AMOUNT OF WATER USED IN IRRIGATION 225 



(8) Method of applying water. The furrow system 

 is usually more economical of water than the flooding 

 system, because less opportunity is given for evapora- 

 tion. 



(9) The fertility of the land, as distinguished from its 

 physical properties, determines the duty of water 

 through its influence on the size of crop which may be 

 produced. A large crop is more economical of water 

 than a small one, but a large crop will require, a larger 

 total amount of water. 



(10) The closeness of planting affects the loss of 

 water in much the same way as a large or a small crop: 

 (a) By determining the total amount of water which 

 must be used directly by the plants; and (b) by shading 

 the ground and cutting down temperature and wind 

 movement more or less, it decreases the loss of water 

 directly from the soil. 



(11) The tillage practice affects the efficiency of 

 water under irrigation as it does the efficiency of rainfall 

 in humid regions. If lax conservation methods are used, 

 much more water will be needed than where the best 

 tillage processes are applied. 



For these reasons, it is not possible to specify any 

 definite amount of water which should be used in the 

 practice of irrigation. It varies widely for different sec- 

 tions of the world and, since it is very common to meas- 

 ure the total amount of water supplied at the head of 

 the intake canal, it is largely determined by seepage from 

 the canals and ditches. (See page 134.) The amounts 

 of water which are applied in different irrigation sec- 

 tions are given by different authorities as follows: 



