Impact of slope length on soil erosion of sloping farmland with crop in red soil hilly region

Abstract: Slope length is one of the important landform factors causing slope erosion. A better understanding of the impact of slope length on the water erosion process would provide theoretical evidence for the prediction of soil loss and improve the use of controlling measures for soil and water conservation. Based on field simulated rainfall events, this study investigated the impact of slope length on soil erosion of sloping farmland with crop cover under different rainfall intensities to determine the critical rainfall intensity and the reasonable slope length for soil conservation in a red soil hilly region. Combined with natural rainfall events of the study region, rainfall intensity in this research varied from 30 to 150 mm/h, which could be adjusted by varying nozzle sizes and water pressure. The simulated rainfall had uniformity of above 85%, similar to natural rainfall in raindrop distribution and size. Calibrations of rainfall intensities were conducted at the beginning of each experiment. The runoff/erosion catchment plots of 2, 4, 6, 8, 10 m (length) by 2 m (width) were established before a crop was planted in Anji County, northwest of Zhejiang province. The soil type at this site is red soil and the slope gradient is 20 deg. The initial water content of the soil was determined to ensure that the soil water content was identical for all simulated experiments. Each rainfall event was replicated two times. The period from rainfall beginning to runoff occurrence was recorded as "runoff occurrence time" during each rainfall event, and the duration of each rainfall simulation was 30 min from the appearance of runoff. All runoff water and sediment samples were collected in the polyethylene bottles (with volume 1 L) at the bottom end of the plot in 3 min intervals. The sloping farmland was dominated by Chili pepper, which was premature, 40 cm in height, covered approximately 65% of the surface of the plots and had high production as a kind of economic crop. The soil was bare under the Chili pepper. Data were analyzed using SPSS to determine if slope length and rainfall intensity had any effect on sediment yield. When there were significant correlations between rainfall intensity (slope length) and sediment yield, statistical processing of the results was further performed using regression techniques. Results indicated that both rainfall intensity and slope length played important roles on runoff formation and soil erosion in the surface runoff. Sediment yield increased as a whole with increasing slope length under the same rainfall intensity, so the relationship between them could be described by using a power function (Determination coefficient R2>0.84). However, the increment of sediment mass was not proportional to the same slope length increase and it even decreased in 4 m intervals. Furthermore, the sediments contained mainly silt particles (47.1%) with diameters less than 0.02 mm and clay particles (40.9%) with diameters less than 0.002 mm. This result further proved that erosion on sloping farmland could reinforce the soil coarsening and decrease the soil fertility. Therefore, this study suggests that engineering measures of soil and water conservation could be constructed or hedgerows could be planted at 4 m intervals to truncate the slope length and decrease soil and water loss, improve soil fertility and prevent farmland from further degradation. Sediment yield increased with increasing rainfall intensity and it could be estimated with a power function of rainfall intensity (Determination coefficient R2>0.76). A quicker increase of runoff velocity as measured during the experiment required a longer slope length. 60 mm/h rainfall intensity was the critical value that resulted in a sudden increase in erosion for red soil hilly region. As a consequence, soil and water conservation measures should be strengthened when rainfall intensity is greater than 60 mm/h. There was positive correlation between sediment yield and rainfall intensity (slope length); moreover, rainfall intensity rather than the slope length played a more notable impact on sediment yield. Research on the impact of slope length on sloping farmland erosion under different rainfall intensities will serve to control soil and water loss on sloping farmland in red soil hilly region.