Runoff change in the Yellow River Basin of China from 1960 to 2020 and its driving factors

Analysing runoff changes and how these are affected by climate change and human activities is deemed crucial to elucidate the ecological and hydrological response mechanisms of rivers.The Indicators of Hydrologic Alteration and the Range of Variability Approach(IHA-RVA)method,as well as the ecological indicator method,were employed to quantitatively assess the degree of hydrologic change and ecological response processes in the Yellow River Basin from 1960 to 2020.Using Budyko's water heat coupling balance theory,the relative contributions of various driving factors(such as precipitation,potential evapotranspiration,and underlying surface)to runoff changes in the Yellow River Basin were quantitatively evaluated.The results show that the annual average runoff and precipitation in the Yellow River Basin had a downwards trend,whereas the potential evapotranspiration exhibited an upwards trend from 1960 to 2020.In approximately 1985,it was reported that the hydrological regime of the main stream underwent an abrupt change.The degree of hydrological change was observed to gradually increase from upstream to downstream,with a range of 34.00%-54.00%,all of which are moderate changes.However,significant differences have been noted among different ecological indicators,with a fluctuation index of 90.00%at the outlet of downstream hydrological stations,reaching a high level of change.After the mutation,the biodiversity index of flow in the middle and lower reaches of the Yellow River was generally lower than that in the base period.The research results also indicate that the driving factor for runoff changes in the upper reach of the Yellow River Basin is mainly precipitation,with a contribution rate of 39.31%-54.70%.Moreover,the driving factor for runoff changes in the middle and lower reaches is mainly human activities,having a contribution rate of 63.70%-84.37%.These results can serve as a basis to strengthen the protection and restoration efforts in the Yellow River Basin and further promote the rational development and use of water resources in the Yellow River.

Quantitative distinction of the relative actions of climate change and human activities on vegetation evolution in the Yellow River Basin of China during 1981-2019

Under the combined influence of climate change and human activities,vegetation ecosystem has undergone profound changes.It can be seen that there are obvious differences in the evolution patterns and driving mechanisms of vegetation ecosystem in different historical periods.Therefore,it is urgent to identify and reveal the dominant factors and their contribution rates in the vegetation change cycle.Based on the data of climate elements(sunshine hours,precipitation and temperature),human activities(population intensity and GDP intensity)and other natural factors(altitude,slope and aspect),this study explored the spatial and temporal evolution patterns of vegetation NDVI in the Yellow River Basin of China from 1989 to 2019 through a residual method,a trend analysis,and a gravity center model,and quantitatively distinguished the relative actions of climate change and human activities on vegetation evolution based on Geodetector model.The results showed that the spatial distribution of vegetation NDVI in the Yellow River Basin showed a decreasing trend from southeast to northwest.During 1981-2019,the temporal variation of vegetation NDVI showed an overall increasing trend.The gravity centers of average vegetation NDVI during the study period was distributed in Zhenyuan County,Gansu Province,and the center moved northeastwards from 1981 to 2019.During 1981-2000 and 2001-2019,the proportion of vegetation restoration areas promoted by the combined action of climate change and human activities was the largest.During the study period(1981-2019),the dominant factors influencing vegetation NDVI shifted from natural factors to human activities.These results could provide decision support for the protection and restoration of vegetation ecosystem in the Yellow River Basin.

Initiatives to clarify mechanisms of hydrological evolution in human-influenced Yellow River Basin

Significant changes in water cycle elements/processes have created serious challenges to regional sustainability and high-quality development in the Yellow River Basin in China.It is necessary to investigate the impacts of climate change and human activities on hydrological evolution and disaster risk from a holistic perspective of the basin.This study developed initiatives to clarify the mechanisms of hydrological evolution in the human-influenced Yellow River Basin.The proposed research method includes:(1)a tool to simulate multiple factors and a multi-scale water cycle using a grid-based spatiotemporal coupling approach,and(2)a new algorithm to separate the responses of the water cycle to climate change and human impacts,and de-couple the eco-environmental effects using artificial intelligence techniques.With this research framework,key breakthroughs are expected to be made in the understanding of the impacts of land cover change on the water cycle and blue/green water redirection.The outcomes of this research project are expected to provide theoretical support for ecological protection and water governance in the basin.

