On the Reliability of Variable-Rate Pumping Test Results: Sensitivity to Information Content of the Recorded Data

Pumping tests are widely used to estimate parameters such as transmissivity and storativity, using aquifer response equations that assume a time-constant pumping rate. However, in actual practice the discharge rate will often vary erratically and follow a generally decreasing trend as the test proceeds. In such cases, if the discharge history is recorded with sufficient temporal fidelity, accurate solutions can be obtained via time domain piecewise-linear numerical integration. However, if the discharge data do not adequately characterize the variability in the dynamics of the pumping rate, the result can be information loss leading to bias in the inferred parameter estimates. Here, we investigate the severity of this problem for six selected aquifer types, including those that are confined, leaky, and unconfined. Our results indicate that the effects of information loss due to inadequate temporal resolution of the discharge data and systematic observational error are significantly more severe than due to random observational error. The implication is that operators should make a concerted effort to record the pumping rate at the highest practical temporal resolution throughout the duration of the test.