The saturated hydraulic conductivity (or permeability) of the soil is a characteristics, which quantifies the ability of the soil to store the water from rainfall or snowmelt. The permeability coefficient varies with the nature of the soil and its texture. Observations of the log profile may show some redoximorphic features, which attest of variations of the groundwater level, and periods of saturation in the soil. Topography or slope gradient do affect the hydraulic conductivity of a given soil.
The graph of K versus time will usually show a fast decrease in the K value, which tend to stabilize with time, when the water infiltration-flow becomes steady.
The K value at saturation is called the Ksat or Kfs.
The time of saturation of the soil will depend of the type of soil, of the initial water content, and water temperature during the test.
A graph of the time necessary to attain a steady state flow which we call saturation phase, shows that most of the silty and sandy soils have a period of saturation not exceeding 4 hours. The sandy clays need 2 to 4 hours, when the sandy silts do not exceed 2 hours.
The clayey soils tend to have longer saturation period, more than 4 hours is not uncommon at all. The maximum time recorded in the region of Armorican massif in France, has been ...12 hours for a clayey silt.