Preparation and Bloating Mechanism of Porous Ultra-lightweight Ceramsite by Dehydrated Sewage Sludge and Yellow River Sediments

To solve the disposal problems of solid wastes, dehydrated sewage sludge and Yellow River sediments were tested as components for production of ultra-lightweight ceramsite. The effects of Yellow River sediments addition on the characteristics of ceramsite were investigated. Ceramsite with different Yellow River sediments additions was characterized using thermal analysis, X-ray diffraction, morphological structures analyses, pore size distributions and porosity analyses. Chemical components, especially ratios of Si O2 ＋ Al2O3/Flux, were used to explain the glassy shell formation, physical properties and pores distribution of ultralightweight ceramsite; physical forces for instance expansion force and frictional resistance which combined with Si O2 ＋ Al2O3/Flux ratios were used to explain the bloating mechanism. Results showed that the maximum addition of Yellow River sediments for making ultra-lightweight ceramsite was 35%. Macropores（between 0.226 μm and 0.554 μm） of ultra-lightweight ceramsite were dominant in the pore structures of ultra-lightweight ceramsite and its porosity was up to 67.7%. Physical force of expansion force was constant with the variation of Yellow River sediments content and physical force of frictional resistance was decreased with the increase of Yellow River sediments addition. The relationship between expansion and frictional resistance could determine the expansion rate of ceramsite. Larger pores inside the ceramsite bodies could be obtained as Yellow River sediments additions ranged from 10% to 30%. Ceramsite with higher Yellow River sediments additions of 40%（Si O2 ＋ Al2O3/Flux ratios 4.25） became denser and have lower porosity. Crystal components analysis proved that the sintering process made some components of raw materials transfer into other crystals having better thermostability.

Impacts of Land-use and Land-cover Changes on River Runoff in Yellow River Basin for Period of 1956–2012

River runoff is affected by many factors, including long-term effects such as climate change that alter rainfall-runoff relationships, and short-term effects related to human intervention(e.g., dam construction, land-use and land-cover change(LUCC)). Discharge from the Yellow River system has been modified in numerous ways over the past century, not only as a result of increased demands for water from agriculture and industry, but also due to hydrological disturbance from LUCC, climate change and the construction of dams. The combined effect of these disturbances may have led to water shortages. Considering that there has been little change in long-term precipitation, dramatic decreases in water discharge may be attributed mainly to human activities, such as water usage, water transportation and dam construction. LUCC may also affect water availability, but the relative contribution of LUCC to changing discharge is unclear. In this study, the impact of LUCC on natural discharge(not including anthropogenic usage) is quantified using an attribution approach based on satellite land cover and discharge data. A retention parameter is used to relate LUCC to changes in discharge. We find that LUCC is the primary factor, and more dominant than climate change, in driving the reduction in discharge during 1956–2012, especially from the mid-1980 s to the end-1990 s. The ratio of each land class to total basin area changed significantly over the study period. Forestland and cropland increased by about 0.58% and 1.41%, respectively, and unused land decreased by 1.16%. Together, these variations resulted in changes in the retention parameter, and runoff generation showed a significant decrease after the mid-1980 s. Our findings highlight the importance of LUCC to runoff generation at the basin scale, and improve our understanding of the influence of LUCC on basin-scale hydrology.

Temporal Change of Runoff and Sediment Load and their Differential Response to Human Activities:A Case Study for a Well-vegetated Mountain Watershed of Southern China

