GRAPHICAL ESTIMATION OF THE THEIS DRAWDOWN FUNCTION

Computer-based curve fitting of the Theis equation can be achieved by specifying the location of two points on a curve sketched or visualised through drawdown data from an observation well penetrating a confined aquifer. Any given pair of points generates unique estimates of storativity and transmissivity, which are calculated by locating the zero of a simple function. This approach is readily extended to observation well data when the later drawdowns are influenced by a single linear aquifer boundary (zero-flux or constant head). Specifying three points on the visualised curve allows an estimate to be made of the distance to the image well. Estimation in the presence of linear boundaries can also be achieved with drawdowns from a pumped well. Examples of the method are illustrated using drawdown data both with and without a linear boundary influence.