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The runoff-sediment relationship is one of the most important aspects to reflect the variation of runoff and sediment. There is a significant impact of the rainstorm on the runoff collection and sediment transport in the watershed. It is a high demand to explore the runoff-sediment relationship under the condition of a single rainstorm in the control of soil and water loss. Heretofore, most research was focused mainly on the northern region of China. It is still lacking in the southern region with frequent rainstorm and flood. Taking the Lianshui watershed located in southern Jiangxi Province as the research area, this study aims to launch the project of forest rehabilitation from slope agriculture, large-scale orchard development, and urbanization construction over the past 30 years. The underlying surface was changed significantly. The characteristics of runoff and sediment were analyzed to determine the relationship between them under different flood types and periods using a cluster, discriminant, and Spearman correlation analysis, according to the hydro-meteorological and land use data from 1965 to 2020 under the condition of a single rainstorm. The hysteretic relationship was then established between the runoff and sediment. The results showed that: 1) The 120 rainstorm floods in the watershed were divided into three types, with the occurrence frequency from the high to the low as Type III, Type I, and Type II, where the proportion were 63.3%, 32.5%, and 4.2%, respectively. The smallest eigenvalue was found in the runoff and sediment transport of Type III flood. The longest flood duration was in the Type I flood. The other eigenvalues were in the middle level. The Type II flood presented the largest eigenvalue, except for the middle-level flood duration. There was a highly correlated runoff depth and sediment transport in the three flood types. The Spearman correlation coefficients were 0.515, 0.800, and 0.481, respectively. 2) Five periods (1965-1975, 1976-1985, 1986-1995, 1996-2005, and 2006-2020) were divided by the whole study period. There was the highest average value of runoff depth and flood duration in 1986-1995, whereas, the average value of peak flow reached the highest in 1976-1985. The eigenvalue of sediment transport in 1996-2005 and 2006-2020 increased significantly, compared with the previous three periods, while the sediment transport capacity of unit runoff raised significantly. 3) The proportion of runoff-sediment loop curves in the type of the clockwise, counterclockwise, positive figure-eight, reverse figure-eight, and compound type were 18.3%, 17.5%, 26.7%, 15.8%, and 21.7%, respectively, among the 120 rainstorm floods. Meanwhile, there was a great variation in the proportion of runoff-sediment loop curves under different flood types. The highest proportion in Type I, Type II and Type III flood were the compound (33.3%), clockwise (40.0%), and positive figure-eight type (30.3%), respectively. However, the highest proportion of counterclockwise and figure-eight type curves was found in the period of 1996-2005 and 2006-2020, respectively, which was closely related to the changes in the location and quantity of sediment sources caused by the large-scale development of orchards in the watershed. The proportion of compound type curve reached the maximum in 1986-1995, due mainly to the high average value of runoff depth and flood duration. The finding can provide the scientific basis for the management and regulation of runoff and sediment in the watershed, in order to implement soil and water conservation measures [ABSTRACT FROM AUTHOR]

Rainstorm has been one of the key factors causing runoff and soil erosion in the Yellow River Basin of China. The check dam can serve as one of the most important engineering measures for soil and water conservation during soil erosion control in the Loess Plateau. However, the check damn has posed a great impact on the formation, evolution, and development of the rainstorm runoff process via the changing runoff, erosion dynamics, and energy distribution in the watershed. Therefore, it is crucial to reveal the various elements nexus in the erosion system of the watershed, in order to explore the disaster mechanism of rainstorm runoff under the action of soil and water conservation measures. This study aims to quantitatively reveal the influence of the check dam configuration on the formation and rainstorm runoff evolution in small watersheds of loess hilly regions. An indoor experiment of artificial simulated rainfall was carried out to analyze the hydrodynamics and erosion dynamic characteristics of runoff at different sections in small watersheds with the different measures (no dam, single, and double dam) and the rainfall intensity as 60 mm/h, according to scale relationship. The results showed that: 1) The sediment concentration of runoff increased significantly, as the time and erosion increased the most in the case of a no-check dam. The maximum increase of erosion (runoff) was more than 114.60 cm³/s (only 70.11 cm³/s) in the case of no (double) dam. The sediment concentration values under the No. 1 single dam, No. 2 single and double dam were reduced by 0.14, 0.14, and 0.19 g/cm³, respectively, compared with that without the dam. 2) The check dams posed a significant impact on the evolution process of rainstorms and runoff in small watersheds. The double dam was the most effective to restrain the increase of velocity, while the limited control effect, compared with the single dam. The check dam had also a great influence on the runoff flow state, where the increasing resistance coefficient determined the runoff flow and sediment transport capacity. There was a time-delayed runoff from the laminar to a turbulent state, which increased the stability of flow state change. The runoff state started to change in 10-15 min without the dam, but without change in the check dam until 50 min later. 3) The check dam reduced the runoff power and runoff erosion power, further delaying the growth trend of runoff erosion power. The runoff power under the no-dam scenario was 1.42-2.45 (1.59-2.64) times that under the No.1 (double) dam scenario. More importantly, the flow pattern changed from the turbulent to laminar slow flow, where the Reynolds number decreased 12.04%-85.85% after the construction of the check dam. 4) The runoff process depended mainly on the runoff power, Reynolds number, runoff erosion power, and average velocity in small watersheds. The dominant parameters were ranked in the descending order of the runoff power, Reynolds number, runoff erosion power, and average velocity. The erosion dynamic parameters also posed a much greater impact on the evolution of runoff than the hydrodynamic force parameters. Consequently, the runoff power and Reynolds number can be expected to serve as the dynamic factors for the erosion sediment yield under different siltation dam configurations. The findings can offer practical implications for the pregnant environment and the disaster mechanism of rainstorms and runoff in the basin. A theoretical basis can also be used to evaluate the effect of soil and water conservation engineering measures on the regulation of the runoff process. [ABSTRACT FROM AUTHOR]