This study examined the temporal trends of runoff and sediment load and their differential response to human activities in the Lishui river,a tributary of the Yangtze river in southern China.The long-term observation data at four gauging stations,generally involving two periods from 1954 to 1985 and from 2007 to 2011,were used.We detected no significant temporal trend for both the annual runoff volume(Q) and the annual suspended Sediment Load(SL) over more than 30 years before 1985.The flow duration curves and the Suspended Sediment Concentration(SSC) also hold constant before 1985.Compared with the period before 1985,SL has decreased by about 80% though Q remains unchanged for the period after 2007.Detailed examination shows that the flow duration curves after 2007 have changed with a significant decrease in the high-flow component,which acts as a major cause for the decreasing SL.In addition,SSC has decreased by several times,which also contributes to the decrease in SL after 2007.Both decreases in high-flow discharges and in SSC can be linked with recent human activities,mainly including vegetation establishment and dam constructions.The constant Q and the decreasing SL are also reported for the main stream of the Yangtze River and other major rivers in southern China,although they are orders of magnitude larger than our study area in drainage area size.The present study highlights the importance of high-flow discharges on SL and suggests that the use of SL is more appropriate to reflect environmental change than Q.

The Effects of Reclamation Activity and Yellow River Runoff on Coastline and Area of the Laizhou Bay,China

Study on morphological changes of a bay can help to identify the effects of anthropogenic activities on coastal environ-ment and guide the exploration of marine resources.In this paper,morphological data including coastline and water areas in five discrete years between 1968 and 2015 were selected and extracted from the remote sensing images and historical marine charts to study the morphological changes in Laizhou Bay(LZB),one of the bays in the southwest of the Bohai Sea.A systematic analysis on spatial variations of the coastline and the surface areas of different types of waters in LZB was conducted.The results showed that the surface area of LZB was decreased by 1253.2km^(2)in the last half century,which is 17.4%of the total in the 1970s.The areas of the natural wetland and the intertidal zone were decreased by 17.2%and 56.1%,respectively,and the average water depth varied from 9.05 m to 8.16m at low tide level from 1968 to 2015.The coastline and shape variations of the bay turned to be complex after the 1980s,and the shape index of LZB showed an increasing trend in more recent years.The centroid of the bay generally migrated to the northeast direction,i.e.,the direction of the center of the Bohai Sea,and the shrinking direction of the bay was consistent with the migration direction of the coastline.The reclamation area during 1968-2015 in LZB was 1201.7km^(2),and 94.1%was in the inter-tidal zone.The overall morphological change of the bay during the last half century was mainly controlled by the coastal reclamation activities,and the Yellow River runoff including the river course change and sediment load variation was also an important control-ling factor.

Diagnostic experiments for transport mechanisms of suspended sediment discharged from the Yellow River in the Bohai Sea

Five diagnostic experiments with a 3D baroclinic hydrodynamic and sediment transport model ECOMSED in couple with the third generation wave model SWAN and the Grant-Madsen bottom boundary layer model driven by the monthly sediment load of the Yellow River, were conducted to separately diagnose effects of different hydrodynamic factors on transport of suspended sediment discharged from the Yellow River in the Bohai Sea. Both transport and spatio-temporal distribution of suspended sediment concentration in the Bohai Sea were numerially simulated. It could be from the Yellow River cannot be delivered in concluded that suspended sediment discharged long distance under the condition of tidal current. Almost all of sediments from the Yellow River are deposited outside the delta under the condition of wind-driven current, and only very small of them are transported faraway. On the basis of wind forcing, sediments from the Yellow River are mainly transported north-northwestward, and others which are first delivered to the Laizhou Bay are continuously moved northward. An obvious 3D structure characteristic of sediment transport is produced in the wind-driven and tide-induced residual circulation condition. Transport patterns at all layers are generally consistent with circulation structure, but there is apparent deviation between the depth-averaged sediment flux and the circulation structure. The phase of temporal variation of sediment concentration is consistent with that of the bottom shear stress, both of which are proved to have a ten-day cycle in wave and current condition.

