332 O. E. MEINZER ESTIMATING GROUND-WATER SUPPLIES 



As methods dealing with intake give no information as to the unavoid- 

 able losses from the gronnd-water reservoirs^ the results obtained by these 

 methods must be used with caution where the question of safe yield is 

 involved. 



DISCHARGE METHODS 



Water is discharged from the zone of saturation by two very distinct 

 processes — by the flow of springs and by evaporation from soil and vege- 

 tation. To estimate the discharge by these diverse processes, entirely 

 different methods are required. 



Most areas yield two kinds of run-off: direct run-off, which is water 

 that has not been beneath the surface since it fell as rain or snow, and 

 ground-water run-off, which is derived either from definite springs or 

 from general seepage along effluent streams. Commonly the two kinds of 

 run-off are mingled, and neither can be measured separately. However, 

 we know that in times of flood the extra water is nearly all direct run- 

 off, and that in the intervals between storms the direct run-off diminishes 

 and the streams carry a constantly increasing proportion of water de- 

 rived from subterranean sources. It is doubtless possible to reach a 

 fairly close approximation as to tlie ground-water run-off of a basin by 

 studying the fluctuations of stream discharge as shown in a hydrograph, 

 and making correlative observations on precipitation, melting of snow, 

 capacity and yield of ponds, lakes, and reservoirs,- total quantity of 

 stream water in the basin at specific stages, turbidity of the stream water, 

 chemical composition of stream water as compared with ground-water, 

 fluctuations of water levels in wells, and fluctuations in the flow of 

 springs that are available for measurement. Such a study is at present 

 being made by A. J. Ellis of the Pomperaug drainage basin, in Connecti- 

 cut. This method is more generally applicable in humid than in arid 

 regions, and hence is not of as much practical value as some other meth- 

 ods. It does not, of course, give total discharge of ground-water, for it 

 takes no account of the discharge from the zone of saturation through 

 plants and evaporation from soil, which in humid regions is very difficult 

 to estimate, though certainly large. 



Where the zone of saturation is near the surface, ground-water may be 

 discharged either through capillary rise and evaporation from the soil or 

 through absorption by the roots of plants and subsequent transpiration. 

 The height to which water in perceptible quantities will rise above the 

 water-table by capillarity depends on the texture of the deposit. It is 

 greater in fine-grained than in coarse-grained materials, but is not com- 

 monly as much as 10 feet. 



