K value with respect to pore size and water content

pretty good answer Melody!

Conor, think about the example in the lecture notes of 4 small pores which have the same total area as one large pore - which will have a higher K?

Answer: the one large pore. As water moves through a pipe (or soil pore) there will be friction along the sides of the pores, thus the velocity of water moving through the large pore will be higher (less friction or drag).

But as Melody indicated, once those large pores drain (e.g. in a sand) and become air filled, the loose their ability to conduct and K drops exponentially. In contrast a finer textured soil will still have significant water fill porosity, and these water filled pores will continue conducting even at low matric potential.