Evolution of the freeze-thaw cycles in the source region of the Yellow River under the influence of climate change and its hydrological effects

As an important water source and ecological barrier in the Yellow River Basin,the source region of the Yellow River(above the Huangheyan Hydrologic Station)presents a remarkable permafrost degradation trend due to climate change.Therefore,scientific understanding the effects of permafrost degradation on runoff variations is of great significance for the water resource and ecological protection in the Yellow River Basin.In this paper,we studied the mechanism and extent of the effect of degrading permafrost on surface flow in the source region of the Yellow River based on the monitoring data of temperature and moisture content of permafrost in 2013–2019 and the runoff data in 1960–2019.The following results have been found.From 2013 to 2019,the geotemperature of the monitoring sections at depths of 0–2.4 m increased by 0.16°C/a on average.With an increase in the thawing depth of the permafrost,the underground water storage space also increased,and the depth of water level above the frozen layer at the monitoring points decreased from above 1.2 m to 1.2–2 m.64.7%of the average multiyear groundwater was recharged by runoff,in which meltwater from the permafrost accounted for 10.3%.Compared to 1960-1965,the runoff depth in the surface thawing period(from May to October)and the freezing period(from November to April)decreased by 1.5 mm and 1.2 mm,respectively during 1992–1997,accounting for 4.2%and 3.4%of the average annual runoff depth,respectively.Most specifically,the decrease in the runoff depth was primarily reflected in the decreased runoff from August to December.The permafrost degradation affects the runoff within a year by changing the runoff generation,concentration characteristics and the melt water quantity from permafrost,decreasing the runoff at the later stage of the permafrost thawing.However,the permafrost degradation has limited impacts on annual runoff and does not dominate the runoff changes in the source region of the Yellow River in the longterm.

Impacts of land use/cover change on water balance by using the SWAT model in a typical loess hilly watershed of China

Land use/cover change(LUCC)plays a key role in altering surface hydrology and water balance,finally affect-ing the security and availability of water resources.However,mechanisms underlying LUCC determination of water-balance processes at the basin scale remain unclear.In this study,the Soil and Water Assessment Tool(SWAT)model and partial least squares regression were used to detect the effects of LUCC on hydrology and water components in the Zuli River Basin(ZRB),a typical watershed of the Yellow River Basin.In general,three recommended coefficients(R^(2)and E ns greater than 0.5,and P bias less than 20%)indicated that the output results of the SWAT model were reliable and that the model was effective for the ZRB.Then,several key findings were obtained.First,LUCC in the ZRB was characterized by a significant increase in forest(21.61%)and settlement(23.52%)and a slight reduction in cropland(-1.35%),resulting in a 4.93%increase in evapotranspiration and a clear decline in surface runoffand water yield by 15.68%and 2.95%at the whole basin scale,respectively.Second,at the sub-basin scale,surface runoffand water yield increased by 14.26%-36.15%and 5.13%-15.55%,respectively,mainly due to settlement increases.Last,partial least squares regression indicated that urbanization was the most significant contributor to runoffchange,and evapotranspiration change was mainly driven by forest expansion.These conclusions are significant for understanding the relationship between LUCC and water balance,which can provide meaningful information for managing water resources and the long-term sustainability of such watersheds.

Spatiotemporal Evolution of Runoff and Sediment of the Taohe River and Its Driving Analysis

In order to explore the spatial and temporal changes of runoff and sediment in the Taohe River and its driving mechanism,Spearman correlation coefficient method,Mann-Kendell mutation test method and ordered clustering method were used to analyze the changes of runoff and sediment discharge and their driving factors in four hydrological stations along the Taohe River from 1957 to 2016.The results showed that the correlation between runoff and sediment of the four hydrological stations along the Taohe River was significant,and the correlation coefficient was 0.728-0.984.The runoff and sediment transport in the interval showed an increasing and decreasing trend.The decrease rate of runoff was 133.82%-216.17%higher than that of Xiabagou station,and the decrease rate of sediment transport was 250.49%-4766.33%higher than that of Xiabagou station.The mutation year of the Taohe River runoff occurred in 1986,and the maximum decrease was 35%.The water-sediment relationship curves of different periods showed that the sediment discharge of the four stations changed abruptly around 1990,and the maximum reduction before and after the mutation was up to 73%,and the sediment discharge in the river channel decreased significantly.The research showed that human activities were the main driving factors for the change of water-sediment relationship in the Taohe River.

