The Duty of Water 197 



place through the foliage of the crop being fed, then a 

 very small rainfall during the growing season, and a 

 very small amount of water added by irrigation, would 

 suffice for the production of large yields. 



In other words, the duty of water in crop produc- 

 tion is determined by the necessary losses: (1) by 

 transpiration through the plant ; (2) by surface evapo- 

 ration from the soil ; and (3) by surface and under- 

 drainage. The more these sources of loss may be cur- 

 tailed, the larger will be the duty of water in both arid 

 and humid regions. 



In countries where irrigation must be practiced in 

 order to successfully grow crops, skillful management 

 may almost wholly prevent loss by drainage, and loss 

 by surface evaporation from the soil can be made 

 relatively very small, so that the major loss may 

 be that which is transpired through the plant itself. 

 So, too, in humid climates, the losses during the grow- 

 ing season by both drainage "and surface evaporation 

 may be greatly reduced through skillful, intelligent 

 practice. 



It will, therefore, be helpful, in forming an estimate 

 of the possible duty of water, to use the data already 

 presented in another place to compute the minimum 

 number of acre -inches of water which may be made to 

 produce yields of different amounts under the condi- 

 tions where no drainage takes place, and where surface 

 evaporation is made as small as it can well be. The 

 results of such a calculation are given in the table 

 which follows: 



