Abstract
Slope farmland has been the predominant land use type in the Three Gorges Reservoir area. There are the significant impacts of slop farmland on the regional ecological environment, due primarily to the soil erosion resulting from agricultural practices. Soil erosion patterns on sloping farmland can depend mainly on the climates and terrains. Additionally, the soil and water conservation can vary greatly in different regions and seasons. Crop coverage can be expected as the key measure for the soil erosion control. This study aims to investigate the relationship between slope sediment and runoff under different rainfall and terrain conditions. The dataset was taken from the 108 natural rainfall observations from 2021 to 2023. Rainfall events were then classified into the patterns using four indicators: rainfall amount, duration, intensity, and the maximum 30-minute intensity. K-means clustering analysis was also employed on the dataset. Data on runoff and sediment yield was collected from five experimental plots with different slope crop coverages, taking the bare slopes as the control. A systematic investigation was implemented on the effects of various rainfall characteristics (type, intensity, and maximum 30-minute intensity) and five slope gradients (5°, 10°, 15°, 20°, 25°) on the runoff and sediment yield. The sediment and runoff reduction benefits of crop coverage also assessed to quantify the contribution rates of rainfall type and slope gradient to sediment and runoff yield on crop-covered slopes. Additionally, Pearson correlation analysis was used to explore the relationship between runoff and sediment yield of different surfaces with rainfall characteristics and slope. The findings indicate that: 1) According to four rainfall characteristic indicators (rainfall amount, duration, intensity, and maximum 30-minute intensity), the 108 rainfall events were categorized into three patterns: A-type (low frequency, long duration, heavy rainfall, and low intensity), B-type (medium frequency, medium duration, moderate rainfall, and moderate intensity), and C-type (high frequency, short duration, light rainfall, and high intensity). C-type rainfall was dominated as the erosive type in the study area. Critical thresholds of rainfall amount, intensity, and maximum 30-minute intensity on the slope erosion were 6~21 mm, 1.1-3.6 mm/h, and 1.2-7.6 mm/h, respectively. 2) Rainfall amount, intensity, and maximum 30-minute intensity were also dominated the runoff generation on both bare and crop-covered slopes (P<0.05). However, the slope sediment yield was correlated with the runoff generation (P<0.01). Furthermore, the sediment yield from C-type rainfall was 4.77 to 17.69 times greater than that from A-type and B-type rainfall under the same runoff. The fitting coefficient of the linear relationship between sediment and water coverage of various crops increased by 1.33 to 5.06 times, as the slope increased. 3) The impact of various rainfall types on the runoff and sediment generation on the bare slopes and crop-covered slopes followed the descending order: C-type > B-type > A-type. The average contributions of C-type rainfall to runoff and sediment production on slopes were 48.19% and 81.82%, respectively. Crop coverage was significantly mitigated the soil erosion on slopes, in order to reduce the runoff and sediment of 61.67% and 77.15%, respectively. The slope erosion shared the critical slope between 15° and 25°, as the slope increased. The stronger the rainfall was, the less the impact of the slope on runoff and sediment production on the slope surface was. The contribution rate of rainfall pattern to runoff and sediment yield on the crop-covered slopes surpassed 60%, where the influence was 2.60 to 5.05 times greater than that of slope gradient. The findings can offer the scientific guidance for the comprehensive management of the soil erosion in sloping farmland in the study area. Furthermore, the scientific basis and practical direction can provide for the standards for erosion control measures on sloping farmland.
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