The sum of specific yield and specific retention is the total porosity. Grain surface area - as grainsize decreases, total surface area increases, leading to smaller specific yields at smaller grain sizes. The highest specific yields come from coarse sands and fine gravels. The lowest specific yields come from clays, which have small particles and voidspaces with large surface areas. Permeability Permeability describes of how easily water is able to move through rock.
Permeability is related to the connectedness of the void spaces and to the grain size of the rock.
Obviously, a rock could be extremely porous, but if each pore was isolated from the others, the rock would be impermeable and thus make a lousy aquifer. Often volcanic rocks will have many vessicles, but the vessicles will be isolated, rendering the rock impermeable. Grain size affects permeability in a mannor similar to the way that it effects specific yield. The thin film of water that clings to the surface of particles is tenaceous. If voidspaces are large, then additional water can easily move past the water coated particle surfaces .