Changes In Sediment Load Research Articles

Quantifying the anthropogenic and climatic contributions to changes in water discharge and sediment load into the sea: A case study of the Yangtze River, China

Based on data from the Datong hydrological station and 147 meteorological stations, the influences of climate change and human activities on temporal changes in water discharge and sediment load were examined in the Yangtze River basin from 1953 to 2010. The Mann–Kendall test, abrupt change test (Mann–Kendall and cumulative anomaly test), and Morlet wavelet method were employed to analyze the water discharge and sediment load data measured at the Datong hydrological station. The results indicated that the annual mean precipitation and water discharge exhibited decreasing trends of −0.0064mm/10yr and −1.41×108m3/yr, respectively, and that the water sediment load showed a significant decreasing trend of −46.5×106t/yr. Meanwhile, an abrupt change in the water discharge occurred in 2003. The sediment load also exhibited an abrupt change in 1985. From 1970 to 2010, the climate change and human activities contributed 72% and 28%, respectively, to the water discharge reduction. The human-induced decrease in the sediment load was 914.03×106t/yr during the 1970s and 3301.79×106t/yr during the 2000s. The contribution from human activities also increased from 71% to 92%, especially in the 1990s, when the value increased to 92%. Climate change and human activities contributed 14% and 86%, respectively, to the sediment load reduction. Inter-annual variations in water discharge and sediment load were affected by climate oscillations and human activities. The effect of human activities on the sediment load was considerably greater than those on water discharge in the Yangtze River basin.

The quantitative estimation of sediment load changes entering Poyang Lake Basin induced by climate change and anthropogenic impacts

The numerical model Hydro Trend is applied to simulate the variations of water discharge and sediment fluxes into Poyang Lake during 1956- 2010 from the five tributaries of Poyang Lake Basin, namely Gan, Fu, Xin, Rao and Xiu rivers. Furthermore, the influences of climate change, vegetation cover variation and reservoirs construction on the sediment load are analyzed, and the contribution of the above three factors to the sediment load entering the Poyang Lake is quantitatively evaluated. The results show that, during the period of 1956-2010,the average annual sediment discharge into Poyang Lake under the influence of climate,vegetation cover and reservoir construction reached 15.5 Mt, 20.8 Mt, 8.5 Mt, respectively;whereas, the sediment load entering the Poyang Lake under the combined- influence of the above three factors is 12.6 Mt a- 1. In addition, during the same period, the sediment load delivered to the Poyang Lake increased by 4.2 Mt a- 1due to water and soil erosion, and decreased by 8.2 Mt a- 1attributed to reservoir interception, which accounted for 32.4% and63.2% of the observed sediment load discharged into the Poyang Lake, respectively. From 1956 to 1989, the increased sediment load caused by water and soil erosion and the decreased sediment load induced by dam emplacement, in terms of both quantity of 5.1 Mt a- 1, is equivalent; However, during 1990- 2010, the sediment load resulting from water and soil loss decreased to 2.7 Mt a- 1, and that intercepted by reservoirs increased to 13.3 Mt a- 1, suggesting that the effect of reservoirs construction on the changes of sediment load entering the Poyang Lake is 5 times of that of water and soil erosion.

Distinguishing the impacts of human activities and climate variability on runoff and sediment load change based on paired periods with similar weather conditions: A case in the Yan River, China

Runoff and sediment loads from river basin are largely affected by the interplay of climate variability and human activities within the basin. However, distinguishing the impacts of climate variability and human activities would vastly improve our knowledge of water resources, climate variability and climate adaptation, and watershed management. We propose a new and simple method to determine the impact of human activities within paired datasets under the same or similar weather conditions (SWC). These weather conditions cover one or more meteorological elements such as precipitation, temperature, or evaporation. If there are two or more periods with similar weather conditions but different runoff, the relative runoff and sediment load changes can be considered a consequence of human-induced land surface changes. This study will report on the application of this new method, using the Yan River Basin in China as a case study. We found 10 sets PPs (paired periods) in 1 year intervals and 12 sets of PPs in inter- vals of 3 years when (1) there was a 2.0% and 1.0% difference of annual precipitation and annual ET0, respectively, (2) the relationship between monthly precipitation and ET0 of PPs was significant (P < 0.05) and, (3) there was no overlap of years for the PPs with intervals of 1 and 3 years. We found that the impact of human activities varied greatly between PPs, with the main trend of declining PPs, matched the trends evident from statistical analysis and land use and land cover (LULC) change evaluation. The method is simple and easily applicable to selected periods in most areas and could be extended when more detailed data are available. The result of this method is the impact of all human activities, allowing for further discussion on the contributions of each kind of human activity over time in determining the range which links the research results at different scales, e.g. to define the sediment delivery ratio (SDR) describing soil erosion on catchment slopes and sediment load in the river.

