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Climate change and human activities exert significant influence on the water–sediment relationship in arid and semi-arid regions. Therefore, comprehending the underlying mechanisms is crucial for the effective management of water and soil resources, as well as integrated watershed management. This research focuses on the Kuye River watershed (KYH_W) in the middle reaches of the Yellow River in China, along with its sub-watersheds Wangdaohengtazi (WDHT_SW) and Xinmiaosi (XM_SW). This paper utilizes the Mann–Kendall non-parametric test and the double cumulative curve method to examine the interannual trends of runoff, sediment transport, precipitation, temperature, and NDVI factors. Furthermore, the method of the slope change ratio of cumulative quantity (SCRCQ) is utilized to quantitatively evaluate the impacts and contribution rates of climate change and human activities on water–sediment changes within each watershed. The results are as follows: (1) From 1969 to 2019, the entire watershed experienced a significant decrease in both runoff and sediment transport, with 1997 marking the year of abrupt change. However, following 2012, the KYH_W and WDHT_SW exhibited a noticeable rebound in runoff. (2) Human activities predominantly contribute to the reduction in water and sediment in the watershed. (3) After the abrupt change, between 1998 and 2011, the contribution rates of climate change and human activities to the annual runoff reduction in the entire KYH_W reached 33% and 64%, respectively. Moreover, these rates for sediment transport reduction reached 26% and 74%, respectively. Subsequently, after 2012, the contribution rates of both factors to the increase in watershed runoff reached 29% and 71%, respectively. Factors other than the NDVI, within human activities, played a dominant role in augmenting the watershed's runoff. (4) Prior to 2011, changes in vegetation cover resulting from the Grain for Green Program, as measured by the NDVI, emerged as the primary factor responsible for reduced runoff in the watershed. Conversely, factors other than the NDVI assumed dominance in reducing sediment transport. The SCRCQ method offers a quantitative approach to assessing water–sediment changes. Based on this method, the study further underscores the substantial impacts of climate change and human activities on variations in runoff and sediment transport within the KYH_W in the middle reaches of the Yellow River. Notably, the water–sediment changes in the KYH_W exhibit distinct stage-wise and spatial discrepancies, which warrant increased attention in future research endeavors. [ABSTRACT FROM AUTHOR]

Investigating the changes in the runoff and sediment coupling relationship in the Middle Yellow River Basin of China can not only deepen the understanding of soil loss control in arid areas, but also help solve key issues of regional ecological protection. Since the 1960s, soil- and water-conservation projects have been implemented in the Middle Yellow River Basin, inducing a significant reduction in runoff and sediment and changes in the relationship between runoff and sediment. The study identified the change points of coupling relationship between runoff and sediment in the Wuding River Basin (WRB) by constructing a diagnostic method based on coupling coordination degree and the Pettitt test; the study validated this using the Copula function and analyzed the impacts of erosion energy and underlying surface factors. The results showed the following: (1) the method based on coupling coordination degree and the Pettit test could accurately reflect the coupling relationship of runoff and sediment and identify two change points (1971 and 1996); (2) runoff and sediment in the WRB decreased gradually over three periods (P1, 1960–1970; P2, 1971–1995; P3, 1996–2020), with an average annual runoff of 15.34 × 108, 10.72 × 108, and 8.32 × 108 m3 and average annual sediment load of 1.84 × 108, 0.82 × 108, and 0.32 × 108 t, respectively; (3) the maximum possible joint design value of runoff and sediment under different return periods in P1 were all the highest, followed by P2 and P3, and the larger the return period, the higher the maximum possible joint design value; (4) runoff erosion power could promote runoff and sediment in PE (1960–2020), P1, P2 and P3 at a significant level, check dams and terrace could decrease runoff and sediment significantly in PE, and the highest contribution to runoff reduction was check dam (95.4%), while the highest contribution to sediment reduction was REP (93.8%). The study can provide a new way to analyze the changes in the runoff and sediment relationship and provide scientific support for runoff and sediment regulation in the Middle Yellow River Basin. [ABSTRACT FROM AUTHOR]

