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[Display omitted] • Developing an integrated sediment source assessment index based on long series hydrologic signals. • Developing Karst Surface Characteristic Index based on karst desertification factors. • Karst Surface Characteristic Index is closely related to hydrological processes. • Clockwise hysteresis is the most frequent and efficient type of sediment transport. The source and transport processes of suspended sediment are issues that require further study. The complex relationship between the integrated surface characteristics and the mechanisms of sediment sources and transport in heterogeneous karst watersheds remains unclear. This study proposes a comprehensive assessment framework for sediment sources that combines the surface characteristics of karst watersheds, such as exposed rock and shallow topsoil in karst areas. Additionally, the Karst Surface Characteristic Index (KSCI) is developed to evaluate the potential supply capacity of sediment in karst watersheds. The hysteresis index (HI mid) is calculated using a long series of hydrological data to study runoff and sediment relationships in eight watersheds in the karst region of southern China. Results indicated that plan curvature, terrain relief, NDVI, and rock exposure contributed most to KSCI, with these four factors contributing 65%. The Q-SSC hysteresis analysis results based on 445 hydrological events showed that a clockwise hysteresis pattern was the most frequent and efficient pattern of sediment transport in the studied watershed (clockwise hysteresis pattern accounted for 72.3% of 445 hydrological events) owing to the limited sediment supply in the karst watersheds. The quantitative analysis results of HI mid and KSCI showed that HI mid was closely related to KSCI. With an increase in KSCI, a clockwise hysteresis pattern was more likely to appear, indicating that watershed characteristics exerted a significant influence on sediment transport velocity and efficiency. In terms of spatial distribution, near the outlet of the watershed or the river channel, high KSCI values were distributed, and clockwise hysteresis was more likely to occur. In contrast, at the far end of the watershed outlet or the river channel, high values of KSCI were distributed, and anticlockwise hysteresis was more likely to occur. Our findings further enrich the theory of soil erosion and the runoff-sediment relationship at the watershed scale in karst areas, and serve as valuable decision-making references for integrated water and soil sources management. [ABSTRACT FROM AUTHOR]

• MEMD revealed scale-specific daily suspended sediment load (SSL) variation. • SSL dynamics were dominated by short time intervals of approximately 4–23 days. • Runoff was the principal driver of daily SSL at both short and long timescales. • Measurements at a single timescale cannot capture sufficient information on daily SSL dynamics. • MEMD can enhance daily SSL predictions. Temporal variation in sediment load is generally scale dependent and affected by many factors operating at different timescales. Due to the complexity of sediment transportation mechanisms and the nonlinear behavior of hydrological processes, identifying multi-timescale effects of variables on daily suspended sediment load (SSL) is challenging. Using multivariate empirical mode decomposition (MEMD), the aims of this study were to investigate scale-dependent relationships between daily SSL and associated variables and to predict daily SSL based on these relationships. Data on daily SSL in four typical karst catchments of southwest China and five potential variables affecting daily SSL (i.e., runoff, precipitation, air temperature, potential evapotranspiration, and Normalized Differential Vegetation Index) were collected during 2009–2012. The results indicated that MEMD decomposed temporal patterns of daily SSL and five potential variables affecting daily SSL into eight or nine intrinsic mode functions (IMFs) and a residue. Short timescale oscillations of IMF1 to IMF4 contributed more than 72 % of the total variance in daily SSL in the four catchments, indicating that temporal variation in SSL was dominated by short intervals of 4–23 days. Runoff was the dominant explanatory variable for the overall daily suspended sediment load prediction at the observation scale. The predictions of daily SSL based on the IMFs and their residuals greatly outperformed those based on the original time series data. This result implies that a single scale (e.g., observation scale) is not sufficient to capture the complex relationships that exist between daily SSL and associated variables at all timescales. The results demonstrate the superiority of MEMD in characterizing scale-specific relationships between daily SSL and associated variables and improving daily SSL predictions. Given the complexity, nonlinearity, and nonstationarity inherent in many hydrological and sediment transport processes, MEMD is recommended for identifying scale-specific variation in daily SSL in future studies. [ABSTRACT FROM AUTHOR]

• We determined the parameters of monthly sediment rating curve (SRC) in karst watersheds. • The SRC parameters of karst watersheds are compared with those of other watersheds in China. • PLS-SEM can decouple the direct and indirect effects of factors on SRC parameters. • Lithology and dam construction complicated the effect of climate and vegetation on SRC parameters. One of the largest contiguous karst areas in the world is southwest China, where special geological conditions, shallow soil layers, the frequency of precipitation extremes, unreasonable land use, and dam construction have rendered the relationship between water discharge and sediment load increasingly complex. Few studies have been devoted to this region to clarify the potential influencing factors of the water–sediment relationship on a monthly basis. Correspondingly, the present study evaluated the temporal variations in sediment rating curves (SRCs: Qs = a Q b , where Q s is the sediment concentration or load, Q is the water discharge, the coefficient a and index b are fitted parameters) and decoupled the effects of climate change and vegetation dynamics on SRC parameters using partial least squares-structural equation modeling (PLS-SEM). To these ends, 2003 to 2017 data on the monthly water discharge, sediment load, climatic and vegetation factors of six karst watersheds were collected and analyzed. No significant trends in interannual variations of SRC parameters occurred. The PLS-SEM showed that the combined effects of climate and vegetation explained 18% to 43% and 8% to 57% of the variations in coefficient a and index b , respectively. In most cases, however, no significant relationship was found among climate, vegetation-related factors, and the SRC parameters. Special hydrogeological conditions and dam construction in the area increasingly complicated the sediment transport process, thus potentially weakening the influence of climate and vegetation on the SRC parameters. This study can serve as guidance for regional ecological management and the sustainable development of soil and water resources. [ABSTRACT FROM AUTHOR]

• We determined the parameters of monthly sediment rating curve (SRC) in karst watersheds. • The SRC parameters of karst watersheds are compared with those of other watersheds in China. • PLS-SEM can decouple the direct and indirect effects of factors on SRC parameters. • Lithology and dam construction complicated the effect of climate and vegetation on SRC parameters. One of the largest contiguous karst areas in the world is southwest China, where special geological conditions, shallow soil layers, the frequency of precipitation extremes, unreasonable land use, and dam construction have rendered the relationship between water discharge and sediment load increasingly complex. Few studies have been devoted to this region to clarify the potential influencing factors of the water–sediment relationship on a monthly basis. Correspondingly, the present study evaluated the temporal variations in sediment rating curves (SRCs: Qs = a Q b , where Q s is the sediment concentration or load, Q is the water discharge, the coefficient a and index b are fitted parameters) and decoupled the effects of climate change and vegetation dynamics on SRC parameters using partial least squares-structural equation modeling (PLS-SEM). To these ends, 2003 to 2017 data on the monthly water discharge, sediment load, climatic and vegetation factors of six karst watersheds were collected and analyzed. No significant trends in interannual variations of SRC parameters occurred. The PLS-SEM showed that the combined effects of climate and vegetation explained 18% to 43% and 8% to 57% of the variations in coefficient a and index b , respectively. In most cases, however, no significant relationship was found among climate, vegetation-related factors, and the SRC parameters. Special hydrogeological conditions and dam construction in the area increasingly complicated the sediment transport process, thus potentially weakening the influence of climate and vegetation on the SRC parameters. This study can serve as guidance for regional ecological management and the sustainable development of soil and water resources. [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]