Delta changes in the Pearl River estuary and its response to human activities (1954–2008)

This paper is concerned with delta evolution in response to human activities in the Pearl River estuary. Utilizing more than 50 years data bathymetric data, together with simulation fluvial discharge data, the net accretion rate in the Lingdingyang subaqueous delta decreased from 16.6 mm/y over the period 1955–1964 to 1.6 mm/y for the period 1998–2008 because there was a dramatic downward trend in the sediment load. In particular, the closure of the Longtan Dam (LTD) in 2006 resulted in the sediment load entering the Pearl River falling from 75 Mt/y (annual average: 1954–2006) to 25.2 Mt/y, a decrease of ∼67%. Another reason may be sand mining in the Lingdingyang estuary. Based on the linear relationship between sediment load and accumulation rate, an annual average sediment load of 21.7 Mt/y carried into the Lingdingyang estuary is the critical threshold value that separates delta progradation from recession. However, the outer subaqueous delta in the Modaomen estuary underwent deposition in the same period, because there is no obvious change in sediment load into the area during 1954–2008. In the coming decades, ongoing human interferences will reduce the fluvial sediment load entering the sea, which probably will result in a phase of general delta recession replacing delta progradation.

Responses of streamflow and sediment load to climate change and human activity in the Upper Yellow River, China: a case of the Ten Great Gullies Basin.

Soil erosion and land desertification are the most serious environmental problems globally. This study investigated the changes in streamflow and sediment load from 1964 to 2012 in the Ten Great Gullies area of the Upper Yellow River. Tests for gradual trends (Mann-Kendall test) and abrupt changes (Pettitt test) identify that significant declines in streamflow and sediment load occurred in 1997-1998 in two typical gullies. A comparison of climatic variability before and after the change points shows no statistically significant trends in annual precipitation and potential evapotranspiration. Human activities have been very active in the region and during 1990-2010, 146.01 and 197.62 km2 of land were converted, respectively, to forests and grassland, with corresponding increases of 87.56 and 77.05%. In addition, a large number of check dams have been built up in the upper reaches of the ten gullies. These measures were likely responsible for the significant decline in the annual streamflow and sediment load over the last 49 years.

Recent changes of suspended sediment yields in the Upper Yangtze River and its headwater tributaries

Abstract. Suspended sediment yields in the Upper Yangtze River and its four headwater tributaries (i.e. Jinsha, Min, Jialing and Wu) have declined significantly during recent decades. Compared with 1956–1970, mean annual suspended sediment yield during 2001–2011 was reduced by 84% in the Upper Yangtze River at Yichang, by 34% in the Jinsha at Pingshan, by 84% in the Jialing at Beibei, by 75% in the Wu at Wulong, and by 48% in the Min at Gaochang. Linking these observed decadal changes of runoff discharge and suspended sediment load to dam construction and multiple environmental rehabilitation projects (e.g. soil-water conservation, reforestation) during the past decades, it can be concluded that the construction of large dams on the main stem and major tributaries of the Upper Yangtze River has played a principal role in the reduction of fluvial suspended sediment yields, while the environment rehabilitation projects may make limited contributions to the changes in suspended sediment yields, except for the Jialing River.

