What would the specific yield of an unconfined aquifer be if the porosity is 0.3 and the effective porosity is 0.18?

The specific yield of an unconfined aquifer is defined as the portion of the total water that can be drained from the aquifer due to gravity, which is typically indicated by the effective porosity. In this context, effective porosity refers to the void spaces in the rock or sediment that contribute to the movement and storage of groundwater.

In this case, with a porosity of 0.3 and an effective porosity of 0.18, the specific yield is determined by the effective porosity value, because it represents the amount of water that can be readily extracted. Therefore, the specific yield is accurately represented by the effective porosity of 0.18.

Choosing the effective porosity as the value for specific yield is consistent with geological principles, as the total porosity encompasses all voids, while effective porosity accounts for only those voids that contribute to groundwater flow. This distinction is crucial in groundwater studies, making the effective porosity the appropriate metric for specific yield in an unconfined aquifer.