The groundwater flow equations are a combination of the continuity equation and Darcy's law.
A model node with its surrounding connected neighbors.
Continuity Equation
The continuity equation states that the transport of mass in a inifitesimal volume must be conserved, see \eqref{ref1}. E.g. what goes in must go out or accumulate within the CV.
\begin{equation}
\frac{\delta M}{\delta t} = m_{in} - m_{out}\label{ref1}
\end{equation}
If you choose a positive direction for the cartesian coordinate system in $x$, $y$, and $z$, $m_{in}$ and $m_{out}$ can be defined as equation \eqref{ref2}:
\begin{equation}
m^* = \rho Q \label{ref2}
\end{equation}