生态恢复对流域水沙演变趋势的影响及其程度分析——以北洛河上游为例

The Loess Plateau in China is characterized by a heavily dissected landscape and severe soil erosion. To fully understand the impacts of soil conservation measures and vegetation restoration on streamflow,sediment load and runoffsediment behaviors,hydrologic data over 47 years(1963 to 2009) in the upper reaches of the Beiluo River( drainage area of 3408 km2) are assessed. The Beiluo is a secondary tributary of the Yellow River,and has had substantial vegetation restorative effects on the Loess Plateau. Based on the Mann-Kendall trend test,Pettit statistical test and flow duration curve(FDC),the daily streamflow and sediment load at Wuqi hydrological station,which is the maximum control station in theupper reaches of the Beiluo River,was chosen to investigate temporal change and evolution characteristics. Precipitation data and rainfall erosion force were interpolated using tessellation polygons from five representative meteorological stations on the Loess Plateau. A dual mass curve and empirically- based hydrological modeling method were also used to estimate the response of streamflow and sediment load to climate change and human activities. The results indicate that:(1) In the past50 years,under a background of precipitation with no significant trend,both streamflow and sediment load had significant negative trends( P 0. 01),with average annual rates of- 0. 28 mm km- 2a- 1and- 180 t km- 2a- 1,respectively. Under the influence of soil conservation measures that have been applied on the Loess Plateau since the 1960 s,there were change points in 1979 and 2002. Thus,the entire study period was divided into a period 1 of base(1963—1979),period 2 of comprehensive harness of soil and water conservation(1980—2002),and period 3 of the Grain for Green project(2003—2009).(2) The FDC showed that,compared with period 1,streamflow in period 2 for 5% and 50% decreased by 17. 0%and 5. 2%,respectively,while it increased by 94. 2% for 95%. Suspended sediment load decreased more significantly,with 34. 6% and 40. 1% for 5% and 50%,respectively,and no suspended sediment days remained almost constant for95%. Period 3 saw more remarkable changes in both streamflow and suspended sediment load. For 5% and 50%,streamflow decreased by 40. 2% and 26. 8%,respectively,while suspended sediment load decreased by 95. 7% and 96.8%,respectively. For 95%,streamflow rose by 128. 1%,and no sediment days increased. The reduction of sediment load was greater than that of streamflow,with the periods altered and negative changes of sediment load.( 3) Compared with period 1,streamflow and sediment load decreased by 0. 10 × 108m3(3 mm) and 0. 04 × 108t(1408 t km- 2a- 1) in period2. It was estimated that human activities accounted for 38. 2% and 51. 4% of the changes of streamflow and sediment load,respectively. In period 3,there was a greater reduction in both streamflow and sediment load,with 0. 52 × 108m3(15. 2mm) and 0. 33 × 108t(9683. 1t km- 2a- 1),respectively,with effects reaching 74. 7% and 86. 7%. Great improvement in the ecological environment of the Loess Plateau has already demonstrated the advantages of biological treatments on a regional scale in reducing flood volumes,and compensating streamflow.

Impacts of Climate Change and Human Activities on Runoff and Sediment Load of the Xiliugou Basin in the Upper Yellow River

Using data of temperature, wind, precipitation, water discharge, and sediment load, the changes in runoff and sediment load of the Xiliugou basin in the upper Yellow River were investigated and the contributions of climate change and human activities to these changes were quantitatively estimated. Results show that the runoff and sediment load of the stream declined gradually in 1960–2012. According to the abrupt change point detected, the runoff and sediment series were divided into two periods: 1960–1998 and 1999–2012. The reductions of runoff and sediment load in 1999–2012 were found to be related to climate change and human activities, and the latter played a dominant role with a contribution of about 68% and 75%, respectively. The effects of rainfall intensity should be considered to avoid overestimating or underestimating the contributions of rainfall changes to the variations of runoff and sediment load in the semiarid region. An inspection of changes in water discharge and sediment regime indicated that the frequency of discharge between 0 and 5 m3/s increased while that between 5 and 1000 m3/s decreased in 2006–2012. This phenomenon can be attributed principally to the soil and water conservation practices.

Geomorphology and evolution of the late Pleistocene to Holocene fluvial system in the south-eastern Llanos de Moxos, Bolivian Amazon

In the Bolivian Amazon several paleochannel generations are preserved. Their wide spectrum of morphologies clearly provides crucial information on the type and magnitude of geomorphic and hydrological changes within the drainage network of the Andean foreland. Therefore, in this study we mapped geomorphological characteristics of paleochannels, and applied radiocarbon and optically stimulated luminescence dating. Seven paleochannel generations are identified. Significant changes in sinuosity, channel widths and river pattern are observed for the successive paleochannel generations. Our results clearly reflect at least three different geomorphic and hydrological periods in the evolution of the fluvial system since the late Pleistocene. Changes in discharge and sediment load may be controlled by combinations of two interrelated mechanisms: (i) spatial changes and re-organizations of the drainage network in the upper catchment, and/or (ii) climate changes with their associated local to catchment-scale modifications in vegetation cover, and changes in discharge, inundation frequencies and magnitudes, which have likely affected the evolution of the fluvial system in the Llanos de Moxos. In summary, our study has revealed the enormous potential which geomorphic mapping and analysis combined with luminescence based chronologies hold for the reconstruction of the late Pleistocene to recent fluvial system in a large portion of Amazonia.

