Laboratory study on soil erosion of loess convex and concave slopes with application of polyacrylamide

The effects of polyacrylamide(PAM) application rate on composite slope were carried out under simulated rainfall condition in the laboratory. The composite slope included convex and concave slopes. The upslope and down slope gradients of convex slope respectively were 5°, 10°, 15° and 20°, 25°, and those of concave slope were 20°, 25° and 5°, 10°, 15°. Polyacrylamide application rates were 0(control), 5, 10, and 20 kg/hm². Total treatment numbers were 48. Results showed that under no PAM application, sediment concentration in runoff on concave slope or the convex slope with a small upslope gradient (5°) was decreasing gradually with rainfall duration, but the sediment concentration in runoff on the convex slope with a big upslope gradient(≥10°) increased sharply at some time during rainfall process because of rill erosion occurrence. The time that rill erosion occurred usually was within 8-10 min. after rainfall began, and it was shortened with the increased upslope gradient for the convex slope with gradient greater than 10°. The possibility of rill erosion occurrence on convex slope was higher than on concave slope under no PAM application. Soil mass loss increased with the increase of slope gradient, and the averaged soil mass loss from all convex slope treatments was 7.4 times greater than that from concave slope treatments. Generally, PAM application significantly increased sediment concentration in runoff and soil mass loss from concave slope or the convex slope with the small upslope gradient of 5° (P<0.05). Polyacrylamide application played a remarkable control role on rill erosion occurrence and no obvious rill erosion occurred after its application, consequently PAM application resulted in the decrease of soil mass loss from the convex slope with the gradient greater than 10°. After PAM application, the combination of different slope gradients did not obviously affect soil mass loss for convex slope and concave slope. When PAM application rates were 5, 10 and 20 kg/hm², the averaged soil mass loss was(1.61±0.30), (2.27±0.77), and(1.83±0.57) kg for convex slopes and(1.09±0.38), (1.82±0.59), and (1.87±0.49) kg for concave slopes, respectively, and the averaged soil mass loss from convex slopes for all PAM application rate treatments was 20% greater than that from concave slopes. Polyacrylamide can be used to control soil loss from convex slope with an upslope gradient bigger than 10° in production practice.