Study on the Operation Mechanism and Effect of the Yellow River Water and Sediment Regulation System

In order to scientifically deal with the problems of less water and more sediment in the Yellow River and the uncoordinated relationship between water and sediment,it is necessary to establish a perfect water and sediment regulation system.Through the calculation of the sediment transport capacity of the Yellow River and the application of the water and sediment regulation system,it is found that the sediment transport efficiency of the Yellow River will increase with the increase of water flow,and there will be an obvious inflection point near the flat discharge.The joint regulation of the backbone reservoir group can discharge the large discharge close to the minimum flat discharge of the downstream river,which improves the sediment transport capacity of the river and alleviates the problem of sediment deposition.In this paper,through the introduction of the Yellow River water and sediment regulation project system,regulation indicators and mechanisms,the author discusses in detail the Yellow River water and sediment regulation scheme and its operation effect,hoping to provide help promote the improvement of the Yellow River governance effect.

Impact of vegetation restoration on hydrological processes in the middle reaches of the Yellow River,China

Sediment discharge from the Yellow River originates mainly from the drainage area between Hekouzhen and Longmen, i.e., the Helong area. Spatial-temporal variations of the vegetation cover in this area during the 1981-2007 period have been investigated using GIMMS and SPOT VGT NDVI data. We have also analyzed the interannual variations in vegetation cover and changes in annual runoff and sediment discharge, the consequences from precipitation change and the Grain for Green Project （GGP）. The results show that vegetation cover of the Helong area has increased during the 1981-2007 period. The northwestern part the Helong area, where the flat sandy lands are covered by grass, has experienced the largest increase. The region where the vegetation cover has declined is largely found in the southern and southeastern Helong area, which is a gullied hilly area or forested. Although precipitation was relatively low during the 1999-2007 period, the vegetation cover showed a significant increase in the Helong area, due to the implementation of the GGP. During this period, the most significant improvement in the vegetation cover occurred mainly in the gullied hilly areas of the Loess Plateau, such as the drainage basins of the Kuyehe and Tuweihe rivers and the middle and lower reaches of the Wudinghe and Yanhe rivers. A comparison of the average annual maximum NDVI between the earlier （1998-2002） stage and the next five years （2003-2007） of the GGP indicates that the areas with increases of 10% and 20% in NDVI account for 72.5% and 36.4% of the total area, respectively. Interannual variation of annual runoff and sediment discharge shows a declining trend, especially since the 1980s, when the decrease became very obvious. Compared with the 1950-1969 period, the average runoff during the 1980-2007 period was reduced by 34.8 × 10^8 m3 and the sediment discharge by 6.4 ×10^8 t, accounting for 49.4% and 64.9% of that in the 1950-1969 period, respectively. There is a positive correlation between the annual maximum NDVI and annual runoff and sediment discharge. This correlation was reversed since the implementation of the GGP in 1999 and vegetation cover in the He- long area has increased, associated with the decrease in runoff and sediment discharge. Less precipitation has been an important fac- tor driving the decrease in runoff and sediment discharge during 1999 2007. However, restoration and improvement of the vegetation cover may also have played a significant role in accelerating the decrease in annual runoff and sediment discharge by enhancing evapotranspiration and alleviating soil erosion.

Contributions of climate and human activities to changes in runoff of the Yellow and Yangtze rivers from 1950 to 2008

