TIMELAG Order No: BAS 22

TIMELAG is a program to estimate hydraulic conductivity from single-well test. The method involves instantaneous raising or lowering of the water level in a well and measuring the water-level recovery through time. The method is based on Hvorslev (1951) and more recent extensions. The method involves fitting a line to reduced field data, using linear least-squares regression on a semilog scale. The induced change in water level must be instantaneous as the derivation of the theoretical basis requires the head in the aquifer to be constant. The well must be constructed so that neither the screen nor the filter pack inhibit ground-water movement. Also, the well should not be developed to the extent that a zone of increased permeability exists around the well. The program allows the standpipe and the well intake (screened or open portion of the well) to have different radii.

The program facilitates interactive data entry and editing, reading from and writing to a data file, graphic display and inspection of the observed timewater level data pairs, and writing results to a file for printing and inclusion in reports. The data editing gives the user the opportunity to correct the data; the graphic display of the data pairs allows the user to remove (bad) data pairs from the early and/or late portions of the curve if they do not confirm to a linear trend. The results are displayed including the regression coefficient to indicate the reliability of the analysis.


Author: D.B. Thompson (SRW Associates)