202 



THE IRRIGATION AGE. 



creek is of sufficient volume this reservoir must be 

 filled. This condition is now doubtless fulfilled, and 

 more than fulfilled, in each average year of rainfall. 



"In years of large rainfall the runoff would be 

 two or three times as much as the reservoir capacity, 

 while in a year of say six and one half inches of rain- 

 fall the contribution to the reservoir must be small. 

 With the invisible escape at the gorge prevented, and 

 but for the occurrence of the occasional drought year, 

 there would be in each April 20,000,000,000 gallons of 

 water available for the next year, equal to 54,000,000 

 gallons of water daily, being three times the amount 

 of water furnished to San Francisco daily. But in 

 order to maintain an even delivery in every year it 

 would be necessary to leave in the reservoir in the 

 spring water enough for thirteen months namely, 

 from the spring of one year to the autumn of the 

 next year, including two summers. This would limit 

 the yearly draft upon the reservoir to something be- 

 tween 10,000,000,000 and 7,000,000,000 gallons, or an 

 average of 23,000,000 gallons lor each of the twenty- 

 four hours." 



The gorge here referred to is at the lower end of the 

 gravel deposits where the hills converge toward the 

 creek. At some future day the company intends to 

 sink a bed-rock dam across the gorge to prevent this 

 invisible escape of the stored waters. For the present, 

 however, the company will construct an infiltration 

 gallery along the right bank of Coyote creek, just 

 above the gorge and about 7,400 feet in length. It 

 will be an open cut, ten feet wide on the bottom with 

 slopes of one to one. At its upper end the grade of 

 the bottom will be 16 feet below the level of the 

 ground-water in the gravel stratum at its lowest stage. 

 From comparisons made with similar galleries in the 

 East and in Europe it is estimated that the daily 



yield in the dry season will reach twenty-five million, 

 gallons. During the rainy season the yield will, of 

 course, be indefinitely augmented. 



From the gallery the water will be carried along 

 the foot-hills in a ditch about eight miles in length^ 

 where the hills merge into the valley. Thence the 

 water will be conducted in plate-iron pipes to all 

 points in the valley where irrigation will be re- 

 quired. 



The elevation of the gallery above sea-level is 225 

 feet, while the average elevation of the main valley 

 is less than 120 feet, thus furnishing ample pressure 

 in the distributing pipes to flow the water by gravita- 

 tion to all desired localities. Eventually it is pro- 

 posed to bring a portion of the water, say four or five 

 million gallons per day, to the City of San Jose, the 

 present supply being quite inadequate, and in the 

 dry season of objectionable quality. The elevation of 

 that city being only about 90 feet above tide, this 

 supply would likewise be furnished by gravitation, 

 all pumping being dispensed with. 



Contracts have been made with a large number of 

 orchards for a term of thirty years. The company 

 agrees to furnish 163,000 gallons per acre for the 

 season, which is equivalent to a rainfall of six inches 

 at seven dollars per acre per annum. The w< rk on 

 the gallery is now under way and it is expected that 

 the works will be in condition to supply the orchards 

 early in the coming spring. The writer has been 

 selected as the ( hief Engineer and Col. E. G. 

 Wheeler as General Manager. The aggregate cost 

 of the complete works has been estimated at about a 

 half million dollars. As the work progresses, or as 

 soon as definite results as to the yield are obtained, I 

 shall be pleased to communicate them to your valu- 

 able magazine. 



MEASUREMENT AND DUTY OF WATER. 



BY A. M. RYON, E. M. 



IT is a matter of considerable importance to know 

 just how much water a given tract of land will 

 require, and statistics on this subject are eagerly 

 sought for by irrigation canal projectors, as well as by 

 the consumers, who pay so much per year for a given 

 quantity of water. 



The amount of irrigation which a given amount of 

 water will accomplish determines the duty of water. 



We express the duty of water by stating the total 

 depth of water applied to a given tract during the 

 season; or by staling the number of acres which a 

 constant flow of a given qxiantity of water will irri- 

 gate. A flow of one cubic foot (7.48 gallons) of water 

 per second will very nearly cover two acres of ground 

 to a depth of one foot in a day of twenty-four hours. 

 As ordinarily expressed we would say that two acres 

 of ground covered with water one foot in depth would 

 require a second foot of water flowing twenty-four 

 hours, or one-half a second foot flowing forty-eight 

 hours. 



We find a great variation in the reported duties 



from different sections, ranging from 2,000 acres for a 

 flow of one cubic foot a second (about forty Montana 

 statutory inches) to a depth of 200 feet per year. 



In the United States the usual answer to an inquiry 

 concerning the amount of water used for irrigation 

 per acre is that one inch is required. 



Notwithstanding these reports it is well understood 

 that the amount of water actually needed varies with 

 many conditions, including the nature of the crop, 

 method of irrigation, character of the soil and sub- 

 soil, amount of rainfall, slope of the ground, climate, 

 skill of the irrigator and the number of previous 

 irrigations. 



As all the irrigating is not done at precisely the 

 game time, it is important to know how much water 

 will be required for a given district during the irri- 

 gating season. It is quite possible that the rate of 

 flow of a stream may be insufficient, providing all 

 irrigators drawing water from it attempt to irrigate at 

 the same time, and yet be ample where rotation is 

 practiced. Our system of prior rights regulates this 



