Conditions Modifying the Duty of Water 199 



as the nearest estimate now attainable of the minimum 

 amount of water the irrigator can hope to deliver to his 

 field where the yields there stated are expected ; and if 

 there are necessary losses in bringing the water to the 

 field, either by seepage or evaporation from the main or 

 lateral ditches, or if the water is badly handled, so that 

 there is a large amount of percolation ; or, again, if 

 unnecessary losses occur through lack of proper tillage 

 after irrigation, then the amounts stated in the table 

 must be exceeded by the amount of these losses. 



CONDITIONS WHICH MODIFY THE AMOUNT OF WATER 

 REQUIRED IN IRRIGATION 



Among the many factors and conditions which increase or 

 diminish the duty of water may be mentioned: 



1. The ^peculiarities of the crop grown. From what has been 

 said regarding the amount of water required for a pound of dry 

 matter and for yields of different amounts for different crops, it 

 will be evident that both the amount of water required by a 

 given crop and the frequency with which it should be applied will 

 depend much upon the crop being grown. 



This variation in the amount of Water required by different 

 crops depends upon many factors, some of which are not well 

 understood. Both the number and size of the breathing pores of 

 the green parts of the plant, through which the air enters and 

 from which the moisture escapes, may be expected to play an 

 important part in determining the necessary loss of water which 

 takes place. So, too, will the character of the foliage and the 

 habit of the plant as influencing the amount of wind movement, 

 and of shade over the soil of the field, effect the necessary loss 

 of water from the soil. 



In illustration of the influence of the shade offered by the 

 crop upon the loss of water from the soil may be cited the differ- 



