Which of the following best defines effective porosity or specific yield?

Effective porosity, also known as specific yield, is specifically defined as the portion of the total pore space in a saturated material that can drain under the influence of gravity. This means that it represents the amount of water that can be extracted from a saturated rock or soil when the water is allowed to drain freely, which aligns perfectly with the first option provided.

This concept is crucial in fields like hydrogeology and environmental health, as it helps in understanding groundwater availability and the behavior of aquifers. The effective porosity is an essential parameter in calculating how much water can be sustainably withdrawn from a groundwater source without depleting it.

While the other options touch upon related subjects, they do not accurately define the term. The second option addresses the flow of water but does not pertain to the concept of porosity or specific yield. The third option focuses on efficiency rather than the specific volume of water that can be drained, leading away from the precise definition needed for effective porosity. Therefore, the best definition of effective porosity is encapsulated in the first choice.