WATER-TABLE METHODS 335 



parts of the area from 10 to 45 feet. The available pore space was esti- 

 mated at about 12 per cent. This gave a total recharge of 34,000 acre- 

 feet, exclusive of loss during the period of rise. 



The most uncertain factor in this method is the percentage of available 

 pore space — that is, the specific yield, sometimes called the effective 

 porosity. There is urgent need for the development of practical methods 

 of determining this quantity. It is well known that the specific yield of 

 a deposit may be very different from its total water content or porosity. 

 In clean gravel, which gives up nearly all of its water by gravity, tlie two 

 are nearly the same, but silt and clay may have high porosities and yet 

 yield little or no water. What is needed is determinations not of porosity, 

 but of specific yield. Reliable determinations of specific yield can not be 

 made by draining small samples, because, as is well known, the amount 

 of water that drains per unit volume from a small sample is less than 

 the amount that drains per unit volume from a large body of the same 

 kind of material. The principle involved could be illustrated by means 

 of a very long capillary tube filled with water. If such a tube were held 

 upright, it would drain down to the level determined by its capillary 

 range, but if it were broken up into small pieces each piece would hold 

 all of its water. 



Although no methods for determining specific retention and specific 

 yield have been well developed and standardized, examination of the lit- 

 erature on the subject shows that at least seven different methods for the 

 determination of these quantities have been suggested. I can not take 

 time here to discuss these methods. Briefly stated, tliey consist of the 

 following procedures: (1) Draining high columns of saturated materials 

 in the laboratory;^ (2) saturating in the field a considerable body of 

 material situated above the water-table and above tlie capillary fringe, 

 allowing it to drain while reasonably protected from evaporation, and 

 then determining its water content and porosity;^ (3) collecting samples 

 immediately above the capillary fringe of the water-table after the water- 

 table has gone down an appreciable distance, as it commonly does in 

 summer and autumn, and determining its water content and porovsity;^ 

 (4) ascertaining the volume of sediments drained by heavy pumping, a 



' F, H. King : Principles and conditions of the movements of ground-water. TJ. S. 

 Geol. Survey, 19th Ann. Kept., part 2, 1899, pp. 86-91. 



* F. J. Alway and G. R. McDole : Relation of the water-retaining capacity of a soil to 

 the hygroscopic coefficient. TJ. S. Dept. Agr., .Tour, of Agr. Research, vol. 9, 1917, pp. 

 27-71. 



» A. T. Ellis and C. H. Lee: Geology and ground-waters of the western part of San 

 Diego roiinty. ralifornia. TJ. S. Geol. Survey Water-supply Paper 446, 1919, pp. 122 

 and 12.'?. This method was applied hy C. H. I^ee. 



XXTII— Bull. Geol. Soc. A>f., Von. 31, 1919 



