The loess hilly area consists of a slope–gully system, which promotes erosion; as such. it is one of the most intensely eroded areas in the world. The construction of check dams can effectively control water and soil loss of slope gullies. However, existing studies focus on the benefits of intercepting runoff and sediments at dam sites, while ignoring the change law of hydrological processes with respect to progressing dam land sedimentation. Moreover, past studies focus on the “runoff–sediment” or “flood–sediment” relationships, but rarely consider the “hydrodynamics–runoff” and “hydrodynamics–sediment” dynamics. Therefore, in this study, we developed five physical models of slope–gully systems for dam land sedimentation depths of 0, 1, 2, 3, and 4 m, in order to explore the effects of sedimentation on runoff–sediment–hydrodynamic processes. The runoff and sediment yield of the slope–gully system decreased with increasing siltation depth. The spatial and temporal distributions of the hydrodynamic parameters were different. The Reynolds number (Re), runoff energy consumption (ΔE), and runoff power (P) increased with rainfall time, whereas runoff shear stress (τ) and Froude number (Fr) did not show a significant trend over time. Re and ΔE could better describe the runoff process of the slope–gully system, while P and ΔE could better simulate the sedimentation process. Notably, our study can provide a scientific basis for establishing effective erosion prediction models to estimate the water erosion process of slope–gully systems.

Soil organic carbon (SOC) plays an essential role in the carbon cycle and global warming mitigation, and it varies spatially in relation to other soil and environmental properties. But the national distributions and the impact mechanisms of SOC remain debated in China. Therefore, how soil texture and climate factors affect the SOC content and the regional differences in SOC content were explored by analyzing 7857 surface soil samples with different land-use. The results showed that the SOC content in China, with a mean value of 11.20 g·kg −1 , increased gradually from north to south. The SOC content of arable land in each geographical area was lower than in grassland and forest-land. Although temperature also played a specific role in the SOC content, precipitation was the most critical climate factor. The SOC content was positively correlated with the silt and clay content. The lower the temperature, the greater the effect of environmental factors on SOC. In contrast, the higher the temperature, the more significant impact of soil texture on SOC. The regional difference in SOC highlights the importance of soil responses to climate change. Temperature and soil texture should be explicitly considered when predicting potential future carbon cycle and sequestration.

Soil erosion is the worldly environmental and ecological problems. How to accurately identify the source of the sediment is important for soil and water conservation. Fingerprint identification technology has been widely used in the extraction of sediment source proportion, but currently most of the methods used to evaluate the accuracy of the results are a-good-fit (GOF) or a mean absolute error (MAE). We propose a new method to evaluate the accuracy of the sediment source mixing model and quantitatively evaluate the two sediment source mixing models. A typical check dam in the Loess Plateau was used to evaluate the new method by combining field sampling and numerical simulation. Collins (C) and Modified Hughes mixing (M-H) models were used to quantitatively analyze the sediment sources in the dam-control watershed. The results showed that the optimum composite fingerprints were Mg, Cr, Ni, and TOC, and they had 97.2% discrimination ability. The contribution rates of sediment source from gully, farmland, grassland and branch ditch were 54%, 24%, 15% and 7%, respectively. The M-H mixing model had a higher comprehensive score (2.26) when compared with the C mixing model (2.20). The comprehensive evaluation method could provide an effective scientific theoretical basis for optimal allocations of water and soil conservation in small watersheds.

Soil erosion poses serious challenge to the sustainable utilization of soil and water resources worldwide. Vegetation restoration can sufficiently improve the environment and reduce soil erosion. It is important to evaluate the effects of vegetation coverage and position on surface runoff and sediment yield. Therefore, this study simulated four vegetation coverage rates (0%, 20%, 40%, and 60%) and two vegetation distribution positions (downslope and upslope) to study the runoff and sediment yield under three rain intensities (60, 90, and 120 mm h(-1)). The results showed that the runoff decreased as vegetation coverage increased: 0% > 20% > 40% > 60%. The minimum total runoff occurred at 60% vegetation coverage rate under 60 mm h(-1)- rain intensity (57.5 L), and the maximum at 0% vegetation coverage rate under 120 mm h(-1) rain intensity (172.2 L). The process of sedimentation was similar to that of runoff. As the rate of vegetation coverage increased, the sediment yield decreased. The total sediment yields under 40% and 60% vegetation coverage rates were significantly lower than that under 0%. In addition, the runoff volume and sediment yield of the upslope vegetation pattern were considerably higher than those of the downslope pattern. Under 60 mm h(-1) rainfall intensity, the total runoff volume and sediment yield of upslope pattern were 70.6 L and 261.1 g, as opposed to 61.6 L and 174.9 g of downslope pattern, respectively. By analyzing the correlations of runoff and sediment yield with vegetation coverage, the interception effect on runoff and sediment yield increased with the increase in vegetation coverage. Moreover, the downslope pattern contributed more in reducing runoff and sediment yield than the upslope pattern. The results can provide scientific basis for vegetation restoration on the Loess Plateau.