What is effective porosity or specific yield?

Effective porosity, often referred to as specific yield, is defined as the amount of water that can drain from saturated rock or soil by the force of gravity. This term specifically pertains to the portion of the voids in the material that can release water, which is essential for understanding groundwater flow and water availability.

When soil is saturated, it contains water in its voids. However, not all of that water can be drained away. Effective porosity focuses on the volume of water that can be extracted under the influence of gravity, as opposed to the total void space that might also retain water through capillary action or adhesion to soil particles. This property is critical in hydrology and is used to determine how much water can be extracted for various uses, such as irrigation and drinking water supply.

The other options describe related but distinct concepts. The total volume of voids refers to all voids present, regardless of whether they can release water. The proportion of water retained after drainage considers water that remains due to adhesion to soil particles, while the measurement of infiltration rates pertains to how quickly water moves into soil rather than the storage capacity that can be drained. Understanding effective porosity helps clarify relationships around water movement and supply in hydrological studies.