Discretize each term of equation [28]:

= DAi » Bi.DZi

=> =

= + Q

q = = q_ij

Equation [28], then becomes:

+ + Q =

+ + DQj - DQi = (qi + qj)

DQi - Bi.DZi = DQj + Bj.DZj + - (qi + qj)

which is of the form:

A21.DQi + A22.DZi = B21.DQj + B22.DZj + B23 [34]

A21 = 1 B21 = 1

A22 = - Bi B22 = Bj

B23 = - (qi + qj)