334 O. E. MEINZER ESTIMATING GROUND-WATER SUPPLIES 



annual discharge amounted to 43 inches in depth of water where the 

 depth to the water-table was 1.3 feet, 30 inches where the depth was 2.9 

 feet, 23 inches where the depth was 4.5 feet, and 8 inches where the 

 depth was 4.9 feet. By applying these and similar results to the segment 

 of Owens Valley which is investigated, he estimated the average discharge 

 of ground-water by evaporation from soil and by transpiration from 

 native vegetation to be between 93 and 114 second-feet. 



The rate of discharge depends on three factors : ( 1 ) Evaporativity of 

 the atmosphere, (2) depth to the water-table, (3) character of soil and 

 vegetation. In the field survey both the areal variations and the seasonal 

 fluctuations in depth to the water-table must be taken into account. 

 As this method gives the surplus actually discharged by nature, it comes 

 nearer than some of the other methods to giving the quantities that can 

 be withdrawn for human use. 



WATER-TABLE METHODS 



The third group of methods, based on water-table fluctuations, are 

 especially well adapted to conditions such as those in California, where 

 the year is divided into a rainy season, when nearly all the recharge takes 

 place, and a dry season, when the heavy withdrawals are made through 

 evaporation and transpiration and by pumping for irrigation. The rise 

 of the water-table in the rainy season represents filling of the aquifer, 

 and the average annual increment to the ground-water supply can bo 

 computed by multiplying the average annual rise by the percentage of 

 available pore space and multiplying this product by the area of the 

 water-table of the given aquifer. If the rise is given in feet and the area 

 in acres, the annual increment is expressed in acre-feet. 



Good examples of the application of this method are afforded by the 

 recent work of W. 0. Clark on the Niles Cone^ and in the Morgan Hill 

 area,^ in California. The Niles Cone was under investigation during 

 1912-1913, an unusually dry year, and 1913-1914, an unusually wet year. 

 On the basis of measurements at about 125 wells, it was estimated that 

 in 1912-1913 the average rise of the water-table was less than 2 feet and 

 the total recharge, exclusive of the loss during the period of rise, was only 

 2,600 acre-feet, but that in 1913-1914 the average rise was 11.3 feet and 

 the recharge 59,000 acre-feet. In the Morgan Hill area the annual fluc- 

 tuation of the water-table in 1914 to 1916 was found to range in different 



5 W. O. Clark : Ground-water resources of the Niles Cone and adjacent areas, Califor- 

 nia. U. S. Geol. Survey Water-supply Paper 345 h, 1915. 



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

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



