Geoscience Reference
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200
0.001
0.1
100
10
0
100
200
T
50
(d)
Figure 5.14
Contour lines showing the fraction of pesticide leached below 1 m depth
as function of distribution coeficient
S
d
and half-life
T
50
. Solid lines correspond to
application in spring (May 25), dashed lines to application in autumn (1 November)
(Boesten and Van der Linden,
1991
).
prevailing hydraulic head gradients and may low in any direction. Sophisticated
two- and three-dimensional models have been developed to simulate water low and
solute transport in the saturated zone. Many environmental studies require interaction
of the unsaturated and saturated zone into one model. Although a few of those models
exist (e.g., MODFLOW, HYDRUS-2D), it is more common either to simplify the
groundwater low and simulate accurately the unsaturated zone, including the inter-
action with vegetation and atmosphere, or simplify the unsaturated zone and simulate
accurately the groundwater low system.
In this paragraph we consider simpliied groundwater low by making the follow-
ing assumptions:
a) Steady groundwater low, for example, the recharge rate multiplied by the total recharge
area, is equal to the discharge of groundwater to surface water.
b) Relative thickness changes of the saturated zone are small compared to the aquifer thick-
ness.
c) The thickness of the aquifer and its porosity are constant.
d) Groundwater low is horizontal, with uniform velocity over depth.
With these assumptions we derive the residence time of solutes in a groundwater
aquifer between parallel canals (
Figure 5.15
). The horizontal, uniform pore water
velocity
v
x
(m d
-1
) at distance
x
(m) from the groundwater divide follows from mass
conservation:
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