The transmissivity (T) of a confined aquifer and the hydraulic conductivity (K) of an unconfined aquifer can be calculated using the equation (16.14) and (16.30) respectively. These two equations were derived for steady state condition. It may be noted that it is difficult to obtain steady state pumping drawdown data as one has to continue the pumping for longer period. The unsteady flow data can be used to calculate both hydraulic conductivity or transmissivity and storage coefficient of an aquifer. In this lecture we will mainly discuss the estimation of aquifer parameters using unsteady flow data.

Theis method

The Theis equation can be written as

                                                                                           (22.1)

Where W(u) is the well function and u is

                                                                                                         (22.2)

                                                                                               (22.3)

In can be observed from the above relations that relation between W(u) and u must be same as S and r²/t. Using this similarity, the aquifer parameters (T,S_s ) of confined aquifer can be estimated. The method for estimation of aquifer parameters can be summarized as follows.

• •  In a logarithmic paper plot the relationship between W(u) and u. This is known as type curve.

•  From the observed time drawdown data, plot the relationship between r²/t and S on another logarithmic paper of same size.

•  The observed r²/t verses S relationship is then superimposed with the type curve in such a way that observed data fall on the segment on the type curve.

•  From the two superimposed relations, the values of W(u), u, S, and r²/t are noted corresponding to a suitable convenient point.

•  Now compute the aquifer parameters (T,S_s ) using the equations (22.2) and (22.3).