Multi-scale variability of water discharge and sediment load into the Bohai Sea from 1950 to 2011

This paper examines the changes in the time series of water discharge and sediment load of the Yellow River into the Bohai Sea. To determine the characteristics of abrupt changes and multi-scale periods of water discharge and sediment load, data from Lijin station were analyzed, and the resonance periods were then calculated. The Mann-Kendall test, order clustering, power-spectrum, and wavelet analysis were used to observe water discharge and sediment load into the sea over the last 62 years. The most significant abrupt change in water discharge into the sea occurred in 1985, and an abrupt change in sediment load happened in the same year. Significant decreases of 64.6% and 73.8% were observed in water discharge and sediment load, respectively, before 1985. More significant abrupt changes in water discharge and sediment load were observed in 1968 and 1996. The characteristics of water discharge and sediment load into the Bohai Sea show periodic oscillation at inter-annual and decadal scales. The main periods of water discharge are 9.14 years and 3.05 years, whereas the main periods of sediment load are 10.67 years, 4.27 years, and 2.78 years. The significant resonance periods between water discharge and sediment load are observed at the following temporal scales: 2.86 years, 4.44 years, and 13.33 years. Water discharge and sediment load started to decrease after 1970 and has decreased significantly since 1985 for several reasons. Firstly, the precipitation of the Yellow River drainage area has reduced since 1970. Secondly, large-scale human activities, such as the building of reservoirs and floodgates, have increased. Thirdly, water and soil conservation have taken effect since 1985.

Discussion of: Olea, R.A. and Coleman, J.L., Jr., 2014. A Synoptic Examination of Causes of Land Loss in Southern Louisiana as Related to the Exploitation of Subsurface Geological Resources.Journal of Coastal Research, 30(5), 1025–1044.

ABSTRACT Turner, R.E., 2014. Discussion of: Olea, R.A. and Coleman, J.L., Jr., 2014. A synoptic examination of causes of land loss in southern Louisiana as related to the exploitation of subsurface geological resources. Journal of Coastal Research, 30(5), 1025–1044. I comment on Olea and Coleman's (2014) conclusion that subsidence was the primary cause of the dramatic rise in Louisiana's coastal land losses in the last 100 years. The focus on subsidence combined with the omission of context for factors not related to subsidence (e.g., dredged canals), leaves the reader with the incorrect conclusion that anthropogenic factors observed to date are insignificant, and that coastal wetland losses are only driven by subsidence. I address this omission by discussing two points about anthropogenic influences: (1) dredged canals and (2) changes in sediment load from the watershed and its distribution. They omit quantitative inclusion of two signature symptoms of the cause-and-effect relationships at temporal and s...

Climate Change Impact on Runoff and Sediment Loads to the Apalachicola River at Seasonal and Event Scales

Chen, X.; Alizad, K.; Wang, D., and Hagen, S.C., 2014. Climate change impact on runoff and sediment loads to the Apalachicola River at seasonal and event scales.In this study, potential climate change impacts on runoff and sediment load in Apalachicola River basin in Florida are assessed using Soil and Water Assessment Tool (SWAT), a semi-distributed hydrologic model. The observed streamflow and sediment load from 1984 to 1994 are used for the model calibration and validation. The streamflow Nash-Sutcliffe Coefficients (NSEs) for the simulation and validation periods (1984–1989 and 1990–1994 years) are 0.92 and 0.88, respectively. The sediment NSEs for the simulation and validation periods are calculated to be 0.46 and 0.36, respectively, with excellent description of trend variability. Rainfall data under climate change effects is applied as the calibrated SWAT model input to estimate the streamflow and sediment load change. The rainfall and temperature data is prepared using two regional climate models (RCM); HRM3-HADCM3, and RCM3-GFDL. Results show that the average daily level of streamflow and sediment load will not vary significantly, but the peak flow and peak sediment load will increase dramatically due to the more intense and less frequent rainfall events. The impact of climate change during an extreme rainfall event is also investigated. A storm event with 25-year return period and 24-hour duration in 1991 is taken as the baseline event. Based on the projection using RCM3-GFDL scenario, the streamflow and sediment load may increase by 50% and 89%, respectively.

