490 Twenty-eighth Annual Report 



and the unit head are interdependent upon each other. Any one 

 may be taken as a function of the others, and the efficiency of irri- 

 gation is a compound function of them all. In many cases of laying 

 out new projects, the head of water has been assumed as one of the 

 independent variables, and the slope for the furrows has been made 

 to conform thereto, but probably the better way would be to run 

 the water down the natural slope and thus save in the cost of grad- 

 ing and obviate the troubles accompanying a heavy cross-slope, and 

 then regulate the length of furrows and the unit head to obtain a 

 uniform distribution of the water. 



The importance of problems in the application of water is indi- 

 cated by the fact that great diversity of practice exists through the 

 State. The Yuma and Salt River valleys use the border method 

 largely, the Upper Gila Valley uses the border method, but often 

 without the borders, the Little Colorado region practices the corru- 

 gation system as in the Northwest, while in Yavapai County the 

 Colorado method of flooding from field laterals prevails. In all of 

 these sections the water is run down the natural slope, except in 

 the Yuma Valley, wdiere it has become the custom to grade the 

 lands level. A discussion of land grading for irrigation has been 

 contributed to the technical press.* 



WATER RESOURCES 



At the time of the entrance of the United States into the world 

 war, a survey was made of the condition of irrigation water sup- 

 plies in Arizona**. On the whole conditions were found to be 

 favorable. Two years of exceptionally heavy rainfall had filled the 

 Roosevelt reservoir and had caused increased flows in all the 

 streams of the State. It was obvious that the limiting factor in the 

 increase of agricultural production in this State would not be the 

 water supply, but rather the amount of labor available. Indeed, it 

 was possible the past summer to grant temporary water to 15,000 

 additional acres in the Salt River Valley. 



Since the time of that survey, the abnormally low rainfall in 

 the important watersheds of eastern Arizona has made our water 

 supplies a matter of grave concern. Already the Upper Gila Valley 

 and northeastern Arizona have felt the water shortage and the 

 Roosevelt stored supply has been reduced 35 per cent. Attention is 

 again called to the need of storage on the Gila River. To the great 



♦Western Engineering- IX, 1. Jan., 1918. 

 **Arizona Magazine VII, 7, May, 1!»17. 



