Editing Aquifer (section)

=== Isotropic versus anisotropic ===
In [[Isotropy|isotropic]] aquifers or aquifer layers the hydraulic conductivity (K) is equal for flow in all directions, while in [[Anisotropy|anisotropic]] conditions it differs, notably in horizontal (Kh) and vertical (Kv) sense.

Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different (see [[Transmissibility (fluid)|hydraulic transmissivity]] and [[hydraulic conductivity#Resistance|hydraulic resistance]]).

When calculating [[drainage equation|flow to drains]]<ref>''The energy balance of groundwater flow applied to subsurface drainage in anisotropic soils by pipes or ditches with entrance resistance''. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [http://www.waterlog.info/pdf/enerart.pdf] {{Webarchive|url=https://web.archive.org/web/20090219221547/http://waterlog.info/pdf/enerart.pdf|archive-url=https://web.archive.org/web/20060522173736/http://www.waterlog.info/pdf/enerart.pdf|archive-date=2006-05-22|url-status=live|date=2009-02-19}} . Paper based on: R.J. Oosterbaan, J. Boonstra and K.V.G.K. Rao, 1996, "The energy balance of groundwater flow". Published in V.P.Singh and B.Kumar (eds.), Subsurface-Water Hydrology, pp. 153–60, Vol. 2 of Proceedings of the International Conference on Hydrology and Water Resources, New Delhi, India, 1993. Kluwer Academic Publishers, Dordrecht, The Netherlands. {{ISBN|978-0-7923-3651-8}} . On line : [http://www.waterlog.info/pdf/enerbal.pdf] . The corresponding "EnDrain" software can be downloaded from : [http://www.waterlog.info/software.htm], or from : [http://www.waterlog.info/endrain.htm]</ref> or [[drainage by wells|flow to wells]]<ref>ILRI (2000), ''Subsurface drainage by (tube)wells: Well spacing equations for fully and partially penetrating wells in uniform or layered aquifers with or without anisotropy and entrance resistance'', 9 pp. Principles used in the "WellDrain" model. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. On line : [http://www.waterlog.info/pdf/wellspac.pdf] . Download "WellDrain" software from : [http://www.waterlog.info/software.htm], or from : [http://www.waterlog.info/weldrain.htm]</ref> in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.