Dynamic changes of sediment load in the middle reaches of the Yellow River basin, China and implications for eco-restoration

Abrupt reduction of the sediment load in the middle reaches of the Yellow River has extracted much attention during the past several decades due to global climate changes and intensive human activities. This paper investigates the spatial and temporal variations of annual sediment load in the section between Hekouzhen and Longmen in the middle reaches of the Yellow River basin from 1962 to 2009 by using annual observations at 21 hydrological gauging stations (4 mainstream and 17 tributaries stations), by making use of Mann–Kendall test and simple linear regression for trend detection, and the sequential cluster analysis method and Student’s t test for changing point detection. The results suggest that sediment load from both mainstream and tributaries show significant decreasing trends at confidence level of 0.01. Evident abrupt changing points are detected around 1970s for most stations, but for Toudaoguai station and Fugu station are found in the mid-1980s, which are mainly attributed to the operation of Longyangxia Dam. In addition, abrupt changes in the sediment series at some stations from the tributaries were found around the late 1990s, this situation may be resulted from the “Green for Grain” project launched in 1999. Sediment budget analysis indicates that the 17 tributaries contributes 70.60% sediment load to the whole research area, which suggests that the majority sediment load contribution to He-Long region is from these tributaries. The comparison among three mainstream sections implies that the sediment is mainly from Wubao–Longmen section. And contribution of tributaries in each mainstream section is mainly from Huangfuchuan River, Kuye River, Wuding River and Yan River. The reduction of the sediment load is mainly caused by human activities especially soil and water conservation projects (such as afforestation, terraces, reservoirs and dams). In summary, the results in this study provide a further evidence for a more exact sediment source region and implementing soil and water conservation measures more effectively in the middle reaches of the Yellow River basin, China.

Hydrological and morphological processes at river mouths of the Caspian Sea Region as possible analogs of expected changes of mouths of other rivers in Russia and the world

Present changes in the regime and structure of mouths of rivers, which empty into the Caspian Sea, are discussed. The similarities and differences of these processes at the mouths of different rivers of the Caspian Region were revealed. Assessment was made of the impact of changes in river water runoff and sediment load and in the Caspian Sea level as well as the nearshore zone relief and local hydraulic engineering activities on the processes under study. Anomalous features of the processes occurring at the mouths of the Volga and Terek rivers were revealed and explained. Hydrological and morphological processes typical of the present mouths of the rivers of Ural, Sulak, and Kura were revealed; these processes could be accepted as universal and possible analogs in assessing the expected changes at the mouths of other rivers in Russia and the world in the XXI century.

Change detection of storm runoff and sediment yield using hydrologic models following wildfire in a coastal redwood forest, California

This study attempted to detect changes in stormflow volumes, peakflows, and sediment loads using hydrologic models within the context of an uncertainty assessment following wildfire. In 2009, after 8 years of study, the Lockheed Fire burned the treatment and control watersheds of a paired watershed study in coastal California, USA, eliminating the ability to continue a paired watershed before–after control–intervention (BACI) study design. An alternative analysis was used to detect stormflow and sediment load changes due to the wildfire by comparing measured posttreatment stormflow and sediment load with simulated predisturbance responses predicted with the hydrologic models HBV-EC and DHSVM. High natural variability of stormflow and sediment measurements compounded with uncertainty associated with the hydrologic models and climate suggest that only large changes can be detected. The fire and subsequent salvage harvest created an approximately 9%–12% reduction in forest overstory canopy and a 70%–90% consumption of understory vegetation. No discernible changes in slopes of regression lines were detected between predisturbance and postfire stormflow volumes, peakflows, or sediment loads. No changes were detected in stormflow volume, peakflow, or sediment loads comparing pre- and post-fire vegetation inputs to the hydrologic model DHSVM. The lack of detected change in streamflow to accelerate stream channel erosion combined with low to moderate fire severity adjacent to stream channels most likely were the reasons for no detected postfire change to sediment loads.

Abrupt changes of runoff and sediment load in the lower reaches of the Yellow River, China

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.

