476 Thirty-first Annual Report 



JRRIGATION BY FLOODING AND THE EFFICIENCY OF 



IRRIGATION 

 A monograph entitled "Irrigation by Flooding and the Effi- 

 ciency of Irrigation," based on the experience of this Departmen: 

 during the past fifteen years, was prepared and read at a conven- 

 tion of the Associated Concrete Pipe Manufacturers of Southern 

 California at Ocean Park, California, in September, 1920. The 

 purpose oi the monograph is to promote better judgment and 

 economy in grading land for irrigation, and economy in the use 

 of water, particularly limited and expensive irrigation water sup- 

 plies. Reprints are available for distribution by the Agricultural 

 Experiment v*^tation. 



SILT CONTENT STUDIES OF GILA RIVER WATER 

 In 1917, silt studies of the waters of the Gila River were 

 initiated by the U. S. Indian Service, and over eleven hundred 

 samples of river water were collected. The samples were so 

 distributed as to show the silt content at six locations on the 

 main stream from Duncan to Kelvin, and at Clifton on the San 

 Francisco tributary. The period of sampling extended over nine 

 months. After correspondence with Federal officials, this Depart- 

 ment obtained possession of the samples in January, 1920. Since 

 then the samples have been analyzed for silt content and for soluble 

 solids, and the silt records at Winkleman have been combined with 

 the stream floAv records in such manner as to show the acre-feet of 

 silt each day that would have been deposited in the San Carlos 

 Reservoir if the reservoir had lH.'cn in use. The total amount ol 

 silt carried by the river in the period was 196.5 acre-feet, which 

 was 0.3 percent of the river discharge for the period. 



A report on these studies has been prepared and a copy was 

 presented to the U. S. Reclamation Service, which is now for the 

 second time investigating the problems of storage, regulation, an 1 

 use of the Gila River waters. 



CASA GRANDE VALLEY 

 Groundwater development increased at a greater rate this year 

 than during any previous year. The acreage under pump irriga- 

 tion, or partial pump irrigation, this year was 9600, as against 5200 

 for 1919. The amount of water pumped is estimated as in direct 

 proportion to the acreages, since there was little rain during the 

 growing season and the increase was planted to cotton. This crop 

 takes less water than alfalfa and on many ranches it replaced it. 

 The volume of water pumped is estimated at about 12,500 acre- 



