Specific Yield
| Material | Specific Yield (%) | ||
|---|---|---|---|
| min | avg | max | |
| Unconsolidated deposits | |||
| Clay | 0 | 2 | 5 |
| Sandy clay (mud) | 3 | 7 | 12 |
| Silt | 3 | 18 | 19 |
| Fine sand | 10 | 21 | 28 |
| Medium sand | 15 | 26 | 32 |
| Coarse sand | 20 | 27 | 35 |
| Gravelly sand | 20 | 25 | 35 |
| Fine gravel | 21 | 25 | 35 |
| Medium gravel | 13 | 23 | 26 |
| Coarse gravel | 12 | 22 | 26 |
| Consolidated deposits | |||
| Fine-grained sandstone | 21 | ||
| Medium-grained sandstone | 27 | ||
| Limestone | 14 | ||
| Schist | 26 | ||
| Siltstone | 12 | ||
| Tuff | 21 | ||
| Other deposits | |||
| Dune sand | 38 | ||
| Loess | 18 | ||
| Peat | 44 | ||
| Till, predominantly silt | 6 | ||
| Till, predominantly sand | 16 | ||
| Till, predominantly gravel | 16 | ||
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
- is the volume of water drained, and
- is the total rock or material volume
It is primarily used for unconfined aquifers, since the elastic storage component, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow.
Read more about this topic: Specific Storage
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