california's water supply for 



Irrigation 



5. Q. BENNETT, Engineer United State* Geological Survey 



The proper solution of the irrigation question is the greatest economic problem 

 now before the people of California. The water supply is the most important factor 

 in the solution of this problem. The Hydrographic Branch of the United States 

 Geological Survey has been making a study of the water supply in this State since 

 1895. On July 1, 1903, the State began to co-operate with the Federal Government in 

 this work. There are now seventy-two gaging stations being maintained. The data 

 being accumulated at each of these points has its specific value in connection with the 

 future development of the magnificent resources of the State. The information will 

 be invaluable in designing and making estimates of cost for storage, irrigation, power 

 and drainage works, and for use in litigation. Its greatest value will be in determin- 

 ing the water supply available for storage and irrigation purposes. 



Before giving some of the results of these stream measurements, an explanation 

 will be made of the units used. The term "second-feet" (sec.-ft.) is an abbreviation 

 for cubic feet per second. It is the number of cubic feet of water flowing by the gag- 

 ing station every second. A continuous flow of one second-foot of water will cover one 

 acre of land two feet deep every twenty-four hours. An "acre-foot" is the quantity 

 of water it would take to cover an acre to the depth of one foot, or it is 43,560 cubic 

 feet of water. This quantity is used in making estimates for irrigation projects, for 

 giving the annual run-off of streams and in expressing the storage capacity of reser- 

 voirs. This is also a convenient term for use in stating the duty of water, or the depth 

 of water necessary to irrigate an acre of land during an irrigation season. For instance, 

 if it is known that two acre-feet are required per acre in one irrigation season in a 

 certain locality, and the capacity of the storage reservoirs in that vicinity is stated 

 in acre-feet, the amount of land that can be irrigated from the reservoirs is readily 

 obtained. 



The principal part of the flow of the streams tributary to the Sacramento occurs 

 during the winter or rainy season, when the water is not needed for irrigation. This 

 is true in a less degree of the streams tributary to the San Joaquin River. The crest 

 of the Sierra Nevada, where these streams have their source, is generally higher than 

 that part of the range where the tributaries of the Sacramento River rise, and more 

 of the precipitation is in the form of snow. This snow is melted in the spring and 

 early summer, and sustains the flow of the streams when the water is most needed for 

 irrigation. The maximum flow of the Colorado River comes nearer coinciding with the 

 season of greatest need for irrigation than any other stream in California. 



These facts will be illustrated by the following table: 



Table Showing the Four Months of Greatest Run-off for Six California Streams and 

 the Percentage this bears to Total Annual Run-off. 



The run-off from the watershed of the Sacramento River for the months of April, 

 May, June and July is less than 28 per cent of the total for the year. The above table 

 shows that the rivers of the San Joaquin Valley are better adapted for irrigation pur- 

 poses than those of the Sacramento Valley. The latter must be supplemented to a 

 great extent by storage reservoirs. Fortunately, nature has been more liberal in pro- 

 viding natural basins suitable for convertion into storage reservoirs in the Sacramento 

 drainage area than in the San Joaquin. The Geological Survey has been surveying these 

 reservoir sites, and to date has surveyed sites in this basin that have a total storage 

 capacity of 5,002,700 acre-feet. Stream gagings are being made at practically all of the 



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