The original units represent volume (quantity) discharged (i.e., collected and measured) through a cross-sectional area per unit time. Flux, however, is not the distance water travels per unit time as the simplification suggests. The flux is the rate of water discharged by the hose, divided by the cross-sectional area of the hose (e.g., gal/hr in 2 or in 3/hr in 2 = in/hr).įlux is commonly expressed on a volume basis (e.g., m 3/m 2 s), which simplifies to a velocity unit (m/s). Where J is the quantity of water ( Q) moving through a cross-sectional area ( A) per unit of time ( t) (figure 1).įigure 1 (Click for higher resolution image)įigure 1.-Water flux ( J) is the quantity of water ( Q) moving through a cross-sectional area ( A) per unit of time ( t).įlux can be thought of as water flowing from a hose. The dimensions assigned to flux and hydraulic gradient determine those of saturated hydraulic conductivity. Hydraulic conductivity ( K) is the constant that defines the proportionate relationship of flux to hydraulic gradient. For simplicity, the minus sign is omitted in the remaining discussion.ĭarcy’s law demonstrates that flux ( J) is proportional to the hydraulic gradient ( i). The minus sign keeps K positive and maintains directional integrity hydraulic gradient always decreases in the direction of water flow.
Where J is water flux (or flow of water), K is hydraulic conductivity, and i is hydraulic gradient. Movement of Water Through Soil Under Saturated Conditionsĭarcy’s law quantitatively describes one-dimensional water flow in saturated soil and is written as: The second section describes the history of the permeability classes and the transition to the current Ks classes used by the National Cooperative Soil Survey.
A discussion is included on how Ks relates to permeability. The first section of this note reviews the concepts of soil water movement (using Darcy’s law) under primarily saturated conditions with an emphasis on saturated hydraulic conductivity ( Ks). Saturated Hydraulic Conductivity: Water Movement Concepts and Class History