| Rivers greatly affected the estuary, the coastal area and the marginal sea by delivering freshwater and terrestrial material to the ocean. Variations of riverine water and sedients will lead to changes in the salinity, turbidity and geomorphic evolution in the coastal zones, as well as the subsequent changes in its function. The global climate has experienced great changes over the last few decades, and human disturbance in the drainage basins enhanced. Many of the river systems worldwide have been experiencing declining water and sediment loads under the combined influence of human activity and climate changes. However, the riverine flux has great different response to climate change and human activities among the river basins, due to the climate and environmental backgrounds, as well as the socio-economic development differ a lot. Therefore, it is necessary to perform comparative studies to disentangle hydrological variations in the river basins.The Yellow River, the Yangtze River and the Pearl River are the top three largest rivers in China, as well as the world’s major rivers (ca.2% of the world’s land surface). The three basins are located in the typical East Asian monsoon region and belong to the most productive and developed regions in China. And they provided most of the water and sediment in the coast of the West Pacific Ocean. In this study, the temporal variations of runoff and sediment load in the three major Chinese river basins in 1956-2013 under impacts of both climate change and human activity were investigate, based on the annually and monthly precipitation, temperature, runoff and sediment data, as well as a variety of mathmatical statistical methods.(1) Variations of temperature and precipitation in the Yellow River, the Yangtze River and the Pearl River basins.The annual mean temperature of the Three Basins increased significantly overall since 1956, and the average warming rate in the Three Basins (0.22℃/10yr) was close to that for the global land surface (0.21℃/10yr). However, considerable spatial disparities in the warming rate existed in the Three Basins:these rates tend to increase with latitude and elevation, ranging from 0.05-0.49℃/10yr, and the most significant warming was found in the Yellow River Basin with higher latitudes and the source region with higher elevations; Additionally, the warming rate in the very large cities with dense population and developed industries (e.g., Shanghai) was considerably greater than surrounding areas. And obvious seasonal difference in warming rates was also detected:it was lower in summer (0.14℃/10yr) and higher in winter (0.29℃/10yr). But the warming was characterized by a staircase form with steps around 1987 and 1998. However, both the annual and seasonal precipitation of the Three Basins showed little change, but with distinct interannual variations over the past 58 years and clear regional differences:A general decrease was found mostly in the central part of the study area (the Middle-Lower basins of the Yellow River, the central area of the Yangtze River and the West River Basin of the Pearl River), in contrast to a slight increase in the source regions of the Yellow River and the Yangtze River on the Qinghai-Tibetan Plateau, as well as in the Middle-Lower basins in the southeastern river basin. In summary, an overall warming-wetting tendency was found in the northwestern and southeastern river basins in 1956-2013, while the central regions tended to become warmer and drier.(2) Coupled variations of seasonal temperature and precipitation in the Yellow River, the Yangtze River and the Pearl River basins with Sea Surface Temperature (SST) in the Pacific and Indian Oceans on both the interannual time scale and the decadal time scale.The interannual variations of seasonal mean surface air temperature over the three river basins were associated with the El Nino-Southern Oscillation (ENSO)-driven variation of SST since 1956. The temperature in most of the river basins was positively correlated with central-eastern equatorial Pacific SST in winter and spring, but negatively correlated in summer and autumn. ENSO SST patterns also affected precipitation variability throughout the year, but with very different response patterns in the different seasons. For instance, ENSO patterns of SST variations were positively correlated with the precipitation in the southeastern basins in winter, but negatively correlated with the precipitation over the Yellow River Basin in summer. In autumn, a negative relationship of ENSO SST to the precipitation in the basin north of the Mainstream of the Yangtze River was found, but a positive correlation with the precipitation in the southern river basins.On the decadal time scale, the seasonal precipitation and temperature variations were strongly associated with the Pacific Quasi-Decadal Oscillation (QDO).(3) Variations of annual runoff and sediment load in the Yellow River, the Yangtze River and the Pearl River.The annual runoff of the Yellow River decreased significantly since 1956, but it hardly changed overall for the Yangtze and the Pear rivers; While the sediment load exhibited significant downward trends in all the three basins. And obvious periodical fluctuations can be detected for runoff and sediment load (but may not be entirely synchronized). For example, the annual runoff and sediment load in the Yellow River decreased significantly in 1964-1997, but an upward trend of runoff was found after 1997, with no clear change in sediment load. The annual runoff of the Yangtze River decreased significantly in 1998-2011, but the significant decrease of sediment load over the last 60 years mainly occurred after 1984. The annual runoff and sediment load of the Pear River slightly increased in 1956-1983, but they both decreased significantly after 1994.