Saturated hydraulic conductivity cannot be used to describe water movement under unsaturated conditions. Ksat is the reciprocal of the resistance of soil to water movement. As the resistance increases, the hydraulic conductivity decreases. Resistance to water movement in saturated soil is primarily a function of the arrangement and size distribution of pores. As illustrated by Poiseuille's law, the resistance to flow in a tube varies as the square of the radius.
Hydraulic conductivity is a highly variable soil property.
Measured values easily may vary by 10-fold or more for a particular soil series. Values measured on soil samples taken within centimeters of one another may vary by 10-fold or more. In addition, measured hydraulic conductivity values for a soil may vary dramatically with the method used for measurement. Laboratory determined values rarely agree with field measurements, the differences often being on the order of 100-fold or more. Field methods generally are more reliable than laboratory methods.