GC: n

CT: Darcy’s Law is an empirical relationship for liquid flow through a porous medium. A common application is groundwater flow through an aquifer. Darcy’s Law gives the relationship among the flow rate of the groundwater, the cross-sectional area of the aquifer perpendicular to the flow, the hydraulic gradient, and the hydraulic conductivity of the aquifer.

S: http://www.brighthubengineering.com/hydraulics-civil-engineering/58490-darcys-law-for-modeling-groundwater-flow/ (last access: 21 December 2014)

N: 1. hydraulic (adj): c.1600, from Greek hydraulikos organon “water organ,” from hydr-, stem of hydor “water” + aulos “musical instrument, hollow tube”. Extended by the Romans to other kinds of water engines.
gradient (n): “steep slope of a road or railroad,” 1835, principally in American English, from grade (n.) by analogy of quotient, etc. It was used 17c. as an adjective, of animals, “characterized by walking;” in that case probably from Latin gradientem, present participle of gradi “to walk.”
2. In porous media: measure of the decrease in head per unit distance in the direction of flow.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=hydraulic&searchmode=none; http://www.etymonline.com/index.php?allowed_in_frame=0&search=gradient&searchmode=none (last access: 21 December 2014). 2. TERMIUM PLUS – https://bit.ly/2PiDgw3 (last access: 21 December 2014).

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CR: geothermal gradient, gradient, hydraulic energy, ocean’s thermal gradient.