Geoscience Reference
In-Depth Information
Sub-irrigation
q
infil
Drainage
q
drain
φ
gwl
φ
drain,1
φ
gwl
φ
drain,2
φ
gwl
φ
drain,3
-
-
-
q
drain
=
+
+
γ
1
γ
2
γ
3
φ
drain,3
Groundwater
height
φ
gwl
φ
gwl
γ
1
φ
drain,1
φ
gwl
γ
2
φ
drain,2
-
-
q
drain
=
+
φ
drain,2
φ
gwl
φ
drain,1
-
q
drain
=
γ
1
φ
drain,1
φ
drain,1
γ
inf
φ
gwl
-
q
infil
=
Figure 9.7
Drainage or sub-irrigation lux as function of groundwater height, drain-
age level
ϕ
and drainage or sub-irrigation resistance
γ
.
conductivity in this region (m d
-1
),
L
is the drain spacing (m) and
r
d
is the hydraulic
radius of the drainage canal (-). The variable
d
is an 'equivalent thickness', which is
smaller than the thickness of the aquifer
D
and which expresses the hydraulic head loss
due to convergent streamlines near the drainage canal. If the required drainage lux is
known, Equation (
9.8
) can be solved iteratively to ind the drain spacing
L
.
Question 9.5:
Consider a ield where the relation between drainage lux and ground-
water level is similar as depicted in
Figure 9.7
. The three drainage resistances are:
γ
1
=
1000 d,
γ
2
= 500 d and
γ
3
= 250 d. The three drainage levels are situated at:
φ
drain,1
= -3.0
m,
φ
drain,2
= -2.0 m and
φ
drain,3
= -1.0 m with respect to soil surface. To solve this ques-
tion, use method the irst method from the above list:
a) How large is the drainage lux (mm d
-1
) at a groundwater level 2.5 m below the soil
surface?
b) How large is the drainage lux (mm d
-1
) at a groundwater level 1.5 m below soil
surface?
c) How large is the drainage lux (mm d
-1
) at a groundwater level 0.5 m below soil surface?
Question 9.6:
At an orchard high groundwater tables occur due to an aquitard at
4 m depth. We want to install subsurface drains such that the maximum groundwater
level is 0.80 m below soil surface at a design discharge of 10 mm d
-1
. The drains
have a hydraulic radius of 5 cm and will be installed at a 2.0 m depth. The saturated
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