Runoff features for interrill erosion at different rainfall intensities, slope lengths, and gradients in an agricultural loessial hillslope

Various interactions, particularly those existing between the rainfall intensity, the slope gradient, the slope length, and the tillage supposedly can affect the runoff features for interrill erosion. Despite numerous studies, their effect on runoff production and pathways and the resulting soil losses have seldom been analyzed. This is especially true under field and tillage conditions. This study investigated the effect of rainfall intensity, slope length, and gradient on runoff amount and pathways for interrill erosion in tilled fields. Runoff features and soil losses were evaluated on bounded plots of 1- and 5-m length located on 4 to 8% slope gradients, and under natural and simulated rainfalls with intensities ranging from 1.5 to 30 mm [h.sup.-1]. Runoff coefficients (R) ranged from 34 to 98% whereas sediment concentrations (SC) varied from 2.9 to 49 g [l.sup.-1]. The runoff coefficient was affected by all three factors: rainfall intensity (r = 0.48; P < 0.0001), slope gradient (r = 0.51; P < 0.0001) and slope length (r = 0.29; P = 0.02); whereas SC was correlated with only rainfall intensity (r = 0.48; P < 0.0001) and slope length (r = 0.44; P = 0.0004). The runoff coefficient and SC ratios between 1- and 5-m long plots were systematically greater for the intermediate rainfall intensity. Runoff features mainly affected by tillage implements may explain higher interrill erosion at longer and steeper slopes. Lower differences between 1- and 5-m plots at high rainfall intensity may reflect greater ponded runoff absorbing raindrop kinetic energy and lowering detachment and transport processes. Finally, the effects of rainfall intensity, slope length, and gradient and tillage are discussed in respect of possible erosion processes operating in the experiments.