• Under the action of concentrated flow, the change of surface runoff of karst gently sloping farmland is mainly controlled by the magnitude of concentrated flow. • Under the action of concentrated flow, the underground runoff of karst gentle slope farmland is significantly negatively correlated with the slope. • Concentrated flow can lead to severe soil erosion in karst gently sloping farmland. Exposed bedrock in karst areas considerably alters runoff paths causing partial runoff to converge into a concentrated flow, which is a potential source of soil erosion risk. However, research is lacking on how concentrated flow affects regional soil erosion. In this study, the surface and underground dual structure in karst areas was simulated in a steel tank. Combined with runoff plot monitoring, the runoff conversion process and erosion characteristics of gently sloping farmland in karst areas under different water discharge rates were examined through indoor concentrated flow discharge scouring tests, and the influence of concentrated flow changes on the runoff spatial distribution and sediment yield in gently sloping farmland was clarified. The results demonstrated the following: (i) there existed a significant positive correlation between surface runoff and the water discharge rate (p < 0.01); the partial correlation coefficient value reached 0.87. The higher the water discharge rate, the more surface runoff formation was facilitated. (ii) When the slope remained the same, the underground runoff was relatively high at 3 L/min. Given the same discharge, a significant negative correlation between underground runoff and slope was found (p < 0.05); the partial correlation coefficient value reached −0.71. (iii) In response to a water discharge rate of 5 L/min, surface soil erosion was serious, and the maximum sediment transport modulus was 174.088 g/min∙m2. A significant cubic function relationship existed between the scouring time and surface sediment yield. Overall, a critical flow rate occurred between 3 and 5 L/min, which controlled changes in surface runoff, subsurface runoff and surface sediment yield. Under highly concentrated flow, drastic soil erosion could occur. These results can provide a scientific basis for the control and prevention of concentrated flow-induced erosion in karst areas. [ABSTRACT FROM AUTHOR]

Earthworms are ecological engineers that play an important role in hydrological processes and soil erosion by their burrowing and casting activities. Little is known, however, about the ecological function of the activities of an anecic species (Metaphire guillelmi) from the Loess Plateau in China where the soil is severely eroded. We conducted rain-simulation experiments on artificial slopes (mesocosms) filled with soil from Loess Plateau. Two rain intensities (90 and 120 mm h−1) and slope gradients (5° and 15°) were used to investigate the influences of earthworm activity on runoff and soil loss under multiple conditions. Data from 15 d of earthworm activity and 1 h of rain indicated that earthworm activity significantly reduced runoff but aggravated soil erosion. Runoff initiation time increased 2.7–3.1-fold, contributing to the promotion of water infiltration, and the amounts and rates of runoff were 35.8–61.6% and 35.1–60.8% lower under earthworm activity than the controls, respectively. Earthworms greatly influenced the physical and chemical properties of the soil, and these properties were strongly correlated with runoff initiation time and cumulative runoff. The surface casts produced by the earthworms on the soil surface were completely dispersed, increasing the cumulative sediment by a maximum of 169%. Both rain intensity and slope gradient greatly influence runoff and soil detachment, but with earthworm activity, slope gradient was more influential due to its superior contribution to sediment transport. This study quantified the effects of M. guillelmi on runoff and soil erosion and provides basic data for developing a more integrated control of soil erosion on the Loess Plateau. • The increase in infiltration related to earthworm activities decreases runoff. • Soil erosion is aggravated by the low stability of surface cast under intense rainfall. • Earthworm impact on soil loss is larger at lower rain intensity and steeper slope. [ABSTRACT FROM AUTHOR]