TR(BR)-130 : Identification of aquifer parameters in Narmada basin

Abstract

The reliability of ground water models (predictive model/ management model) is enhanced by rapid and reasonable identification of aquifer parameter. The important aquifer parameters are transmissivity, T and storage coefficient, 0. Pump-test occupies a prominent position among the various methods available for identification of aquifer parameters. Graphical methods of identifying the aquifer parameters have been proposed by many investigators (Theis, 1935; Cooper and Jacob. 1946: and Chow, 1952) but these methods are time consuming and involve considerable subjectivity on the part of analyst. Numerical methods that use the objective criteria for the matching of theoretical and observed drawdown have been suggested by McElwee(1980), Chander et al.(1981), Goyal(1984) and Swamee and Ojha(1990) for constant rate pumping.

Though, several investigators (Abu-Zeid and Scott, 1963; Abu-Zeid et al., 1964; and Sternberg, 1967, 68) have presented analytical and graphical methods for analysis of variable rate pumping test data, only a few numerical methods for variable rate pumping, have been proposed (Butt and McElwee; 1985). For the identification of the aquifer parameters using the drawdown observation in the pumped well, the 'well loss' and the 'effective radius' of pumped well need to be taken into account. Therefore, using only the observation data at the pumped well. instead of ø, r ø² e (re =effective radius of well) should be determined ( Jacob: 1947).

In the present report the identification of T and ø by analyzing the observed drawdown in a confined aquifer due to pumping a tube-well at a variable rate has been attempted. Aquifer parameters have been identified for the five test sites in Narmada basin. The main conclusions drawn from the study are;

1. A model has been developed for identification of aquifer parameters, T and ø, utilizing the drawdown observations in a confined aquifer due to pumping a tube well at a variable rate. Marquardt algorithm has been used with analytical derivatives for nonlinear parameter optimization.

2. Explicit expressions for T and 0 have been obtained making use of the least square regression. and the Theis equation for constant rate pumping. In this method. the well loss is accounted automatically.

3. Well loss affects only the identification Of r ø² e and does not affect the identification of T.