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 Tan Minh Village (20º 55’ 49” N, 105º 7’ 3.6” E) Da River Watershed Hoa Binh Province Northern Vietnam Fragmented landscape (intensification in swiddening in last 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 Translocated soil from any subplot (TS i ) (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 range 0.55 – 0.85 m m -1 Ultisols

SF RELATIONSHIP with SLOPE Linear over narrow range of slopes investigated ( m m -1 ) but Non-linear over wide range of slopes ( m m -1 ) Field differences: texture and weed cover SF (kg m -1 tillage pass -1 ) Slope (m m -1 ) RESULT

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

VarUnitsNESWHG Slope † m m ± ± SF or TSkg m -1 pass ± ± 2.3 SLRMg ha -1 pass ± ± 1.1n.a. L 50 m1.5 ± ± 0.6< 0.5 Ravel-0.38 ± ± 0.12n.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 pass -1 )

Contribution to Total Erosion on “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. Simulated Soil Loss (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 Ziegler U. Hawaii

Contribution to Total Erosion on “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. Simulated Soil Loss (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 2 Physico-chemical properties for all six fields on each of the two hillslopes investigated HillslopepH † OM † P†P† bb Sand † SiltClay † -g kg -1 mg kg -1 kg m -3 g kg -1 NE5.8 ± ± ± ± ± ± ± 10 SW4.3 ± ± ± ± ± ± ± 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 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) (Gabet, 2003)

Application to other studies