Specific yield

Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:

where

V_wd is the volume of water drained, and

V_T is the total rock or material volume

	Specific Yield (%)
Material	Maximum	Minimum	Average
coarse gravel	26	12	22
medium gravel	26	13	23
fine gravel	35	21	25
gravelly sand	35	20	25
coarse sand	35	20	27
medium sand	32	15	26
fine sand	28	10	21
silt	19	3	18
sandy clay	12	3	7
clay	5	0	2
(Johnson (1967) as quoted by C.W. Fetter)