 

 tity of water between the grains. The longer the period 

 that has elapsed since the last rain, the greater the quantity 

 of water that will be absorbed. 



Percolation. Some soils are of an open, porous nature 

 and permit the water to percolate down to a considerable 

 depth. In this way water may work its way in under- 

 ground channels and finally find an outlet to the surface 

 as springs. When the underlying stratum is of an open 

 nature, the flow of water is facilitated, and when this stratum 

 is overlaid by a porous covering, it may be possible for prac- 

 tically all the rainfall to be disposed of by percolation. 

 Damage to crops will not occur from excessive rains in such 

 a locality. However, under ordinary conditions there is a 

 limit to the amount of rainfall which can be disposed of in 

 this way. 



Evaporation. Certain conditions of air and tempera- 

 ture tend to evaporate water on the surface and remove 

 it as vapor in the air. Great quantities of moisture are at 

 times removed in this way. Evaporation tables indicate 

 that the annual quantity of water drawn back into the air 

 from a body of water by this agency is frequently as large 

 as, if not larger than, the rainfall. Hasty conclusion might 

 lead to the error that evaporation is sufficient to prevent 

 damage to growing crops from excessive moisture. If the 

 evaporation and rainfall are uniformly distributed through- 

 out the year, evaporation will remove the surplus rain; but 

 excessive moisture, when conditions are right, can per- 

 manently injure a crop in twenty-four hours. Conditions 

 may be such that at the time of the falling of the rain 

 evaporation is practically stopped for a period of twenty- 

 four hours. High winds and warm temperature are con- 

 ducive to rapid evaporation, and when a rainstorm is 



