ABSTRACT: Although diagnostic plots are commonly used in petroleum engineering, they have rarely been adapted for hydrogeological investigations. This article therefore focuses on the advantages of diagnostic plots determined from pumping test evaluation. Our study centred on geological investigation of the lignite deposit in the Čáry mine in the southeast part of Slovakia. This is an appropriate method to combine with groundwater modelling to determine pumping effects on drawdown. These are mainly influenced by boundary conditions, collector joining, changes in discharge rates or heterogeneous aquifers. This methodology is also advisable for model calibration because the first derivative enables precise analysis of slope angle and also alterations in the drawdown curve slope during pumping.