Agriculture Reference
In-Depth Information
Factor
Relevant field condition
Typical values
Assumed value
K
Soil erodibility factor
: The soil is
loam
0.013-0.053
0.03
LS
Slope length and steepness factor
:
5% slope, 400 m long
0.2-6.0
4.0
C
Cover and management factor
:
Crop is corn, moderate coverage
0.01-1.4
0.5
P
Support practice factor
: Contour
farming
0.6-0.9
0.6
Putting the values,
A
=
2, 027
×
0.03
×
4.0
×
0.5
×
0.6
=
72.9 t/ha/yr (Ans.)
Example 6.5
Mr. John Mitchell has a 100 ha watershed in Melbourne. The watershed has 20 ha of
good pasture in silty-clay soil of 2% slope (CN
=
65), 30 ha of pasture on silt-loam
soil of 0.5% slope (CN
=
70) and remaining 50 ha of wheat on loam soil of 1%
slope (CN
=
85). Determine the weighted curve number for the watershed.
Solution
We know
(CN
i
×
A
i
)
CN
w
=
A
The data are arranged in tabular form as follows:
A
×
Sl no.
Area,
A
(ha)
Curve no., CN
CN
1
20
65
1,300
2
30
70
2,100
3
50
85
4,250
A
= 100
(
A
×
CN)
=
7, 650
Putting the values in the above equation, CN
=
7,650/100
=
76.5
≈
77 (Ans.)
Example 6.6
In a humid sub-tropic climatic zone in India, a farmer has sown winter wheat. The
subsequent crop in the field is broadcasted (dry-seeded) Aman rice. The soil is silt-
loam having a moderate organic matter. The average grade of the field is 1% and the
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