The second edition of this title encompasses the numerous significant developments in groundwater modeling, since the 1st edition was published in 1992. The increased computational speed and capacity of present day multi-core computers as well as the availability of sophisticated graphical user interfaces (GUIs) and geographical information systems (GISs) have transformed groundwater modeling. In this 2nd edition many of the important advances in applied groundwater modeling introduced since 1992 are discussed, along with the update to treatment of fundamentals of groundwater flow modeling covered in the 1st edition. The chapters on model calibration and forecasting (Chapters 9 and 10 in the 2nd edition) are entirely new and include discussion of new tools for parameter estimation and uncertainty analysis in forecast simulations