Calculation of Discharge in Confined Aquifer Using Theim’s Formula for Steady Radial Flow:

Discharge through confined aquifer can be calculated from the formula

K=[ 2.303 x Q x log(r/rw) ] / [2πm (h-H2) ]

Consider that the recharge to the aquifer within the influence zone of the pumped well equals the rate of discharge of the well so that the drawdown remains stabilised and therefore steady state exists.

where

K = coefficient of permeability

m = thickness of aquifer

rw = radius of the well

T = transmissibility of aquifer = K.m

The above equation is called equilibrium or Thiem’s equation and is used to determine piezometric head at any point at a radial distance r from the centre of the well. Carrying the logic further if the piezometric heads in two observation wells say h1 and h2 at two points r1 and r2 distance radially away respectively from the centre of the pumped well are measured during the pumping test, coefficient of permeability ‘K can be easily calculated. The formula can be written as follows (r2 > r1) The above method is popularly called Thiem’s method.