WATER SUPPLIES FOR IRRIGATION. 



reference to bodies of water, mountain 

 ranges, etc., also has a marked effect 

 upon climate and consequently affects the 

 crop season and time for irrigating to quite 

 a considerable degree. Again a portion 

 of the season at which crops are growing 

 may be abundantly supplied with moisture 

 by nature, while the portion which is 

 deficient in natural humidity may require 

 more or less irrigation. It therefore be- 

 comes a matter of the highest importance 

 to study conditions and definitely fix the 

 time of the irrigation season in a locality 

 for which a supply of water is being sought. 

 If this is done much time and labor will 

 be saved in analyzing and studying the 

 water question, as the investigations can 

 be confined to a stated period of the year, 

 and ignored as to the time when we are 

 aware that no water will be required. 



LENGTH OF IRRIGATION SEASON. 



While the time of the irrigation season 

 varies in different localities, the duration 

 of the period in each year when irrigation 

 is necessary varies even more. In cool 

 climates crops grow and mature very slow- 

 ly, while in warm regions their progress 

 is more rapid. As a perfectly natural con- 

 sequence to this the irrigation season in 

 one place may be of double the length 

 that it is in another. Some crops require 

 a longer period for their growth than others 

 even in the same locality, and frequently 

 two crops of different kinds are taken 

 from the same land; especially is this lat- 

 ter statement true in relation to lands ly- 

 ing in the tropical and semi-tropical zones. 



All such matters will have to be care- 

 fully studied and determined in reference 

 to any particular locality, before the ex- 

 tent of the water supply needed can be 

 fully ascertained. 



In such investigations, the doubt, if any 

 there be, should be given to make the 

 water supply safe and ample, so that no 

 error is committed for want of conservative 

 judgment and action. If different crops 

 can be grown the amount of water should 

 be figured for that particular kind of crop 

 which requires the most water and con- 

 sumes the longest time for its growth; and 

 if two crops can be raised on the same 

 land each season, enough water should be 

 provided so that this can be done if de- 

 sired. Every precaution should be taken 

 to make the water supply ample for all re- 

 quired purposes and possible future de- 



mands upon it, to the end that the lands 

 watered will be able to support the largest 

 possible population and yield the greatest 

 production. 



As will be seen later on, the length of 

 the irrigation season does not affect some 

 classes of water supplies, such as running 

 streams, where it is only necessary to de- 

 termine the minimum flow of the stream 

 during the period covered by the irriga- 

 tion season. But on the other hand it is 

 a most important consideration in some 

 cases as for instance in the case of storage 

 reservoirs, in which the water is collected 

 during one part of the year, when no irri- 

 gation is required, and expended during 

 another part of the year, i.e. during the 

 irrigation season. 



The physical conditions affecting the 

 question of the selection of a water sup- 

 ply are many, and can only be determined 

 by careful observations and accurate sur- 

 veys. They are not the same for all 

 sources of water supply and can therefore 

 be best discussed hereafter in connection 

 with the separate discussion of each mode 

 of water supply known for irrigation pur- 

 poses. 



CLASSIFICATION OF IRRIGATION WATER SUP- 

 PLIES. 



The different classes of water supplies 

 for irrigation purposes may be grouped in 

 two main divisions : (1) Gravity Supplies; 

 (2) Pumping Plants. 



A gravity supply is any water supply, 

 which has sufficient head or elevation to 

 enable the water to flow and be discharged 

 upon the land where it is to be used for 

 irrigation without the application of power 

 for raising it above its level at the point 

 of diversion. A pumping plant for irriga- 

 tion is used only where the land to be 

 watered lies at such an elevation that the 

 supply of water proposed for its irrigation 

 can not reach it without being raised by 

 means of power. 



Gravity supplies for irrigation come 

 from a number of different sources, the 

 principal of which are : (1) Flow of na- 

 tural streams. (2) Underflow of rivers 

 or creeks. (3) Storage reservoirs. (4) 

 Springs and swamp lands. (5) Artesian 

 wells. 



Pumping plants may be erected to pump 

 water from any of the above sources, when 

 the water from them is to be used on 

 lands which are higher than can be 



