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Soil translocation by weeding on swidden fields in northern Vietnam

Soil translocation by weeding on swidden fields in northern Vietnam. Alan D. Ziegler & T.W. Giambelluca, R.A. Sutherland, M. Nullet Geography Department, University of Hawaii, Honolulu, HI USA & Tran Duc Vien Hanoi Agricultural University, Gia Lam, Vietnam. STUDY SITE.

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Soil translocation by weeding on swidden fields in northern Vietnam

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  1. Soil translocation by weeding on swidden fields in northern Vietnam Alan D. Ziegler & T.W. Giambelluca, R.A. Sutherland, M. Nullet Geography Department, University of Hawaii, Honolulu, HI USA & Tran Duc Vien Hanoi Agricultural University, Gia Lam, Vietnam

  2. STUDY SITE Fragmented landscape (intensification in swiddening in last 30-50 years) Tan Minh Village (20º 55’ 49” N, 105º 7’ 3.6” E) 1998-1999 (2100 ha) Da River Watershed Hoa Binh Province Northern Vietnam Little forest; abundance of swidden-related covers of various stages

  3. Taicomposite swidden agriculture system: rice paddies, household gardens, orchards, livestock, & swiddens

  4. Manual Tillage Erosion Ngheo used for field preparation and weeding TREND:Shorter fallows, longer rotations = more weeds = increased tillage erosion

  5. Soil Translocation Investigation EXPERIMENTS three 20-m plots in 2 fields on Hillslope 1 (NE) Hillslope 2 (SW) 12 Experimental plots SW Hillslope Hypothesis Manual erosion was on the same order of importance as water erosion Investigation Quantify soil translocation by weeding with the ngheo compare tillage erosion rates with water erosion estimates

  6. Translocation Experiments “BACKSTOP” METHOD Slope range 0.55 – 0.85 m m-1 Ultisols CALCULATIONS 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)

  7. SF RELATIONSHIP with SLOPE Field differences: texture and weed cover RESULT SF (kg m-1 tillage pass-1) 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) Slope (m m-1)

  8. Long translocation distances ½ material from > 1.5 m upslope Ravel contribution Cumulative density function of TS Slope (m m-1) Dry RAVEL: rolling, sliding, & bouncing of material downslope Distance upslope from backstop (m)

  9. SummaryResults Summary data for the NE and SW hillslope experiment sites low high 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

  10. Soil Flux versus Other Studies Thailand (Turkelboom+ 1997,1999) China (Zhang+ 2004) Tanzania – GT (Kimaro+ 2005) Tanzania – trap (Kimaro+ 2005) SF (kg m-1 pass-1) Lao PDR – UR (Dupin+ 2002) Lao PDR – JT (Dupin+ 2002) Slope (m m-1) Why so low? Small hoe; few weeds

  11. Contribution to Total Erosion on “average” field ………….. CONCLUSION Acceleration in total erosion in recent past but Low contribution from tillage erosion vs water erosion Cooperative: Government decisions on land allocation and planting (shorter fallow periods, more weeds) Pre-cooperative: Low population (true swidden = long fallows; low weeds) Post-cooperative: Market forces (cash crops); intensification of cultivation; shorter fallows; and many weeds. BASIS: Guided assumptions (crop rotation length, fallow time; weeding frequency) & experiment-derived erosion rates Simulated Soil Loss (cm) Year

  12. Thank You! Alan Ziegler U. Hawaii adz@hawaii.edu

  13. Contribution to Total Erosion on “average” field ………….. CONCLUSION Acceleration in total erosion in recent past but Low contribution from tillage erosion vs water erosion Cooperative: Government decisions on land allocation and planting (shorter fallow periods, more weeds) Pre-cooperative: Low population (true swidden = long fallows; low weeds) Post-cooperative: Market forces (cash crops); intensification of cultivation; shorter fallows; and many weeds. Guided assumptions (crop rotation length, fallow time; weeding frequency) and experiment erosion rates Simulated Soil Loss (cm)

  14. Table 2 Physico-chemical properties for all six fields on each of the two hillslopes investigated 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.

  15. Ravel Model (Gabet, 2003) 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: (cf. Kirkby and Statham, 1974)

  16. Application to other studies

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