AbstractAn approximation of the drawdown function developed by Hantush (1964) for late times was used to estimate the aquifer diffusivity directly from measured data from a constant‐head pumping test. This method of evaluation is especially useful in cases where early time data are absent or are obscured by other phenomena The use of the constant‐head time type curves allows ambiguity in the choice of the time match point because of the shallow slopes of the type curves for later times.Using the described approximation, constant‐head time type curves specific to the dimensionless distance from the pumping well may be compared with the measured data to find anomalies and bias in the measured data. A case study is presented to compare the results of the analyses with the constant‐head time type curves and with the approximation of the drawdown function. The results show that the approximation is valid after the first 100 minutes of meas