WATER SUPPLIES FOR IRRIGATION. 



causes the condensation and subsequent 

 reappearance of the evaporated water in 

 the form of rain or snow. 



We now have all the conditions neces- 

 sary to the creation of a flowing stream of 

 water. As soon as the ability of the soil 

 to absorb the rain or melted snow is ex- 

 hausted, it begins to flow along the inclined 

 surface, always seeking its lowest level, 

 until the union of one small stream with 

 another in a common channel produces a 

 creek or river of importance. 



ABOUT RAINFALL IN GENERAL. 



Owing to the irregularity and uneven- 

 ness of the surface of the earth, causing 

 differences in the area and elevations of 

 watersheds, and variations in the tempera- 

 ture of the atmosphere together with the 

 varying distances between bodies of water, 

 the length, size and the volume of flow of 

 streams at different seasons of the year is 

 a matter of much uncertainty. It depends 

 upon the amount of rainfall on the area 

 tributary to the stream and its distribution 

 throughout the year either by the constant 

 recurrence of rains or the melting of ac- 

 cumulated snows. In the case of large 

 rivers, where the volume of flow is derived 

 from a large watershed having a copious 

 rainfall, the supply is probably so much 

 greater than all possible demands, that in- 

 vestigations of the rainfall are unnecessary. 

 But such rivers do not frequently exist in 

 arid regions, and the irrigation engineer 

 is generally called upon to obtain a supply 

 from streams having a limited as well as a 

 poorly and unevenly watered drainage 

 basin. 



In such cases it is necessary to proceed 

 with the utmost caution, and to carefully 

 investigate the amount and distribution of 



the rainfall throughout a sufficient number 

 of years to determine the available supply 

 of the stream. 



VALUE OF STATISTICS. 



All civilized countries have statistics of 

 the rainfall on its principal river basins 

 and watersheds and of the flow of its prin- 

 cipal streams at different seasons of the 

 year. The value of these statistics, of 

 course, depends largely upon the efficiency 

 of the service under which they are pre- 

 pared, but it is safe to recommend the re- 

 ports of the signal service and meteoro- 

 logical departments and the reports of the 

 geological survey of most countries, as a 

 source from which a large amount of reli- 

 able information can be drawn regarding 

 the capacity and discharge of the principal 

 streams of those countries. 



Statistics prepared by private individ- 

 uals and corporations, who are or have 

 been the projectors and owners of water 

 works or irrigation enterprises, may be 

 said to be equally as valuable if not more 

 so than those prepared under government 

 supervision, and when they are obtainable 

 much valuable information can be obtained 

 from them. But as a rule these only re- 

 late to watersheds or streams which have 

 already been improved or appropriated, 

 and are useful only in making examinations 

 and reports on existing works, or for 

 drawing conclusions and making estimates 

 on works projected in the immediate vicin- 

 ity. No tables of statistics relating to 

 rainfall or the areas of drainage basins of 

 streams will be reproduced in this work, 

 as this would simply be copying public rec- 

 ords, which are open to all, and would 

 occupy space which is more valuable for 

 the discussion of principles. 

 (To be continued.) 



