119 

 OVENS V.AT.t.f:y, CALIFORNIA 



Closed Basins 



Determination of the safe yield from closed rock basins is 

 of considerable importance to sections of the Southwest where re- 

 liance is placed upon ground water as the principal source of sup- 

 ply. The recharge into the basin results from percolation from 

 stream flow and from precipitation. The normal loss is from evap- 

 oration from water surfaces and moist areas, transpiration from 

 vegetation, and surface and underflow from the basin. Underflow 

 is generally a slow movement through a limited cross section of 

 alluvial material and may sometimes be omitted from consideration. 



Under natural conditions the recharge and discharge will be 

 about evenly balanced over a long period of time. In times of 

 drought the moist area of the basin will contract owing to lower 

 ground water, and in periods of above-normal precipitation it will 

 expand and there will be increased flow out of the basin. 



Under natural conditions the discharge by evaporation and 

 transpiration may be considered as the theoretical yield which may 

 be pumped from the basin without greatly changing ground-water 

 levels. In actual practice, however, the safe yield is less than 

 the theoretical owing to loss of water by plant use in low areas 

 and evaporation of moisture from the soil surface. 



Measurement of the quantity of water which may be safely 

 extracted from a basin of the closed alluvial type may be arrived 

 at through estimating the natural losses resulting from evapora- 

 tion and from consumptive use by natural vegetation. The ground- 

 water discharge by plants applied to areas of known depth to water 

 will provide a measure of quantities recoverable for other uses. 



The pioneer work of this nature by Lee (16, 1?) in deter- 

 mining the safe yield of water in the Owens Valley, Calif. , prior 

 to the construction of the Los Angeles aqueduct, opened the way 

 for other investigations described elsewhere in this report. 



