204 Irrigation and Drainage 



and the various distributaries will, of course, be relatively greater 

 in regions of soils of coarse texture than where the soils are finer, 

 so that here is a factor modifying the duty of water as con- 

 sidered from the standpoint of the water company and irrigation 

 engineer especially, but also with the large irrigator, who has 

 extensive distributaries, through which the water must be con- 

 veyed before it is finally taken out upon the land. It should be 

 emphasized that our discussion has reference to the duty of water 

 after it has reached the field where it is used. 



If it shall be found true that the continued growth of large 

 crops upon a piece of land, and the consequent more complete 

 evaporation of all water brought to the soil, thus curtailing the 

 drainage, tends to develop alkalies to an injurious extent, or 

 other prejudicial salts, so that flooding or leaching by irrigation 

 shall be found necessary in order to restore fertility, then here, 

 again, the character of the soil will modify the amount of water 

 required. 



3. The character of the rainfall will necessarily modify in a 

 marked manner the amount of additional water which may be 

 used to advantage in the production of crops. It has already 

 been pointed out on page 103 that the difference in the character 

 of the rainfall in parts of California, Oregon and Washington, as 

 compared with that of western Kansas and Nebraska, may explain 

 why equivalent amounts of rain are much more effective in the 

 former than in the latter regions, and if it is true that the fre- 

 quent summer rains east of the Rocky Mountains do tend to hold 

 the development of the roots of crops closer to the surface, and 

 also to destroy the effectiveness of soil mulches, it is clear that 

 the duty of water in climates where most of the growing season 

 is an uninterrupted rainless period will be relatively higher than 

 where frequent but inefficient showers tend to reduce the effi- 

 ciency of mulches, and to hold the roots of crops closer to the 

 surface. It is, therefore, likely to be found true that more water 

 will be required for like results in western Texas, Oklahoma, 

 Kansas, Nebraska, and the Dakotas, and similar climates, than 

 will be required where the whole summer season is one con- 

 tinuous interval of no rain. 



