soil translocation by weeding on swidden fields in northern vietnam alan d. ziegler & t.w....
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Soil translocation by weeding on swidden fields in northern Vietnam
Alan D. Ziegler
& T.W. Giambelluca, R.A. Sutherland, M. NulletGeography Department, University of Hawaii, Honolulu, HI USA
& Tran Duc VienHanoi Agricultural University, Gia Lam, Vietnam
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STUDY SITE
Tan Minh Village (20º 55’ 49” N, 105º 7’ 3.6” E)
1998-1999
Da River Watershed Hoa Binh Province
Northern Vietnam
Fragmented landscape (intensification in swiddening in last 30-50 years)
(2100 ha)
Little forest; abundance of swidden-related covers of various stages
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Tai composite swidden agriculture system: rice paddies, household gardens, orchards, livestock, & swiddens
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Manual Tillage Erosion
Ngheo used for field preparation and weeding
TREND: Shorter fallows, longer rotations = more weeds = increased tillage erosion
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Soil Translocation Investigation
Quantify soil translocation by weeding with the ngheo
compare tillage erosion rates with water erosion estimates
Manual erosion was on the same order of importance as water erosion
three 20-m plots in 2 fields on Hillslope 1 (NE)Hillslope 2 (SW)
12 Experimental plots
EXPERIMENTS
SW Hillslope
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Translocation Experiments
“BACKSTOP” METHOD
2
iiMS
TS
20
1i
iTSTSSF
1020
SF LR S
Translocated soil from any subplot (TSi) (kg m-1 tillage pass-1)
Soil Flux (SF) = Total translocation (TS) (kg m-1 tillage pass-1)
Soil Loss Rate (SLR) (Mg ha-1 tillage pass-1)
CALCULATIONS
Slope range0.55 – 0.85 m m-1
Ultisols
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SF RELATIONSHIP with SLOPE
Linear over narrow range of slopes investigated
(0.50-0.85 m m-1)
but
Non-linear over wide range of slopes
(0.0-1.0 m m-1)
SNE eSF 148.6021.0
SSW eSF 400.5116.0
Field differences: textureand weed cover
SF
(kg
m-1 t
illa
ge
pas
s-1)
Slope (m m-1)
RESULT
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Long translocation distances
Dry RAVEL: rolling, sliding, & bouncing of material downslope
Cu
mu
lati
ve d
ensi
ty f
un
ctio
n o
f T
S
½ material from > 1.5 m upslope
Distance upslope from backstop (m)
Slope (m m-1)R
avel
co
ntr
ibu
tio
n
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Var Units NE SW HG
Slope† m m-1 0.76 ±0.06 0.66 ± 0.04 0.05
SF or TS kg m-1 pass-1 2.6 ± 1.6 3.9 ± 2.3
SLR Mg ha-1 pass-1 1.3 ± 0.8 2.0 ± 1.1 n.a.
L50 m 1.5 ± 0.4 1.5 ± 0.6 < 0.5
Ravel - 0.38 ± 0.06 0.37 ± 0.12 n.a
Summary Results
Values are medians ± one median absolute standard deviation
SLR is the soil loss rate; SF is total sediment transported from the 20-m field; L50 is the upslope distance above which 50% of the translocated material originated; Ravel is the percentage of transported material that was ravel
Summary data for the NE and SW hillslope experiment sites
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Soil Flux versus Other Studies
Why so low? Small hoe; few weeds
Tanzania – GT(Kimaro+ 2005)
Tanzania – trap(Kimaro+ 2005)
Lao PDR – JT(Dupin+ 2002)
China (Zhang+ 2004)
Lao PDR – UR(Dupin+ 2002)
Thailand (Turkelboom+ 1997,1999)
Slope (m m-1)
SF
(kg
m-1 p
ass-1
)
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Contribution to Total Erosionon “average” field …………..
Pre-cooperative: Low population (true swidden = long fallows; low weeds)
Cooperative: Government decisions on land allocation and planting (shorter fallow periods, more weeds)
Post-cooperative: Market forces (cash crops); intensification of cultivation; shorter fallows; and many weeds.
Sim
ulat
ed S
oil L
oss
(cm
)
Acceleration in total erosion in recent past
but
Low contribution from tillage erosion vs water erosion
CONCLUSION
BASIS: Guided assumptions (crop rotation length, fallow time; weeding frequency) & experiment-derived erosion rates
Year
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Thank You! Alan ZieglerU. [email protected]
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Contribution to Total Erosionon “average” field …………..
Pre-cooperative: Low population (true swidden = long fallows; low weeds)
Cooperative: Government decisions on land allocation and planting (shorter fallow periods, more weeds)
Post-cooperative: Market forces (cash crops); intensification of cultivation; shorter fallows; and many weeds.
Sim
ulat
ed S
oil L
oss
(cm
)
Acceleration in total erosion in recent past
but
Low contribution from tillage erosion vs water erosion
CONCLUSION
Guided assumptions (crop rotation length, fallow time; weeding frequency) and experiment erosion rates
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Table 2Physico-chemical properties for all six fields on each of the two hillslopes investigated
Hillslope pH† OM† P† b Sand† Silt Clay†
- g kg-1 mg kg-1 kg m-3 g kg-1 g kg-1 g kg-1
NE 5.8 ± 0.2 6.3 ± 1.3 240 ± 100 1035 ± 89 670 ± 50 160 ± 40 160 ± 10
SW 4.3 ± 0.1 4.2 ± 0.4 30 ± 10 987 ± 81 520 ± 50 230 ± 40 260 ± 40
Values are medians ± median absolute deviations from the median; OM is organic matter (carbon content via Walkley and Black method * 1.724); P is available phosphorus (Oniani method), b is bulk density.† indicates significant difference at α = 0.05 (non parametric Mann-Whitney U Test); n = 12 for OM and P; n = 24 for b; n = 6 for sand, silt, and clay.
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Ravel Model
220 tan
tan
QQt
μ is a kinetic friction coefficient that encompasses friction from rolling, bouncing, and particle collisions down a slope; and
is a constant that accounts for the distribution of initial velocities, gravitational acceleration, the frequency and spatial density of tillage disturbance, and average mass of displaced material.
Initial basis: is distance traveled relationship:
)sincos(2
2
g
vl id
(cf. Kirkby and Statham, 1974)
(Gabet, 2003)
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Application to other studies