336 O. E. MEINZER ESTIMATING GROUND-WATER SUPPLIES 



record being kept of the quantity of water that is pumped ;^^ (5) ascer- 

 taining the volume of sediments saturated by a measured amount of 

 seepage from a natural or artificial stream; (6) making indirect de- 

 terminations in the laboratory with small samples by ascertaining the 

 moisture equivalent — that is, the percentage of water held against a 

 centrifugal force 1,000 times as great as the force of gravity ;^^ and (7) 

 making mechanical analyses of the water-bearing material and computing 

 therefrom the specific retention and specific yield.^^ 



It will be noted that the fourth and fifth methods are essentially com- 

 binations of the water-table method with the discharge and intake meth- 

 ods of determining recharge, with a shift in the factor that is the un- 

 known quantity. In the Morgan Hill area Clark calculated the average 

 specific yield by applying the laboratory results obtained by King to the 

 local deposits as revealed by 72 well logs. This calculation gave an aver- 

 age specific yield of a little over 12 per cent. Later he had opportunity 

 to obtain a check on this figure by means of an entirely independent 

 method. In a pumping test made by the Bay Cities Water Company 

 1,643 acre-feet of water were withdrawn, and, according to the records 

 of the lowering of the water level in a series of observation wells situated 

 within a few miles of the pumped wells, the total volume of sediments 

 drained was 14,195 acre-feet, giving a specific yield of 11.6 per cent as 

 against 12 per cent estimated by the other method. As Clark states in 

 his report, "the close agreement is, of course, accidental, but the fact that 

 the two methods lead to the same general result is probably significant." 



In the highly developed San Bernardino Valley, in California, Men- 

 denhalP^ used a skillful modification of the ordinary water-table method 

 in which he eliminated specific yield as a factor. He obtained data on 

 the total annual pumpage and also found that in years having about 2% 

 inches more of precipitation than the average there was no net rise or 

 fall of the water-table. On the assumption that the recharge varies in 

 proportion to the precipitation, he calculated that about 115 second-feet 

 may be withdrawn without permanently reducing the reserve supply. 

 Such a method is, of course, applicable only in areas having extensive 

 pumping developments. 



^<> W. O. Clark : Ground-water for irrigation in the Morgan Hill area, California. 

 U. S. Geol. Survey Water-supply Paper 400 e, 1917, pp. 84-86. 



" L. J. Briggs and J. W. McLane : The moisture equivalent of soils. U. S. Dept. Agr. 

 Bur. Soils Bun. 45, 1907, p. 22. 



" L. J. Briggs and H. L. Shantz : The wilting coefficient for different plants and its 

 indirect determination. U. S. Dept. Agr. Bur. Plant Industry Bull. 230, pp. 68 and 73. 



13 W. C. Mendenhall : The hydrology of San Bernardino Valley, California. U. S. 

 Geol. Survey Water-supply Paper 142, 1905, pp. 56-65. 



