K value with respect to pore size and water content

Hey guys,

Just wondering if anyone can clarify the relationship between water content and pore size and the value of K. Still a bit confused with this. Thanks!

I think - when pore size increases, more water can flow rapidly through the large pores, so hydraulic conductivity (K) is high. SO sand has higher hydraulic conductivity than clay initially (until all water drains out then pores are filled with air, then sand will have a lower K than clay) - when there is more water (saturated soil), pores are filled with water so K is high. If pores are filled with air (unsaturated) then it's harder for water to flow through so K in that case will be low

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.