As a computer modeler you should be able to write small programs. Coding graphics is not a necessity but the ability to code in some modern computer language is important for a modeler. Fortran is especially useful to ground-water hydrologists because many commonly used codes are coded in Fortran. It is not likely that you will write elaborate ground-water modeling codes such as MODFLOW. However the ability to write codes to manipulate your data or solve an analytical solution will make you a good hydrologist. If you plan to use models in your career, and you lack programming skills, you should strive towards developing some basic programming skills.