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Soil translocation by weeding on swidden fields in northern Vietnam

Study on soil translocation by weeding in swidden fields, investigating erosion rates and impact of tillage erosion compared to water erosion. Experiment results, calculations, and conclusions drawn from field research in Northern Vietnam.

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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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