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The effect of stubble level on runoff and soil loss under simulated rainfall

R.J. Loch1 and T.E. Donnollan2

1Queensland Wheat Research Institute, Holberton Street, Toowoomba. Qld 435C
Department Primary Industries, Toowoomba. Qld 4350.

The level of stubble mulch needed to reduce runoff and soil loss is a major consideration in the development of mulch-tillage systems. Rainulator studies in the U.S. (Mannering and Meyer 1963; Meyer et al. 1970) showed that relatively low stubble levels greatly reduced soil loss, but this effect has not been shown for Queensland soils and tillage methods.

Rainulator plots 32.5 x 4 m were established on a shallow black earth of 6% slope, with standing wheat stubble treatments of 3,3 and 1 t/ha, and a burnt stubble treatment of 0.2 t/ha. These plots were ploughed on the contour using a blade plough fitted with 900 mm sweeps. To minimize differences in antecedent soil moisture all plots were pre-wet to the point of surface runoff and allowed to drain overnight before 79 mm of artificial rain was applied at 95 mm/hr. Results of runoff and soil loss measurements are shown in Table l.

TABLE l. Runoff and soil loss under four stubble levels.

The results show large differences in surface runoff even after the pre-wetting treatment. These differences are attributed to heavy lateral water movement downslope through the ploughed layer, rather than to differences in deep infiltration.

Soil losses from the 0.3 t/ha and 3 t/ha treatments differed greatly, with spatial variability in stubble distribution affecting the intermediate treatment results. Rills did not develop on any of the plots, and soil loss was by inter-rill erosiononly. Soil losses shown in Table 1 are quite low, suggesting that even under intense rainfall, inter-rill erosion is unlikely to cause appreciable soil loss.

Other studies have shown that rill erosion can transport much greater quantities of soil than inter-rill erosion (Mosley 1974; Loch unpubl. data). Further studies of stubble effects obviously need to consider the effect of stubble on rill development. The results also show that lateral sub-surface water movement could be an important phenomenon in some cultivated soils.

Mannering, J.K. and Meyer, L.D. (1963). Soil Sci. Soc. Amer. Proc. 37 : 84.

Meyer, L.D., Wischmeier, W.H. and Foster, G.R. (1970). Soil Sci. Soc. Amer. Proc. 34: 938.

Mosley, M.P. (1974). Trans.A.S.A.E. 17: 909.

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