Runoffs in the Yellow River and Yangtze River basins, China, have been changing constantly during the last half century. In this paper, data from eight river gauging stations and 529 meteorological stations, inside and adjacent to the study basins, were analyzed and compared to quantify the hydrological processes involved, and to evaluate the role of human activities in chang- ing river discharges. The Inverse Distance Weighted （IDW） interpolation method was used to obtain climatic data coverage from station observations. According to the runoff coefficient equation, the effect of human activities and climate can be ex- pressed by changes in runoff coefficients and changes in precipitation, respectively. Annual runoff coefficients were calculated for the period 1950-2008, according to the correlation between respective hydrological series and regional precipitation. An- nual precipitation showed no obvious trend in the upper reaches of the Yellow River but a marked downward trend in the mid- dle and downstream reaches, with declines of 8.8 and 9.8 ram/10 a, respectively. All annual runoff series for the Yellow River basin showed a significant downward trend. Runoff declined by about 7.8 mm/10 a at Sanmenxia and 10.8 ram/10 a at Lijin. The series results indicated that an abrupt change occurred in the late 1980s to early 1990s. The trend of correlations between annual runoff and precipitation decreased significantly at the Yellow River stations, with rates ranging from 0.013/10 a to 0.019/10 a. For the hydrologic series, all precipitation series showed a downward trend in the Yangtze River basin with de- clines ranging from about 24.7 mm/10 a at Cuntan to 18.2 mm/10 a at Datong. Annual runoff series for the upper reaches of the Yangtze River decreased significantly, at rates ranging from 9.9 to 7.2 mm/10 a. In the middle and lower reaches, the run- off series showed no significant trend, with rates of change ranging from 2.1 to 2.9 ram/10 a. Human activities had the greatest influence on changes in the hydrological series of runoff, regardless of whether the effect was negative or positive. During 1970-2008, human activities contributed to 83% of the reduction in runoff in the Yellow River basin, and to 71% of the in- crease in runoff in the Yangtze River basin. Moreover, the impacts of human activities across the entire basin increased over time. In the 2000s, the impact of human activities exceeded that of climate change and was responsible for 84% of the decrease and 73% of the increase in runoff in the Yellow River and Yangtze River basins, respectively. The average annual runoff from 1980 to 2008 fell by about 97%, 83%, 83%, and 91%, compared with 1951-1969, at the Yellow River stations Lanzhou, San- menxia, Huayuankou and Lijin, respectively. Most of the reduction in runoff was caused by human activities. Changes in pre- cipitation also caused reductions in runoff of about 3%, 17%, 17%, and 9% at these four stations, respectively. Falling precipi- tation rates were the main explanation for runoff changes at the Yangtze River stations Cuntan, Yichang, Hankou, and Datong, causing reductions in runoff of 89%, 74%, 43%, and 35%, respectively. Underlying surface changes caused decreases in runoff in the Yellow River basin and increases in runoff in the Yangtze River basin. Runoff decreased in arid areas as a result of in- creased water usage, but increased in humid and sub-humid areas as a result of land reclamation and mass urbanization leading to decreases in evaporation and infiltration.

Simulating cross-sectional geometry of the main channel in response to changes in water and sediment in Lower Yellow River

To understand the non-equilibrium morphological adjustment of a river in response to environmental changes,it is essential to(i)accurately identify how past conditions of water and sediment have impacted current morphological adjustment of the river,and(ii)establish a corresponding simulation for non-equilibrium conditions.Based on discharge and suspended sediment concentration(SSC)as well as 82 cross-sectional data items for the Huayuankou-Lijin reach of the Lower Yellow River in the period 1965-2015,the process of adjustment of the geometry of the main channel(area,width,depth,and geomorphic coefficient),and its responses to changes in discharge and SSC for different reaches are statistically analyzed.Following this,a delayed response model(DRM)of the geometry of the main channel subjected to variations in discharge and SSC is established using a multi-step analytical model,with the discharge and SSC as the main controlling factors.The results show that the area,width,and depth of the main channel decreased initially,then increased,decreased again,and finally increased again.These features of the geometry of the channel were positively correlated with the 4-year moving average discharge and negatively with the 4-year moving average SSC.The geomorphic coefficient for the Huayuankou-Sunkou reach exhibited a trend of decrease,whereas that of the Sunkou-Lijin reach decreased initially,then increased,decreased again,and finally increased again.Except for the Huayuankou-Gaocun reach in 1965-1999,the coefficient was negatively correlated with the 4-year moving average discharge and positively with SSC.The simulated values of the morphological parameters of the main channel for all sub-reaches obtained using the DRM agreed well with the measured values.This indicates that the DRM can be used to simulate the process of response of the cross-sectional geometry of the main channel to variations in the water and sediment.The results of the model show that the adjustment of the geometry of the main channel was affected by the discharge and the SSC at present(30%)as well as for the previous 7 years(70%).The proposed model offers insights into the mechanism whereby past water and sediment influence the current morphological adjustment of the river,and provides an effective method for predicting the magnitude and trend of the geometry of the main channel under different flow conditions.

