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

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

Tai composite swidden agriculture system: rice paddies, household gardens, orchards, livestock, & swiddens

Manual Tillage Erosion

Ngheo used for field preparation and weeding

TREND: Shorter fallows, longer rotations = more weeds = increased tillage erosion

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

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

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

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

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

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

)

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

Thank You! Alan ZieglerU. Hawaiiadz@hawaii.edu

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

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.

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)

Application to other studies