Units in groundwater transmissivity calculator: cm=centimeter, ft=foot, gpd=U.S. gallon/day, gph=U.S. gallon/hr, gpm=U.S. gallon/min, gal=gallon (U.S.), hr=hour, km=kilometer, m=meter, min=minute, s=second.
Groundwater Drawing and Transmissivity Equation
Unconfined Aquifer: T = K h
Confined Aquifer: T = K b
Aquifer transmissivity is a useful parameter in groundwater flow modeling.
Transmissivity includes the aquifer hydraulic conductivity which is a property of the aquifer.
Hydraulic conductivity is a function of the fluid moving through the porous medium (i.e. aquifer, formation)
and the permeability of the formation. Many experiments over many years
have been conducted of water flowing through various porous media.
Hydraulic conductivities determined from many experiments have been tabulated.
The table below is typical. In viewing the table, it is apparent that hydraulic
conductivity values cover a wide range even for a given formation. If field or
laboratory test results of hydraulic conductivity are not available for your
particular formation, then use of published values like the ones below are helpful guides.
In addition to hydraulic conductivity, transmissivity is a function of aquifer thickness.
In unconfined aquifers, the aquifer thickness is a height h. h is the water depth above
an impermeable boundary. If a well is pumped, h can vary with time and location.
In confined aquifers, the aquifer thickness b is independent of any effects of pumping since
the aquifer is bound between two impermeable layers. Water in a confined aquifer is under pressure between
the layers. If the water level drops below the upper impermeable layer, then the aquifer
would no longer be confined.
Hydraulic Conductivity Table (Freeze and Cherry, 1979, p. 29)
The values in the drop-down menu in our transmissivity calculator are typical numbers
within the ranges given below for clayey, silty, sandy, and gravelly soil.
||Hydraulic Conductivity (m/s)
|Unweathered Marine Clay
||10-12 - 10-9
||10-12 - 10-6
||10-9 - 10-5
||10-7 - 10-3
||10-6 - 10-2
||10-3 - 1
|Unfractured Metamorphic and Igneous Rocks
||10-14 - 10-10
||10-13 - 10-9
||10-10 - 10-6
|Limestone and dolomite
||10-9 - 10-6
|Fractured Igneous and Metamorphic Rocks
||10-8 - 10-4
||10-7 - 10-2
||10-6 - 10-2
Notation (SI units are shown. Transmissivity calculator allows variety of units)
b = Confined aquifer thickness (m).
h = Height of water table in unconfined aquifer (m).
K = Aquifer hydraulic conductivity (m/s).
T = Aquifer tranmssivity (m2/s).
Freeze, R. A. and Cherry, J. A. 1979. Groundwater. Prentice-Hall, Inc.
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