Spatial variation and driving mechanism of soil organic carbon components in the alluvial/sedimentary zone of the Yellow River

Alluviation and sedimentation of the Yellow River are important factors influencing the surface soil structure and organic carbon content in its lower reaches.Selecting Kaifeng and Zhoukou as typical cases of the Yellow River flooding area,the field survey,soil sample collection,laboratory experiment and Geographic Information System(GIS)spatial analysis methods were applied to study the spatial distribution characteristics and change mechanism of organic carbon components at different soil depths.The results revealed that the soil total organic carbon(TOC),active organic carbon(AOC)and nonactive organic carbon(NOC)contents ranged from 0.05–30.03 g/kg,0.01–8.86 g/kg and 0.02–23.36 g/kg,respectively.The TOC,AOC and NOC contents in the surface soil layer were obviously higher than those in the lower soil layer,and the sequence of the content and change range within a single layer was TOC>NOC>AOC.Geostatistical analysis indicated that the TOC,AOC and NOC contents were commonly influenced by structural and random factors,and the influence magnitudes of these two factors were similar.The overall spatial trends of TOC,AOC and NOC remained relatively consistent from the 0–20 cm layer to the 20–100 cm layer,and the transition between high-and low-value areas was obvious,while the spatial variance was high.The AOC and NOC contents and spatial distribution better reflected TOC spatial variation and carbon accumulation areas.The distribution and depth of the sediment,agricultural land-use type,cropping system,fertilization method,tillage process and cultivation history were the main factors impacting the spatial variation in the soil organic carbon(SOC)components.Therefore,increasing the organic matter content,straw return,applying organic manure,adding exogenous particulate matter and conservation tillage are effective measures to improve the soil quality and attain sustainable agricultural development in the alluvial/sedimentary zone of the Yellow River.

Numerical simulation on seasonal transport variations and mechanisms of suspended sediment discharged from the Yellow River to the Bohai Sea

Based on sediment and discharge flux data for the Yellow River, realistic forcing fields and bathymetry of the Bohai Sea, a suspended sediment transport module is driven by a wave-current coupled model to research seasonal variations and mechanisms of sus- pended load transport to the Bohai Sea. It could be concluded that surface sediment concentration indicates a distinct spatial distribution characteristic that varies seasonally in the Bohai Sea. Sediment concentration is rather high near the Yellow River estuary, seasonal variations of which are controlled by quantity of sediment from the YeUow River, suspended sediment concentration reaches its maximum during summer and fall. Furthermore, sediment concentration decreases rapidly in other seas far from the Yellow River estuary and maintains a very low level in the center of the Bohai Sea, and is dominated by seasonal variations of climatology wind field in the Bohai Sea. Only a small amount of sediments imported from the Yellow River are delivered northwestward to the southern coast of the Bohai Bay. Majority of sediments are transported southeastward to the Laizhou Bay, where sediments are con- tinuously delivered into the center of the Bohai Sea in a northeastward direction, and part of them are transported eastward alongshore through the Bohai Strait. 69% of sediments from the Yellow River are deposited near the river delta, 31% conveyed seaward, within which, 4% exported to the northern Yellow Sea through the Bohai Strait. Wind wave is the most essential contributor to seasonal variations of sediment concentration in the Bohai Sea, and the contribution of tidal currents is also significant in shallow waters when wind speed is low.

小浪底水库运行以来对黄河下游河道河床演变特性的影响

小浪底水库自1999年10月下闸蓄水,23年来深刻改变了进入黄河下游河道的径流泥沙过程,进而引起了下游河道的再造床。本研究以1960—2022年长序列水文泥沙、断面地形、遥感影像等多源资料为基础,从径流泥沙、冲淤演变、形态调整三个方面阐明了小浪底水库运行以来对下游河道河床演变特性的影响。结果表明:(1)水库拦沙效果显著,下泄水量在年内分配更加均匀、沙量则向汛期更加集中;大流量和高含沙事件的严重程度和发生频率均有所降低;悬沙调整不明显,床沙粗化显著。(2)扭转了黄河下游河道持续淤积的趋势,冲淤调整主要发生在花园口至高村的游荡性河段;冲刷效率持续下降,主槽过流能力得到显著恢复。(3)游荡性河段横断面形态向窄深发展,二级悬河发育得到有效缓解,整体河势处于相对稳定状态。研究成果丰富了多沙河流修建水库对下游河道影响的科学认识,也可为小浪底水库优化调度提供科技支撑。

