Minggu 3-4

Tanah dan Bentuk Lahan
Didik Suprayogo
Hujan
Mudah tidaknya tanah suatu tererosi = Erodibilitas Tanah
Erodibiltas Tanah
Kehilangan tanah (ton/ha/th)
0 – 14,6
Tingkat Erodibiltas
Sangat Rendah
14,7 – 29,2
Rendah
29,3 – 46,9
Sedang
47,0 – 63,0
Agak Tinggi
63,1 – 80,6
Tinggi
> 80,6
Sangat Tinggi
Erodibiltas dan Sifat
Fisik Tanah
• Ketahanan tanah terhadap
daya rusak dari luar
• Kemampuan Tanah untuk
menyerap air (inflitrasi dan
perkolasi)
Ketahanan tanah terhadap daya
rusak dari luar
• Tekstur tanah
• Kemantapan Agregat
• Kandungan bahan organik
• Bahan semen lainnya
SOIL ERODIBILITY - K
• Effect of texture
– clay (0.1 - 0.2) resistant to detachment
– sand (0.05 - 0.15) easily detached, low runoff,
large, dense particles not easily transported
– silt loam (0.3 - 0.5) moderately detachable,
moderate to high runoff
– silt (0.4 -0.6) easily detached, high runoff,
small, easily transported sediment
Time Variable K
•
•
•
•
Varies during year
High when rainfall is high
Low when temperature is high
Very low below about 25 oF
Time Variable K
Base K value = 0.37
0.8
0.6
0.5
CA
SD
0.4
MA
TN
0.3
0.2
0.1
Day in Year
361
337
313
289
265
241
217
193
169
145
121
97
73
49
25
0
1
Daily Soil Erodibility Value
0.7
Menetapkan
Erodibiltas Tanah
SOIL ERODIBILITY - K
• Measure of soil erodibility under standard
unit plot condition
– 22.2 m long, 9% steep, tilled continuous fallow,
up and down hill tillage
• Independent of management
• Major factors
– Texture, organic matter, structure, permeability
(runoff potential)
L = 22 m S = 9%
K = A/R
Erodibiltas Tanah
Kelas dan Nilai K
Tingkat Erodibiltas
1. 0,00 – 0,10
Sangat Rendah
2. 0,11 – 0,20
Rendah
3. 0,21 – 0,32
Sedang
4. 0,33 – 0,43
Agak Tinggi
5. 0,44 – 0,55
Tinggi
6. >0,56
Sangat Tinggi
Nomograph
Erodibiltas Tanah
Klasifikasi Struktur Tanah
Kelas
Keterangan
1
Granuler sangat halus
2
Granuler halus
3
Granuler sedang-kasar
4
Masif kubus, lempeng
Erodibiltas Tanah
Klas dan Permiabilitas
(cm/jam)
1. >12,5
Tingkat Erodibiltas
Cepat
2. 6,25 – 12,5
Agak Cepat
3. 2,00 – 6,25
Sedang
4. 0,50 – 2,00
Agak Lambat
5. 0,125 – 0,50
Lambat
6. < 0,125
Sangat Lambat
TOPOGRAPHY
FAKTOR LERENG DAN KEMIRINGAN
(LS)
TOPOGRAPHY
• Overland flow path length
• Slope lengths for eroding portions of
hillslopes
• Steepness
• Hillslope shape
Hillslope Shape
Convex
Uniform
Concave
ComplexConvex:concave
ComplexConcave:convex
Overland Flow Path Length
• Distance from the origin of overland flow to
a concentrated flow area
• This length used when the analysis requires
that the entire flow path length be
considered.
