Aquifer Transmissivity Calculator

Transmissivity is the volume of water an aquifer transmits through a vertical strip of unit width, spanning the full saturated thickness, under a unit hydraulic gradient. It is the product of hydraulic conductivity K and saturated thickness b, so it collapses two properties into the single parameter that actually governs well performance: a thin but very permeable gravel and a thick but sluggish silt can share the same T and deliver similar yields. Multiplying a value in square metres per second by 86,400 converts it to square metres per day, and the older United States unit of gallons per day per foot is about 80.5 times the value in square metres per day. Practitioners commonly rate an aquifer as highly productive above 500 square metres per day, moderate between 50 and 500, low between 5 and 50, and marginal below 5, with alluvial gravels often in the hundreds and fractured crystalline rock frequently below 10. The simple product K times b strictly applies to a confined aquifer of uniform thickness. In an unconfined aquifer the saturated thickness itself falls as pumping proceeds, so transmissivity is not constant during a test and drawdown must be corrected before analysis. The Thiem option here assumes steady-state radial flow to a fully penetrating well in a homogeneous aquifer and uses an assumed borehole radius, which makes the result sensitive to the poorly constrained radius of influence appearing inside a logarithm. Real aquifers are anisotropic and layered, so a single number represents a bulk average over whatever volume the test stressed.

The mathematics of flow toward a well began with Gunther Thiem, whose 1906 work in Germany produced the steady-state equation still bearing his name, relating discharge to the logarithm of the ratio of distances between two observation wells. The decisive advance came in 1935 when Charles Vernon Theis of the United States Geological Survey published a non-equilibrium solution for transient drawdown, drawing on the mathematical analogy with heat conduction in a solid that his colleague, the mathematician Clarence Lubin, pointed out to him. Theis introduced the coefficient of transmissibility, expressed in gallons per day per foot, together with the storage coefficient, giving hydrogeology its two fundamental aquifer parameters. Field practice became far more usable in 1946 when Hilton Hazen Cooper and Charles E. Jacob published a semi-logarithmic straight-line approximation to the Theis solution, allowing transmissivity to be read from the slope of drawdown plotted against the logarithm of time. As the discipline adopted metric units the older transmissibility terminology was retired in favour of transmissivity with dimensions of length squared per time, a convention set out in Stanley Lohman's Ground-Water Hydraulics, published by the Geological Survey in 1972. Compilations such as the pumping test manual issued by Kruseman and de Ridder through the International Institute for Land Reclamation and Improvement extended the family of solutions to leaky, partially penetrating and unconfined conditions, and those analytical methods remain the reference against which numerical models are calibrated.