Specific Yield Calculator

Specific yield, written Sy, is the volume of water a saturated material releases by gravity drainage divided by its bulk volume. It partitions the total porosity into two parts, since Sy plus the specific retention Sr must equal the porosity n. Specific retention is water held by capillary and adsorptive forces in small pores and as films on grains, and because those forces scale with surface area rather than pore volume, fine-grained materials retain a large share. That is why clay can have a porosity of 45 to 55 percent yet a specific yield of only 1 to 5 percent, while clean sand and gravel near 30 to 35 percent porosity yield 20 to 30 percent. Silt and fine sand fall between, roughly 5 to 20 percent. In groundwater practice specific yield is the storage coefficient of an unconfined aquifer, so the volume of water released per unit area for a unit fall in the water table is essentially Sy. Values of 0.05 to 0.30 for water table aquifers are two to four orders of magnitude larger than the elastic storativity of a confined aquifer, which is typically 0.00001 to 0.001, and this contrast dominates any comparison of available storage. The main limitations are that gravity drainage is never instantaneous or complete, so a value derived from a short pumping test can be markedly lower than the long-term drainage value, an effect known as delayed yield. Laboratory column drainage is sensitive to sample disturbance, drainage time and column height, and hysteresis means wetting and drying paths give different results.

Quantitative groundwater hydrology in the United States took shape at the Geological Survey around the turn of the twentieth century, when Charles Slichter published theoretical investigations of the motion of groundwater in 1899 and began field tests of aquifer behaviour. The specific terminology used today, however, is largely the work of Oscar Edward Meinzer, who headed the Survey division of ground water from 1913 and set out a systematic vocabulary in Water-Supply Paper 494, Outline of Ground-Water Hydrology with Definitions, published in 1923. Meinzer defined specific yield and specific retention as complementary fractions of porosity and stressed that only the yielding fraction represents water that can actually be developed by wells. The distinction became operationally important once transient well hydraulics matured. Charles Theis published his non-equilibrium equation for confined aquifers in 1935, and its extension to unconfined conditions exposed a problem: drawdown data from water table aquifers did not fit a single storage value, appearing elastic at early time and gravity-controlled at late time. Norman Boulton addressed this in the 1950s and early 1960s with the theory of delayed yield, and Shlomo Neuman published a widely used solution for unconfined aquifers with delayed gravity response in the early 1970s. Meanwhile compilations of laboratory and field values, notably the Geological Survey work summarised by Allen Johnson in Water-Supply Paper 1662-D in 1967, gave engineers the tables of specific yield by material type that remain in reference use.