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III. FACTOR INFLUENCING
EROSION
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Secara hubungan fungsional, erosi terjadi
karena interaksi kerja dari iklim, topografi,
vegetasi dan manusia terhadap tanah
yang dinyatakan dalam persamaan
E= (i, r, v, t, m)
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CLIMATE
Climate factors affecting erosion: Rainfall
Temperature
Wind
Humidity Solar radiation
Soil loss is closely related to rainfall partly through the detaching power of raindrops
partly through the contribution of rain to runoff Rainfall parameters related to erosion
Rainfall intensity the quantity of rainfall for a certain time period. i.e: mm/hr
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Rainfall duration
Raindrop size and
distribution
Terminal velocity
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Rainfall Erosivity
Is the potential ability of rain to causeerosion
Closely related to energy of rainfall (kineticenergy)
Formulas for kinetic energy of rainfall: Wischmeier and Smith (1958)
KE is kinetic energy in J/m2.mm and I is rainfallintensity in mm/h
In metric unit: Where KE in ton.m/ha.cm and I in cm/h
IKElog73.887.11
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Hudson (1965) for tropical area
Zanchi and Torri (1980)
Onaga, Shirai and Yoshinaga (1988)
IKE
5.1278.29
IKE log25.1181.9
IKE log6.1081.9
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Wischmeier and Smith (1958) : soil loss by
splash, overland flow and rill erosion is related to
a compound index of kinetic energy and
maximum 30-minute intensity (I30)called EI30 index
Hudson (1965) : erosion is almost entirely
caused by rain falling at intensity greaterthan 25 mm/h called KE>25 index
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EI30 = 2,34 (CHm)1,98
Dimana R = erosivitas hujan bulanan,
(Rain) m = curah hujan bulanan (cm), nilai
R setahun diperoleh dengan
menjumlahkan RM selama setahun.
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RE menyatakan rata-rata indeks erosivitas
hujan (unit/bulan), Q menyatakan rata-rata
jumlah hujan bulanan (cm/bulan), Pm
menyatakan rata-rata curah hujan
maksimal per hari (cm), dan D
menyatakan jumlah hari hujan per bulan.
349,0
678,0263,2
*056,40
*
D
PmQRE
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SOIL
Correspond to soil detachability and soiltransportability
Erodibility : the resistance of the soil to erosion
Erodibility will suffer more erosion Erodibility varies with :
Soil texture
Aggregate stability
Organic matters
Shear strength
Infiltration capacity
Chemical content
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Erodibility indices Wischmeier and Mannering (1969)
M : particle size parameter (% silt+% very finesand)x(100-% clay)
a : percent organic matter
b : soil structure code (1. very fine granular; 2. finegranular; 3. medium or coarse granular; and 4.
blocky, platy or massive c : profile permeability class ( 1. rapid; 2. moderate to
rapid; 3. moderate; 4. slow to moderate; 5. slow; and6. very slow)
)3(103.3)2(103.4)12(108.21003314.17
cxbxaMxK
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VEGETATION
Vegetation acts as a protective layer or buffer betweenthe atmosphere and the soil
The major effects of vegetation in reducing erosion : Interception of rainfall by adsorbing the energy of the raindrops
canopy cover, and thus reducing surface sealing and runoff Retardation of erosion by decreased surface velocity
Physical restraint of soil movement
Improvement of aggregation and porosity of the soil by roots andplant residue
Increase biological activity in the soil
Transpiration, which decrease soil water resulting in increasedstorage capacity and less runoff
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Varies with:
Season
Crop
Degree of maturity of vegetation Soil
Climates
The mean annual soil loss was 43 t/ha forbare plot, and 3.8 t/ha for the covered plot(Zanchi, 1983)
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TOPOGRAFI
Topographic features thatinfluence erosion: Degree of slope
On steep slope resultingrunoff water is more
erosive and can moreeasily transport detachedsediment downslope
Mc Cool et al (1987) :slope steepness factor
for slope < 4 m for slope> 4 m and s < 9 % for
slope > 4 m and s 9 %where : field slopesteepness in degrees =tan-1(s/100)
s = field slope in percent
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Length of slope
Longer slope were
more runoff
accumulates and
greater erosive forces
McCool et al (1989) :
length factor
l : slope length in m
m
lL
22
05.0)(sin269.0sin
sin8.0
m
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Wischmeier and Smith (1978) formulating
slope and length factor as:
n : 0.5 for convex slope, 0.4 for straight
slope and 0.14 for concave slope
)0065.0045.0065.0(13.22
2
ss
l
LS
n
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Shape of slope
Convex. The intensity
of erosion processes
reach the maximum at
the bottom part Concave. The intensity
of erosion processes
reach the maximum at
the end of the upper
part straight (uniform)
combine
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HUMAN
Dari semua faktor di atas, faktor manusialahyang sebenarnya paling menentukan apakahtanah yang diusahakannya akan rusak ataulestari. Banyak faktor dari manusia yang
menentukan perlakuan manusia terhadapsumber daya tanah, antara lain: (1) luas tanahpertanian yang diusahakan; (2) sistempengelolaan yang dilakukan; (3) statuspenguasaan tanah; (4) tingkat pengetahuan dan
penguasaan teknologi; (5) harga produkpertanian; (6) pajak; (7) pasar dan inputpertanian; dan (8) infrastruktur dan fasilitas.