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Glossary - H - hydraulic conductivity

Hydraulic conductivity (K), is a property of soil or rock, in the vadose zone or groundwater, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation. Saturated hydraulic conductivity, K_s, describes water movement through saturated media.

Hydraulic conductivity is the proportionality constant in Darcy's law, which relates the amount of water which will flow through a unit cross-sectional area of aquifer under a unit gradient of hydraulic head. The hydraulic conductivity (k — the English letter "kay") is specific to the flow of a certain fluid (typically water, sometimes oil or air); intrinsic permeability (κ — the Greek letter "kappa") is a parameter of a porous media which is independent of the fluid. This means that, for example, k will go up if the water in a porous medium is heated (reducing the viscosity of the water), but κ will remain constant. The two are related through the following equation:

Where γ is the specific gravity of water (with units of force per volume, N/m³ or lbf/ft³), μ is the dynamic viscosity of water (with units of pascal seconds (Pa·s), poise, or lbf·s/ft²) and κ is the intrinsic permeability (units of m² or the hydrogeology unit of the darcy).

Another important aquifer parameter is transmissivity, (T) - a measure of how much water an aquifer can transmit horizontally. It depends on the aquifer's saturated hydraulic conductivity and thickness, (b):

Hydraulic conductivity (K) and transmissivity (T) are indirect aquifer properties (they cannot be measured directly). T is the K integrated over the vertical thickness (b) of the aquifer (T = K . b). These properties are measures of an aquifer's ability to transmit water. Hydraulic conductivity has units with dimensions of length per time (e.g., m/s, ft/day); transmissivity then has units with dimensions of length squared per time.

Because of their high porosity and permeability, sand and gravel aquifers have higher hydraulic conductivity than clay or unfractured granite aquifer. Sand or gravel and gravel aquifers would thus be easier to extract water from (e.g., using a pumping well) because of their high transmissivity, compared to clay or unfractured bedrock aquifers.

Hydraulic conductivity (k) is the most complex and important of the hydrogeologic aquifer properties; values found in nature:

• range over many orders of magnitude (the distribution is often considered to be lognormal),
• vary a large amount through space (sometimes considered to be randomly spatially distributed, or stochastic in nature),
• are directional (in general k is a symmetric second-rank tensor; e.g., vertical k values can be several orders of magnitude smaller than horizontal k values),
• are scale dependent (testing a m3 of aquifer will generally produce different results than a similar test on only a cm3 sample of the same aquifer),
• must be determined indirectly through field pumping tests, laboratory column flow tests or inverse computer simulation, (sometimes also from grain size analyses), and
• are very dependent (in a non-linear way) on the water content, which makes solving the unsaturated flow equation difficult. In fact, the variably saturated k for a single material varies over a wider range than the saturated k values for all types of materials (see chart below for an illustrative range of the latter).

General overview - conductivity of geological materials

Source: modified from Bear, 1972