• Calculation unit division induced soil spatial aggregation in SWAT model. • Effect of soil spatial aggregation on simulation was separated and quantified.Soil spatial aggregation caused a significant increase in annual runoff and sediment load. • The effect of soil spatial aggregation on sediment load was greater than on runoff. • Optimal sub-watershed division scheme was discussed. Distributed hydrological model divides watersheds into distinct calculation units to simulate hydrological processes. Within the model, different calculation unit division schemes induce input elements, such as soil and land use, to certain degrees of generalization, thus affecting the simulation result. Analyzing the effect of the calculation unit division on the simulation result is an important methodology to reduce uncertainty and improve the simulation accuracy of the model. Previous researchers have focused on the comprehensive effect of underlying surface generalization (such as soil, land use, and terrain generalization) caused by the calculation unit division on the simulation results, while it did not separate the effect of the generalization of the individual input factor on the simulation result. In this paper, the Dongjiang headwater region in southern China was selected as the study area. The single type of land use and slope were input into the Soil and Water Assessment Tool (SWAT) model to fix the effects of land use and terrain caused by calculation unit division on the simulation results. Eleven calculation unit division schemes were used to analyze the impact of soil spatial aggregation on runoff and sediment load simulations. The calculation unit division schemes included 11 sub-watershed division schemes and their hydrological response unit (HRU) thresholds were set at 8 %. The drainage area thresholds of the 11 sub-watershed division schemes were 30 ha, 60 ha, 120 ha, 240 ha, 480 ha, 1000 ha, 1600 ha, 2200 ha, 2800 ha, 3400 ha and 4000 ha. The results demonstrated that the soil spatial distribution showed an obvious aggregation with an increase in the drainage area threshold in the SWAT model. Of these, the Krasnozem soil type which covers the largest area in the watershed, increased significantly with an increase in the drainage area threshold. However, the areas of all the other soil types decreased significantly. The Lioth soil type covers the smallest area in the watershed decreased and even disappeared when the threshold in the SWAT model was greater than 1000 ha. Furthermore, soil spatial aggregation caused a significant increase in the annual runoff and sediment load (P < 0.05), with the sediment load (Re = 29.78 %) increasing at a greater rate compared to runoff (Re = 1.74 %). Increased soil aggregation significantly increased the sediment loads in the maximum 1-day, the continuous maximum 5-day, the continuous maximum 7-day, and the sediment load in flood processes (P < 0.001). Refining the calculation unit division scheme will cause a significant increase in the number of calculation units. Due to the computer processing power limitations, we cannot further refine the calculation unit division scheme. Based on our current operational calculation unit division scheme, we did not identify the optimal division scheme in the study region. These results can provide reference information for future research into hydrological model uncertainty and can improve the SWAT model simulation accuracy. [ABSTRACT FROM AUTHOR]

Summary: Annual runoff and annual suspended sediment loads of hydrological gauging stations along the mainstream of the Yangtze River basin (Pingshan station, Yichang station, Hankou station and Datong station) and main tributaries (Beipei station in Jialingjiang River, Wulong station in Wujiang River and Huangzhuang station in Hanjiang River) were analyzed with the help of Mann–Kendall trend analysis and linear regression analysis. Research results indicate that (1) changing patterns of runoff and sediment loads are different in different parts of the Yangtze River basin. No significant trend is detected for annual runoff at all stations at >95% confidence level. Changes of sediment loads, however, demonstrate different pictures in the Yangtze River basin. The sediment loads are in increasing trend in Pingshan station- the most upstream station on the Yangtze River basin (this increasing trend is significant at >95% confidence level after about 1990), but are in decreasing trend at other stations (including stations in the tributaries studied in this paper). This decreasing trend becomes more obvious from Yichang station to Datong station. (2) Water reservoirs exerted more influences on changes of sediment loads than on runoff, which is the main reason for the decreasing trend of sediment loads found in most stations. (3) Influences of water reservoirs on sediment loads are more obvious in the tributaries than in the mainstream of the Yangtze River basin, while in the mainstream the variation patterns of sediment loads are determined by multiple factors. [Copyright &y& Elsevier]

Investigation of the variations in runoff, sediment load, and their dynamic relation is conducive to understanding hydrological regime changes and supporting channel regulation and fluvial management. This study is undertaken in the Xihanshui catchment, which is known for its high sediment‐laden in the Jialing River of the Yangtze River basin, southern China, to evaluate the change characteristics of runoff, sediment load, and their relationship at multi‐temporal scales from 1966 to 2016. The results showed that runoff changed significantly for more months, whereas the significant changes in monthly sediment load occurred from April to September. The contributions of runoff in summer and autumn and sediment load in summer to their annual value changes were greater. Annual runoff and sediment load in the Xihanshui catchment both exhibited significant decreasing trends (p < 0.05) with a significant mutation in 1993 (p < 0.05). The average annual runoff in the change period (1994–2016) decreased by 49.58% and annual sediment load displayed a substantial decline with a reduction of 77.77% in comparison with the reference period (1966–1993) due to climate change and intensive human activity. The power functions were satisfactory to describe annual and extreme monthly runoff–sediment relationships, whereas the monthly runoff–sediment relationship and extreme monthly sediment‐runoff relationship were changeable. Spatially, annual runoff–sediment relationship alteration could be partly attributed to sediment load changes in the upstream area and runoff variations in the downstream region. Three quantitative methods revealed that the main driver for significant reductions of annual runoff and sediment load is the human activity dominated by soil and water conservation measures, while climate change only contributed 22.73%–38.99% (mean 32.07%) to the total runoff reduction and 3.39%–35.56% (mean 17.32%) to the total decrease in sediment load. [ABSTRACT FROM AUTHOR]

Determining abrupt changes in runoff and sediment load may not only enhance identification of the principal driving factors for such changes but also help establish effective countermeasures for serious water deficit by managers in the Yellow River basin. We used the Mann-Kendall trend test and linear regression to determine trends and abrupt changes of runoff and sediment load during the period between 1950 and 2005, based on monthly hydrological data. Results show that runoff and sediment load decreased from 1950 to 2005, on annual or monthly time scales. Their changes are divided into three stages: fluctuating stage (1950-1970), slowly decreasing stage (1970-1980) and accelerated decreasing stage (1980-2005). The relationship between runoff and sediment load was most significant, and it can be expressed as a linear regression function. Precipitation was one of the most important climate factors affecting runoff before 1985, and the impact of human activities on runoff decrease grew strongly after 1985. Water balance analysis of the Yellow River basin indicates that natural climate change contributed about 55.3% and human activities about 44.7% to the runoff decrease after 1986. [ABSTRACT FROM AUTHOR]