20
by the volume of soil in each grid and then multiplying by the time
step. Finally, the model calculated the new water contents from the
previous water contents plus the calculated water content changes.
Model results compared favorably with field measurements.
Zazueta et al. (1985) developed a simple explicit numerical model
for the prediction of soil water movement from trickle sources. The
model was based on the mass balance equation and an integrated form of
Darcy's law. The model produced good agreement with other results
obtained with more complicated numerical methods and analytical
solutions.
Soil Water Extraction
Water uptake by plant roots has been investigated by many
researchers. Among the first researchers to attempt to describe plant
water relations were Gradmann (1928) and van den Honert (1948). Two
approaches to modeling water extraction have been utilized to describe
the water extraction by plant roots. The first, called the microscopic
approach, describes water movement to individual roots. The second
approach, called the macroscopic water extraction model, describes water
uptake by the whole root zone, and the flow to individual roots is
ignored.
Gardner (1960) developed a microscopic water uptake model. He
described the root as an infinitely long cylinder of uniform radius and
water-absorbing properties, assuming that water moves in the radial
direction only. The flow equation for such a system is
8 e
8 t
1 3 80
( r D )
r 8 r 8 r
r 8 r
(9)