16
The sink term is used to represent the loss or gain of water from
the soil by root extraction or by application of irrigation water from a
point source.
For implicit solutions, equation (4) must be written in terms of
only one variable, soil water content or potential. By introducing the
specific water capacity, C, defined as
d e
C = (5)
d h
and using the chain rule of calculus, equation (4) may be written in
terms of the soil water potential as
9h 3 3 h 3 K(h)
C = (K(h) ) - S(z,t) (6)
3 t 3 Z 3 Z 3 Z
For two-dimensional flow, equation (3) becomes
3 h 3 ah 3 3 h 3K(h)
C = (K(h)) + (K(h)) S(x,z,t) (7)
at 3 X 3 X 3 Z 3 Z 3 Z
Th