Agriculture Reference
In-Depth Information
WP
i
−
1
)f rWP
i
−
1
Z
i
=
Z
i
−
1
+
(
ζ
i
−
ζ
i
−
1
)WP
i
−
1
+
ζ
(
δ
t
i
)(WP
i
−
(3.19)
or
for WP
i
Z
i
=
Z
i
−
1
+
(
ζ
i
−
ζ
i
−
1
)WP
i
−
1
<
WP
i
−
1
(3.20)
where WP
=
the wetted perimeter, and
ζ
is given by the extended Kostiakov
equation:
a
+
C
ζ
=
K
τ
τ
where
field-measured coefficients; and
subscripts
i
and
i
-1 denote consecutive time lines at which the solution is computed.
the fixed time step or the fixed space step.
τ
=
the intake opportunity time;
k, a, C
=
3.4.8 General Guideline/Thumb Rule for Furrow Design
Furrow irrigation has limitations or field constraints to acknowledge as a guide-
line when considering this method. The general limits of different parameters are
described below.
3.4.8.1 Furrow Length
Row length should not exceed 400 m on heavy soils and, depending on soil texture
and slope, can be shorter. Although furrows can be longer when the land slope is
steeper, the maximum recommended furrow slope is 0.5% to avoid soil erosion. The
coarser the soil or the steeper the slope, the shorter will be the run length.
3.4.8.2 Slope
Row slopes should be between 0.05 and 0.5%. Cross slope should be less than or
equal to row slope, except in a permanent-graded furrow design. Furrows can also
be level and are thus very similar to long narrow basins. However, a minimum grade
of 0.05% is recommended so that effective drainage can occur following irrigation
or excessive rainfall. If the land slope is steeper than 0.5%, then furrows can be set
at an angle to the main slope or even along the contour to keep furrow slopes within
the recommended limits. Furrows can be set in this way when the main land slope
does not exceed 3%. Beyond this there is a major risk of soil erosion following a
breach in the furrow system. On steep land, terraces can also be constructed, and
furrows should be constructed along the terraces.
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