Discussion of “Aquifer Parameters from Drawdowns in Large-Diameter Wells: Unsteady Pumping” by Sushil K. Singh

Abstract

The author proposed the use of an optimization method for the aquifer parameter identification during a pumping large diameter well with unsteady pumping discharge. The method is specified as new with reliable parameter estimates, which cannot be acceptable because of the following points: 1. There are different methodologies for variable pumping discharge calculations as given in the manuscript by the authors as well as additional ones not cited in the paper Sen 1995; Sen and Altunkaynak 2004 . The necessary analytical expressions are derived with incorporation of the variable discharge and then aquifer performance hydraulic parameters are evaluated through a simplified straight line method. The methodology presented was the generalization of the Aron and Scott 1965 approach. Hence variations are considered between initial and final pump discharge values. 2. The author states that any pump discharge variation can be analyzed with his approach reliably, but there are many physically inconsistent imbedded ingredients in such an approach. For instance, in the “method” section is the statement, “The effective radius accounts for the variation in geometric dimensions of well and factors governing flow at the interface of the well and aquifer.” In fact, it is the horizontal distance from the axis of a well to the outside of the gravel pack or the zone of increased permeability that has been developed around the well by pumping. So the author does his own arbitrary definition of the effective well radius as if it takes into account any irregularities in the wellaquifer interface, which is not correct. Variation in geometric dimension causes variations in the well vicinity flow lines. 3. All equations cited from author’s previous publications are in physical suspect but are another mathematical versions of physically sound formulations, such as the groundwater movement equation solution in terms of well function W u in Eq. 15 and dimensionless time function as implied in other formulations. Incorporation of local slope values as in Eqs. 11 and 16 implies account of variations in the aquifer parameters Sen 1986a , whereas the author obtained single aquifer parameter values. 4. The optimization approach is convenient if there are no errors measurement, random, etc. in data; otherwise minimum objective function does not mean the best all the time. It is not mentioned in the manuscript how the data reliability is considered. The SEE values are almost equal to zero, because it is obvious from Fig. 1 that discharge variations are