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.