tests. When calculating flow to drains or to a well field in an aquifer with the aim to control the water table, the anisotropy is to be taken into account, otherwise the result may be erroneous. Relative properties Because of their high porosity and permeability, sand and gravel aquifers have higher hydraulic conductivity than clay or unfractured granite aquifers. Sand or 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 has units with dimensions of length per time (e.g., m/s, ft/day and (gal/day)/ft² ); transmissivity then has units with dimensions of length squared per time. The following table gives some typical ranges (illustrating the many orders of magnitude which are likely) for K values. Hydraulic conductivity (K) is one of the most complex and important of the properties of aquifers in hydrogeology as the 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). Ranges of values for natural materials Table of saturated hydraulic conductivity (K) values found in nature Values are for typical fresh groundwater conditions — using standard values of viscosity and specific gravity for water at 20 °C and 1 atm. See the similar table derived from the same source for intrinsic permeability values. Source: modified from Bear, 1972 See also Aquifer test Hydraulic analogy Pedotransfer function – for estimating hydraulic conductivities given soil properties References External links Hydraulic conductivity calculator Hydrology Hydraulic engineering