Effects of rainfall intensity and slope on surface and subsurface runoff in red soil slope farmland

Abstract: The surface and subsurface runoff in the red soil slope farmland result in soil moisture and nutrient loss. In addition, the surface and subsurface runoff are important components contributing runoff at the watershed scale. An extensive literature shows that rain intensity and slope are two main factors playing an important role in the surface and subsurface runoff. In this paper, the surface and subsurface runoff in the red soil slope land under different rainfall intensities and slopes were studied by stimulated rainfall experiments in the flume (3.0 m length×1.0 m width×0.5 m depth) with variable slopes. The experiments were conducted in the Jiangxi Soil and Water Conservation Ecological Science and Technology Park. Stratified soil samples from different profiles including 0-20 cm (the plow horizon) and 20-40 cm (the plow pan) were collected and then filled respectively into the flumes by controlling the bulk density. For the rain intensity of 90 mm/h, we set four slopes including 5°, 10°, 15° and 20° in the simulated rainfall experiment. Three rain intensity levels (30, 60 and 90 mm/h) were set for the plots with slope of 10°. The results indicated that: 1) the subsurface runoff lagged behind the surface runoff, and the initiation time of the surface and subsurface runoff decreased with the increase of rainfall intensity from 30 mm/h to 90 mm/h. The lag time of the subsurface runoff initiation increased with the increasing rain intensity, and then tended to be stable; 2) the initial and steady surface runoff increased with the increase of the rain intensity; and the initiation subsurface runoff intensity increased with the increase of rainfall intensity. An increase trend was observed in the runoff produced by per unit rainfall; 3) the initial intensity of the surface runoff increased with the increase of rainfall intensity from 30 to 90 mm/h. However, no significant differences were found for the peak values of the subsurface runoff under different rainfall intensities. The attenuation curves of the surface runoff under different rainfall intensities were similar; 4) the obvious differences were observed between the runoff process curves of the surface runoff and subsurface runoff. The surface runoff increased firstly and then tended to be stable, but the soil subsurface runoff increased firstly and then decreased; 5) the initiation time of the surface runoff decreased with the increase of slope from 5° to 20°, but the initiation time of the subsurface runoff decreased firstly and then increased with the increase of the slope. The lag time of the subsurface runoff to the surface runoff also decreased firstly and then increased with the increasing slopes; 6) the surface runoff intensity first increased and then decreased with the increasing slope from 5° to 20° with a critical slope of 10°. The peak values of the subsurface runoff in soils increased firstly and then decreased with the increasing slopes. Moreover, the time to reach the peak value of the subsurface runoff decreased gradually with the increase of the slopes.