Tillage erosion in developing countries in Asia
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Tillage erosion, which is the net redistribution of soil within the landscape as a result of farming activities, is one of the most important soil degradation processes on sloping croplands world-wide (Govers et al., 1999; Lindstrom et al., 2001). Tillage contributes to the denudation of upper portions of hillslopes and causes accumulation of soil on lower portions of the hillslope (Lobb et al., 1995). It therefore plays an important role in soil redistribution across the landscape, along with the traditionally recognized processes of wind and water erosion (De Alba et al., 2004; Van Oost et al., 2005). Tillage erosion can be of greater importance than water erosion on fields with negligible rill erosion, which is the formation of small channels by concentrated overland flow (e.g., Quine et al., 1999; Van Muysen et al., 1999; Figure 1).
Experimentally, tillage erosion is often expressed as a soil flux, which has units of mass of material moved per unit width of a plot or field per tillage pass (e.g., kg m-1 pass-1). The relationship between soil flux and slope vary from site to site depending on tillage intensity, implement size, and surface conditions related to crop choice, surface contact cover, soil properties, and other surface conditions. From a process standpoint, soil flux (Qt) on relatively moderate slopes can be modeled adequately as a linear process (Lobb et al., 1999):
Qt = α + βS 
where α is the tillage transport constant; β is the tillage transport coefficient, and S is slope gradient. Simply stated, α is the soil flux caused by the implement alone, without the influence of slope; and β represents soil erosivity.
Hoeing with hand-held implements during plot preparation, sowing, and weeding is an important agent of tillage erosion on sloping lands in developing countries. Turkelboom et al. (1997; 1999) investigated tillage erosion on hillslopes cultivated by ethnic Akha villagers in Chiang Rai Province of northern Thailand. There, traditional shifting agriculture (also called slash-and-burn agriculture) had been given way to semi-permanent cultivation of annual crops. Because the length of the fallow period had reduced weed pressure was high, requiring substantial hoeing. Tillage erosion was therefore greatly accelerated over that during former times before agriculture intensification. Tillage erosion contributed to terrace formation and fertility gradients within fields; and soil loss was particularly important on short (< 20 m) fields that did not produce rill erosion (Figure 1). Estimates of soil fluxes caused by tilling with traditional hoes ranged from about 20-100 kg m-1 pass-1 across a wide range of slopes (Figure 2). In comparison, even higher soil fluxes were caused by tillage with large hoes in Sichuan Province, China (Zhang et al. 2004; Figure 2).
In Lao PDR, tillage-induced soil fluxes in upland rice and Job's tears fields ranged from about 10 to 30 kg m-1 pass-1 (Figure 2; Dupin et al., 2009). In comparison, soil fluxes caused by weeding by the Da Bac Tay ethnic group in northern Vietnam were much smaller, primarily because the tillage depth was very shallow (ca. 0.01 m) and weed density was low (Figure 2). Tillage erosion rates associated with weeding were an order of magnitude lower than reported water erosion rates; however, it was still an important process affecting soil distribution in some fields (Figure 3). Combined water and tillage erosion estimates indicated a possible unsustainable increase in soil loss on some steep fields within the last few decades, owing to intensification of cultivation.
The studies conducted in Asia on tillage erosion caused by hand-held implements have demonstrated that as field slope approaches the angle of repose for resting aggregates, the increase in soil flux becomes nonlinear as soil translocation is affected by a secondary ravel process, which is simply the rolling, bouncing, and sliding of soil clods down slope (Figure 4). Because of the occurrence of ravel on steep slopes, soil flux from manual tillage is best described as a non-linear process (Ziegler and Sutherland, 2009; Figure 2):
Qt = aebS 
where a is analogous to the tillage transport constant α; and b controls the non-linear increase in soil flux with increasing slope gradient (S).
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