Slope Length for Eroding Portion
of Slope
• Only works for simple slopes
• Traditional definition
– Distance from origin of overland flow to concentrated
flow or to where deposition begins
– Definition is flawed for strips and concave:convex
slopes
• Best approach: Use overland flow path length and
examine RUSLE2 segment erosion rate values
Slope Length for Concave Slope
Overland flow path length
Eroding portion
slope length
Deposition
Rule of Thumb for Deposition Beginning
on Concave Slopes
Average steepness of
concave portion
Example:
Assume average slope of
concave section = 10%
½ of 10% is 5%
Deposition begins at location
where the steepness is 5%
Deposition begins at location
where steepness = ½ average
steepness of concave portion
Deposition begins
Slope Length for Concave:Convex Slope
Overland flow path length and slope
length for lower eroding portion of slope
Slope length for upper
eroding portion of slope
Deposition
Insert figures from AH703 to illustrate field
slope lengths
Basic Principles
• Sediment load accumulates along the slope
because of detachment
• Transport capacity function of distance
along slope (runoff), steepness at slope
location, cover-management, storm severity
(10 yr 24 hr precip)
• Deposition occurs where sediment load
becomes greater than transport capacity
Detachment Proportional to Slope
Length Factor
• Slope length effect
– l= (x/72.6)n
– x = location on slope
– n = slope length exponent
• Slope length exponent
– Related to rill:interrill ratio
– Slope steepness, rill:interrill erodibility, ground cover,
soil biomass, soil consolidation
• Slope length factor varies on a daily basis
Slope Length Effects
• Slope length effect is greater on slopes
where rill erosion is greater relative to
interrill erosion
• Examples:
–
–
–
–
Steep slopes
Soils susceptible to rill erosion
Soils recently tilled
Low soil biomass
Detachment Proportional to Slope
Steepness Factor
Not affected by any other variable
4.5
4
Factor Value
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
Slope Steepness (%)
25
30
35
Effect of Slope Shape on Erosion
100 ft long, 1% to 19% steepness range
200
Erosion rate (t/ac)
150
100
Concave
Convex
50
Uniform
0
1
2
3
4
5
6
7
-50
-100
Segment Along Flow Path
8
9
10
Sifat lereng vs energi penyebab
erosi:
·
Kemiringan
·
Panjang Lereng
·
Bentuk lereng
Hubungan Erosi dan kemiringan
EaS
b
Zing (dalam Baver, 1961) a= 0.065, b= 1.49
à sifat hujan Lal (1979) b= 1.1 – 1.2 Hudson b = 2
à tanaman Djorovic (1978)
·
rumput a= 0.75 dan b = 2.788
·
weat a= 32 dan b 2.121
·
jagung a= 160 dan b= 1.163
300
Runoff / Erosi
250
200
Runoff
150
Erosi
100
50
0
0
10
20
30
40
Kemiringan (%)
(0.43  0.30s  0.043S )
S
6.574
2
Panjang Lereng
l x
L( )
22
x= konstan, 0.5 untuk s>4%, 0.4 unt 4% dan 0.3 <3%
3. Measurement of
soil macroporosity:
Methyelen Blue
Technique
Macropore distribution
1.5 m
Forest
Coffee 7 yr
Coffee 1 yr
Coffee 10 yr
Coffee 3 yr
Macroporosity
Vertical macropores
PORI MAKRO HORISONTAL
(horizontal
maps)
Horizontal macropores
PORI MAKRO VERTIKAL
(vertical maps)
16
20
14
18
landuse
BNT
16
hutan
kopi 1th
kopi 3th
kopi 7th
kopi 10th
10
fraction
Macropore
Jumlah Makropore
(%)
fraction
Macropore
Jumlah Makropore (%)
12
8
6
4
2
0
BNT
14
12
10
8
6
4
2
10
30
50
Kedalaman Tanah (cm)
70
Depth of soil layer
90
hutan
kopi 1th
kopi 3th
kopi 7th
kopi 10tj
landslidg
Forest 1 3 Landuse7
10yr old
coffee garden
ROOT DISTRIBUTION
160
0.4
140
0.35
Number anecic +
endogeic
0.3
120
100
0.25
80
0.2
60
0.15
40
0.1
20
0.05
0
0
Remnant
Forest
Multistrata
Shaded
Monoculture
Forest: Amynthas gracillis & Peryonix excavatus (bigger size)
Coffee based: Dichogaster bolaui (smaller size)
-2
0.45
No anecic + endogeic, indiv m
Ratio tot. biomass to population, g / indiv.
Earthworm population
Improvement of soil macropore
Anecic
Outlayer from the forest
Linear (Endogeic)
Endogeic
Linear (Anecic)
Number of macropore, %
30
25
20
y = 10.042x + 2.9368
2
R = 0.2741
15
10
y = 2.162x + 2.4942
5
2
R = 0.6722
0
0
1
2
3
4
Earthworm FW, g per indiv
5
6
0
Soil depth, cm
20
40
60
80
100
Forest
Multistrata
Soil depth, cm
0
20
40
60
80
100
Monoculture
Soil macropore on vertical plane
14
12.3
Vertical macropore, %
12
10
8
6
4
2.99
3.47
3.6
Shaded
Monoculture
2
0
Remnant
Forest
Multistrata
Macropore on vertikal
plane, %
Soil surface cover, %
120
y = 4.1193x + 3.5336
100
R2 = 0.823
80
60
40
20
0
0
5
10
15
20
R2 = 0.4631
-1
5
4
3
2
1
0
0
5
10
15
Litter Thickness, mm
20
10
y = 0.7503x - 6.4402
8
R2 = 0.7407
6
4
2
0
5
10
15
20
25
Littter thickness, mm
Infiltration, mm min
-1
Infiltration, mm min
y = 0.1522x + 0.5532
12
0
25
Litter thickness, mm
6
14
25
6
5
4
3
y = 0.2161x + 1.7855
2
R2 = 0.7097
1
0
0
5
10
Macropore on vertical plane, %
15
Infiltration Rate (cm/min)
Infiltration Rate under various Coffee-based
Systems
10
8
6
5.2
4.7
4
2
2.2
1.9
C+G
MiC
2
0
For
MoC
C+P
Coffee-based Systems