This presentation by ACWADAM on hydraulic conductivity deals with the hydrologic properties of rocks that signify the status of a rock as a good or poor aquifer. The status of a rock with regard to groundwater occurrence and movement is decided by its porosity (specific yield, more practically) and its hydraulic conductivity. These properties decide whether a rock can store and transmit groundwater.

Hydraulic conductivity is also commonly called as permeability of the rock or rock material and is its ability to allow the flow of groundwater through it. A rock with good hydraulic conductivity allows groundwater to easily flow through it.  

The presentation describes Darcy’s Apparatus, which comprises of a glass cylinder filled with porous sand. The glass cylinder with the sand bed had an inlet for inflow of water and an outlet for outflow of water. Two vertical glass tubes (manometers) are fitted in the sand tube placed at a fixed distance from each other to measure the respective levels (hydraulic heads).

Darcy’s experiment led to the formulation of Darcy’s Law, which states that the rate of flow of water through the porous sand bed is directly proportional to the hydraulic head difference (h) between the two points and inversely proportional to the distance between the two points (l).

v = k (h/l); where v = velocity of water, h = hydraulic head difference between the two points, l = distance between the two points and k is the coefficient of permeability, now more popularly called hydraulic conductivity.

Hydraulic conductivity depends upon the characteristics of the medium, i.e. rocks or rock material. It also depends upon the characteristics of the water, although these are fairly constant. Hence, different rocks possess different hydraulic conductivities and the presentation discusses the hydraulic conductivity of unconsolidated sediments, of sediments & sedimentary rocks and wells tapping hard rocks. 

This presentation is part of the training modules on planning, development and management of groundwater with special reference to watershed management programmes by ACWADAM. Please write to ACWADAM at acwadam@vsnl.net for sourcing these presentations.