# 6 Mathematical Modeling

Numerical modeling of groundwater flow is a complex subject, and variable-density models are significantly more complex than constant-density models. An exhaustive discussion of all the details of these complexities is impossible here. Readers are referred to the works of, amongst others, Holzbecher (1998), Segol (1994), Voss (1999, 1984) and Zheng and Bennett (2002) for information on important aspects of variable-density flow such as governing equations, numerical methods, boundary conditions, initial conditions and parametrization. This section touches upon some of the basics and discusses the challenges associated with modeling unstable systems.