Adjustment in the main-channel geometry of the lower Yellow River before and after the operation of the Xiaolangdi Reservoir from 1986 to 2015

Based on the measured discharge,sediment load,and cross-sectional data from 1986 to 2015 for the lower Yellow River,changes in the morphological parameters(width,depth,and cross-sectional geomorphic coefficient)of the main channel are analyzed in this paper.The results show that before the operation of the Xiaolangdi Reservoir(XLDR)from 1986 to 1999,the main channel shrunk continually,with decreasing width and depth.The rate of reduction in its width decreased along the river whereas that of depth increased in the downstream direction.Because the rate of decrease in the width of the main channel was greater than that in channel depth,the cross-sectional geomorphic coefficient decreased in the sub-reach above Gaocun.By contrast,for the sub-reach below Gaocun,the rate of decrease in channel width was smaller than that in channel depth,and the cross-sectional geomorphic coefficient increased.Once the XLDR had begun operation,the main channel eroded continually,and both its width and depth increased from 2000 to 2015.The rate of increase in channel width decreased in the longitudinal direction,and the depth of the main channel in all sub-reaches increased by more than 2 m.Because the rate of increase in the depth of the main channel was clearly larger than that of its width,the cross-sectional geomorphic coefficient decreased in all sub-reaches.The cross-sectional geometry of the main-channel of the lower Yellow River exhibited different adjustment patterns before and after the XLDR began operation.Before its operation,the main channel mainly narrowed in the transverse direction and silted in the vertical direction in the sub-reach above Aishan;in the sub-reach below Aishan,it primarily silted in the vertical direction.After the XLDR began operation,the main channel adjusted by widening in the transverse direction and deepening in the vertical direction in the sub-reach above Aishan;in the sub-reach below it,the main channel adjusted mainly by deepening in the vertical direction.Compared with the rates of decrease in the width and depth of the main channel during the siltation period,the rate of increase in channel width during the scouring period was clearly smaller while the rate of increase in channel depth was larger.After continual siltation and scouring from 1986 to 2015,the cross-sectional geometry of the main-channel changed from wide and shallow to relatively narrow and deep.The pattern of adjustment in the main channel was closely related to the water and sediment conditions.For the braided reach,the cross-sectional geomorphic coefficient was negatively correlated with discharge and positively correlated with suspended sediment concentration(SSC)during the siltation period.By contrast,the cross-sectional geomorphic coefficient was positively correlated with discharge and negatively correlated with SSC during the scouring period.For the transitional and meandering reaches,the cross-sectional geomorphic coefficient was negatively correlated with discharge and positively correlated with SSC.

Response of water budget to recent climatic changes in the source region of the Yellow River

Discharge in the source region of the Yellow River significantly declined after 1990.China Meteorological Administration(CMA) data show that precipitation in this region was low in the 1990s but returned to above normal after 2002;in recent decades there has been rapid warming of surface air,moistening and wind speed decrease.To investigate the influences of recent climatic changes on the water budget,this study simulates the surface water budget at CMA stations within and surrounding the source region during 1960-2006,using an improved land surface model.Results indicate that the spatial pattern of precipitation change is an important factor(except for precipitation amount and intensity) in determining the response of runoff to precipitation changes.Low runoff in the 1990s was consistent with precipitation amount and intensity.The recovery of precipitation after 2002 is mainly from increased precipitation in the dry area of the source region.Evaporation was mainly limited by water availability in this dry area,and thus most of the precipitation increase was evaporated.By contrast,energy availability was a more important influence on evaporation in the wet area.There was more evaporation in the wet area because of rapid warming,although precipitation amount partly decreased and partly increased,contributing to the reduction of runoff after 2002.This control on evaporation and its response,together with the modified spatial pattern of precipitation,produced a water budget unfavorable for runoff generation in the source region during recent years.