48 IRRIGATION FARMING. 



Evaporation and Run- Off.— When the rain falls 

 upon the surface of the earth, a part is evaporated and 

 carried away in the clouds, a part sinks into the soil 

 to be slowly evaporated, and a very large part is carried 

 away by vegetation itself. Plants drink water and 

 transpire it into the air in very large quantities. That 

 which is not evaporated from the earth's surface sooner 

 or later, or transpired by plants, is gathered into the 

 rivers; we call that which ultimately flows out to sea 

 the ' ' run-off ' ' water; and that which is evaporated and 

 which drifts away in the air we call ' * fly-off ' ' water. 

 These are two very common, simple terms. In cal- 

 culating the requirements of modern irrigation, the 

 best authorities hold that the water supply for a given 

 acre should be sufficient to cover it twenty-one inches 

 deep during the course of an irrigating season of lOO 

 days. Some experts place the maximum as high as 

 twenty-four inches, which is an estimate that is 

 certainly liberal enough. 



Other conditions being equal, the drier the soil the 

 greater its absorptive powers. An ordinary rain falling 

 upon a dry, cultivated field will be almost entirely 

 absorbed, but if the ground is already charged with 

 moisture nearly all the rain will run off the surface 

 and be carried away through the ordinary drainage 

 channels. The per cent, of the total rainfall which 

 joins the run-off in humid climates is therefore much 

 greater than in arid regions unless other conditions 

 modify the results. In general, therefore, the pro- 

 portion of the total rainfall which may be depended 

 upon for filling reserv^oirs in arid and semiarid climates 

 is much less than in humid sedtions. The rapidity of 



