INTAKE METHODS 331 



or by evaporation from soil and plants; the third, in observing the fluc- 

 tuations in the water-table, which represent filling or emptying of the 

 ground-water reservoir; the fourth, like the gaging of surface streams, 

 in measuring the flow of ground-water at selected cross-sections. 



INTAKE METHODS 



Intake methods are applicable chiefly with respect to influent streams 

 in arid regions, especially the mountain streams that flow out over grav- 

 elly alluvial fans, where they lose water rapidly. 



If nearly the entire supply comes from one stream, as in the San Jose 

 district of Santa Clara Valley, California,^ the recharge can be ascer- 

 tained by establishing a gaging station where the stream enters the valley, 

 and one or more gaging stations farther do^vnstream. 



In an area such as Big Smoky Valley, Nevada,^ however, which re- 

 ceives 54 perennial mountain streams and the flood discharge of innu- 

 merable dry canyons, the problem is more complex. In the latter part 

 of April, 1915, measurements showed that the discharge of 33 of the 

 streams entering Big Smoky Valley amounted to 163 second-feet, and it 

 was estimated that the aggregate discharge of the 54 perennial streams 

 was about 170 second-feet. Similar measurements were made in July 

 and September, and a daily record for several months was obtained on 

 one of the streams. Certain of the streams were investigated with respect 

 to seepage losses. For example, in April, Belcher Creek flowed 6.25 

 second-feet at the mouth of its canyon and 4.73 second-feet at a point 

 farther down the valley, thus losing 1.52 second-feet in the intervening 

 distance. On the basis of the evaporation area afforded by the creek and 

 the knovm rate of evaporation, the loss by evaporation was calculated to 

 be only 0.02 second-foot, leaving for ground-water recharge a flow of 

 1.50 second-feet, which is at the rate of 1,086 acre-feet a year. On the 

 basis of similar studies for other creeks and for this creek at other sea- 

 sons, it was estimated that the perennial streams contribute to the ground- 

 water supply at an average rate somewhere between 15,000 and 30,000 

 acre-feet a year. The contributions by floods from dry canyons, by 

 underflow through the gravel deposits in the canyons, by precipitation 

 in the vallciy, and by percolation from the bedrock could not well be 

 measured, though the data obtained indicates that these contributions 

 amount to several tens of thousands of acre-feet a year. 



' W. O. Clark : Ground-water for irrigation in the Morgan Hill area, California. U. S. 

 Geol. Survey Water-supply Paper 400 e, 1917. 



8 O. E. Meinzer : Geology and water resources of Big Smoky, Clayton, and Alkali 

 Spring valleys, Nevada. U. S. Geol. Survey Water-supply Paper 423, 1917. 



