Evaluation of aquifer yields with the conventional time- drawdown method is based on the assumption of Theisian or infinite radial flow (IRF) of ground water to a well. However, long-term aquifer yields are controlled by heterogeneities and boundary conditions, which lead to departures from the assumptions underlying the IRF. Accurate prediction of long-term aquifer yields therefore requires evaluation of aquifer heterogeneities. This study involves estimation of Shiraz aquifer parameters from aquifer tests in Fars province, Iran. Aquifer-test responses indicate internal heterogeneity at a scale below the resolution attainable with the available well control. Reliable estimates of aquifer parameters are obtained by applying a derivative technique to the analysis of time drawdown data. Derivative analysis allows us to identify test segments for which the assumption of IRF is valid. Conventional time-drawdown analyses and derivative curves are then integrated with geological information to identify the nature of heterogeneities and assess their impact on long-term aquifer response to pumping. Finally a conceptual model is proposed for the aquifer.