**
These are the finite difference equations that relate
heads at a cell to surrounding heads. **

**If we
solve for the future head (head at next time step) at a given cell, using the
current heads at the surrounding cells, we call this explicit or forward difference
and we can solve one equation to determine the future head at that cell, because
there is only one unknown.**

**If we
solve for the future head (head at next time step) at a given cell, using the
future heads at the surrounding cells, we call this implicit or backward difference
and we must solve simultaneous equations to determine the future head at that
cell, because there are numerous unknowns.**

**An alternative
answer:**

**On
page
http://www.mines.edu/~epoeter/583/06/discussion/basis_of_fd.htm **

**eqtn
8 is the fundamental finite difference equation**

**having
this we have 2 choices:**

**we
can set n = t, the present time at the beginning of the time step, and solve
each equation independently for the heads at i-1 i i+1 (we call this explicit,
or forward difference because we use the past heads to solve for future heads)**

**OR**

**we
can set n = t+^t, the future time at the end of the time step, and solve the
equations (3 in the case of a 1D problem) simultaneously for the heads at i-1
i i+1 (we
call this implicit, or backward difference because we use the future heads to
solve for future heads ... this requires simultaneous solution because we do
not know the future heads)**