Specific Yield Compilation of Specific Yields for Various Materials

Specific yield is defined as the ratio of (1) the volume of water that a srturated rock or soil will yield by gravity to (2) the total volume of the rock or soil. Specific yield is usually expressed as a percentage. The value is not definitive, because the quantity of water that will drain by gravity depends on variables such as duration of drainage, temperature, mineral composition of the water, and various physical characteristics of the rock or soil under consideration. Values of specific yield, nevertheless, offer a convenient means by which hydrologists can estimate the water-yielding capacities of earth materials and, as such, are very useful in hydrologic studies. The present report consists mostly of direct or modified quotations from many selected reports that present and evaluate methods for determining specific yield, limitations of those methods, and results of the determinations made on a wide variety of rock and soil materials. Although no particular values are recommended in this report, a table summarizes values of specific yield, and their averages, determined for 10 rock textures. The following is an abstract of the table: Specific yields, in percent, of various materials [Rounded to nearest whole percent] Number of _______Specific yield______ Material determination* Maximum Minimum Average Clay.----..-----.---.------.-15 5 0 2 Silt_------------------_------16 19 3 8 Sandy clay....__________ 12 12 3 7 Fine sand__. ___. ______ 17 28 10 21 Medium sand____. __ ___ 17 32 15 26 Coarse sand____...__..__ 17 35 20 27 Gravelly sand......__--__--._-15 35 20 25 Fine gravel-______--._.___ 17 35 21 25 Mediumgravel -----------14 26 13 23 Coarse gravel.__.___.__.. 14 26 12 22

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