Construction of lake bathymetry from MODIS satellite data and GIS from 2003 to 2011

In recent years, sedimentation conditions in Dongting Lake have varied greatly because of signifi cant changes in runoff and sediment load in the Changjiang (Yangtze) River following the construction of Three Gorges Dam. The topography of the lake bottom has changed rapidly because of the intense exchange of water and sediment between the lake and the Changjiang River. However, time series information on lake-bottom topographic change is lacking. In this study, we introduced a method that combines remote sensing data and in situ water level data to extract a record of Dongting Lake bottom topography from 2003 to 2011. Multi-temporal lake land/water boundaries were extracted from MODIS images using the linear spectral mixture model method. The elevation of water/land boundary points were calculated using water level data and spatial interpolation techniques. Digital elevation models of Dongting Lake bottom topography in different periods were then constructed with the multiple heighted waterlines. The mean root-mean-square error of the linear spectral mixture model was 0.036, and the mean predicted error for elevation interpolation was −0.19 m. Compared with fi eld measurement data and sediment load data, the method has proven to be most applicable. The results show that the topography of the bottom of Dongting Lake has exhibited uneven erosion and deposition in terms of time and space over the last nine years. Moreover, lake-bottom topography has undergone a slight erosion trend within this period, with 58.2% and 41.8% of the lake-bottom area being eroded and deposited, respectively.

Temporal Variation of Streamflow, Sediment Load and Their Relationship in the Yellow River Basin, China

Variation of streamflow and sediment load in the Yellow River basin has received considerable attention due to its drastic reduction during the past several decades. This paper presents a detailed investigation on the changes of streamflow and sediment load from 1952 to 2011 using monthly observations at four gauging stations along the Yellow River. The results show significant decreasing trends for both streamflow and sediment load at all four gauging stations over the past 60 years. The wavelet transform demonstrated discontinuous periodicities from 1969 to 1973 and after 1986 due to the construction of large reservoirs and implementation of numerous soil and water conservations practices. The sediment rating curves with the power-law function was applied to investigate the relationship between discharge and sediment load. The results indicate distinct variations of the relationship between streamflow and sediment and implied significant hydro-morphological changes within different periods. The reducing sediment supply from the source region and the increased erosive power of the river are detected at Lanzhou station, while the decrease of the transport capacity at Toudaoguai is caused by severe siltation. Significant changes in the relationship between streamflow and sediment load are found at Huayuankou and Gaocun stations, which are largely induced by evident sediment income and trapping effects of large reservoirs. It is estimated that numerous reservoirs have strongly altered the regime and magnitude of streamflow and trapped large amount of sediment, leading to severe siltation and evident reduction of their total volumes. A decrease in precipitation, incoming water from the upper reaches, soil and water conservation measures as well as water consumption contribute most to the significant reduction of streamflow. The decrease of sediment load mainly resulted from various soil and water conservation measures and trapping in reservoirs from 1986 to 2011.

Changes in runoff and sediment load from major Chinese rivers to the Pacific Ocean over the period 1955–2010

Changes in runoff and sediment loads to the Pacific Ocean from 10 major Chinese rivers are presented in this paper. To quantitatively assess trends in runoff and sediment loads, a parameter called the “Trend Ratio T” has been defined in this paper. To summarize total runoff and sediment load from these rivers, data from 17 gauging stations for the duration 1955 to 2010 has been standardized, and the missing data have been interpolated by different approaches according to specific conditions. Over the observed 56-year study period, there is a quite stable change in total runoff. Results show that the mean annual runoff flux entering the Pacific Ocean from these rivers is approximately 1,425 billion cubic meters. It is found that all northern rivers within semi-arid and transitional zones including the Songhua, Liaohe, Haihe, Yellow and Huaihe rivers present declining trends in water discharge. Annual runoff in all southern rivers within humid zones including the Yangtze, Qiantang, Minjiang, Pearl and Lancang rivers does not change much, except for the Qiantang River whose annual runoff slightly increases. The annual sediment loads of all rivers show significant declining trends; the exceptions are the Songhua and Lancang rivers whose annual sediment loads have increasing trends. However, the mean annual sediment flux carried into the Pacific Ocean decreased from 2,026 million tonnes to 499 million tonnes over the 56-year period. During this time there were 4 distinct decreasing phases. The decrease in annual sediment flux is due to the integrated effects of human activity and climate change. The reduction in sediment flux makes it easy for reservoir operation; however, the decrease in sediment flux also creates problems, such as channel erosion, river bank collapse and the retreat of the delta area.

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.