Which property is defined by Sy = Vw drained / total volume ×100%?

• A. Specific Yield
• B. Specific Retention
• C. Porosity
• D. Permeability

Answer: (A)

This is about how much water a saturated soil or rock can lose due to gravity per unit bulk volume. Specific yield is defined as the volume of water that drains from the material by gravity divided by the total volume, multiplied by 100 percent. It represents the portion of the pore water that is readily available as it drains downward, rather than water held by capillary forces.

This differs from porosity, which is simply the total pore space as a fraction of the bulk volume—water that could be present if the material were fully saturated. It also differs from specific retention, which is the water that remains in the pores against gravity after drainage has ceased due to capillary forces. Permeability, meanwhile, describes how easily water can flow through the material under a hydraulic gradient, not how much water can be drained under gravity.

For example, if a 100 cm^3 sample loses 25 cm^3 of water when allowed to drain by gravity, the specific yield is 25%.