Transmissivity from Conductivity and Thickness

Hydraulic conductivity says how easily a material passes water, per unit of thickness. Transmissivity says how much water the whole aquifer can move, and it is nothing more than the conductivity multiplied by the saturated thickness: 25 m/d through 20 metres of clean sand is 500 m²/d. The distinction matters because a well does not care about the conductivity of a formation, it cares about the total capacity of everything the screen intersects.

The range is enormous and worth carrying in your head. Gravel runs 100 to 1000 m/d, clean sand 1 to 100, silt 0.001 to 1, and clay below 0.0001, which spans nine orders of magnitude from one end to the other. Nothing else in civil engineering varies over such a range, and it is why a groundwater estimate that is right within a factor of three is often considered good work. On the transmissivity side, anything above 500 m²/d is a productive municipal aquifer, and below about 10 m²/d you have domestic supply at best.

Two things to get right. In a layered aquifer, transmissivity is the sum of the products layer by layer, and a single half-metre gravel seam can carry more than fifty metres of the silt around it, so an average conductivity applied to the total thickness is badly misleading. And in an unconfined aquifer the saturated thickness is not a fixed property: it shrinks as the water table drops, so transmissivity falls during pumping and falls further in a dry season. That is the feedback which makes unconfined well hydraulics nonlinear and which turns a marginal well into a dry one over the course of a hot August.