(4) Contributions of the subbasins to the water and sediment flux of the Yellow River, the Yangtze River and the Pearl River.The water and sediment yield in the subbasins of the Yellow River changed significantly over time. The runoff from the upstream above Lanzhou hydrological station accounted for about 69% of the Yellow River since 1956, and the Mid-Basin contributed about 77% of the sediment and 31% of the runoff. But the runoff and sediment yield in the Mid-Basin significantly reduced since 1956 (especially after 1985); the Mid-Basin contributed about 86% of the sediment in 1956-1985, but this proportion dropped to 68% after 1986. The annual runoff and sediment load showed complex changes in the sub-basins:the runoff of the Jinshajiang River basin increased, but the sediment load decreased since 1956; The annual runoff of Minjiang Jianglingjaing and Hanjiang basins in the region north of the Mainstream decreased (it was significant in Minjiang basin), together with consistent and significant downward trend in sediment; On the south bank of the mainsteam, the annual runoff showed a downward trend in the Wujiang Basin, but upward trends in Poyang and Dongting basins, and general decreases of sediment in all the three sub-basins. The mountainous area located in the up-basin contributed most of the sediment in the Yangtze River before the impoundment of the Three Gorges Reserviour (1956-2002), but the sediment load was mainly from the mid-lower mainstream after that (especially the riverbed in the mid-mainstream). The West River contributed about 77% of annual runoff and 89% of sediment respectively of the Pearl River in 1956-2013; And no significant trends of annual runoff were found in the West River, the North River and the East River, but the sediment load showed downward trend in the three sub-basins, and it was significant in the West River and the East River.(5) Impacts of climate change and human activities on the water and sediment flux of the Yellow River, the Yangtze River and the Pearl River.Impacts of climate change on the water and sediment flux of the three basins have great temporal and spatial disparities:The impacts of precipitation changes on the water and sediment fluxes of the Yellow River was more pronounced than the Yangtze River and the Pearl River; and the water and sediment flux were hardly affected by climate change overall for the last 58 years, but considerable effects can be detected over decadal periods. For example, the precipitation reduction in 1975-1997 explained about 20% of the decrease of runoff and sediment of the Yellow River over the same period.50% of the downward of runoff in the Yangtze River in 1998-2011 was attributed to the reduction in precipitation, which can also explain 20% of the decrease of sediment. The upward trend of runoff of the Pearl River in 1956-1983 was mainly caused by the increasing precipitation, while 50% of the downward trend of runoff and 20% of the significant reduction in sediment in 1994-2013 were explained by the reduction in precipitation.The double mass curves showed that the annual runoff and sediment load in the three rivers were both affected by external factors other than precipitation (such as human activities) since 1956.And this kind of variation in the Yellow River was more obvious in the Mid-Basin. Precipitation changes were mainly responsible for the variations in runoff in the Yangtze and Pearl rivers, but the significant reduction in sediment load in the three basins was mainly caused by human activities. And the impact of human activities on runoff and sediment increased in different decades. For example, the observed annual sediment load in the Yellow River decreased about 21% than the predicted values due to human disturbance in 1960s, and this ratio soared to 89% after 2000. The annual sediment load in the Yangtze River decreased significantly after 1969 due to human activity, and the observed annual sediment load decreased by 74% than the predicted values in 2010-2013. The sediment reduction of the Pearl River tended to increase after 1990s due to human interference, and the observed annual sediment load was 66% lower than the predicted values in the Pearl River in 2010-2013.Of all the human activities, the impoundment of reservoirs was the main reason for the gradually decreasing sediment in the three basins. For example, the sediment of Hanjiang River was reduced significantly after 1968 due to Danjiangkou reservoir, and the annual sediment load of Hanjiang River in 1969-2013 was only 14% of that in 1956-1968. The sediment in Jinshajiang River decreased by about 42% after the impoundment of Ertan reservoir in 2000; A large amount of sediment was intercepted by the Three Gorges reservoir after 2003, and the average sediment of the Yangtze River in 2003-2013 (143 Mt/yr) was only 28% of that in 1956-1968. The sediment in the West River of the Pearl River was only 23% of in 2007-2013 after the impoundment of Tianshengqiao reservoir in 1997 and Longtan reservoir in 2006.The changes in land use types also have great impacts on the water and sediment fluxes of the three rivers. For example, the sediment yield of the Yellow River decreased significantly since 1980s with the implementation of the water and soil conservation measures in the Loess Plateau of the the Yellow River. The surface erosion enhanced with the development of the Yangtze River Basin, and the sediment yeild increased before 1980s, but it decreased after that in response to the water and soil conservation in the basin (the Changzhi Project). The large-scale agricultural production and deforestation since the late-1970s in the Pearl River Basin exacerbated the soil erosion, and the observed sediment yield in the West River and the East River was larger than the predicted values in 1980s; but the sediment decreased gradually with the implementation of reservoirs and the water and soil conservation measures after the 1990s.Additionally, the water demand for industry and agriculture has increased dramatically with the increasing of population, together with the water diversion and sand mining activities in the basin, were important reasons for the reduction in runoff.The seasonal distribution of the runoff and sediment was also greatly afftected by human activities. The difference of the runoff and sediment in the flood and dry greatly reduced in the three basins. For example, the summer runoff and sediment load in the Yellow River had sharp declines due to the impoundment of Longyangxia Reservoir in 1986 and Xiaolangdi reservoir in 1999, and the ratio of the maximum monthly runoff (August) and the minimum monthly runoff (January) was 11:1 in 1956-1959 (the natural period), but it was 7:1 in 2002-2013 under great human influence; The ratio of the maximum monthly sediment (August) and the minimum monthly sediment (January) was 495:1 in 1956-1959, while the ratio of the maximum monthly sediment (July) and the minimum monthly sediment (February) was only 71:1 after 2002. The ratio of the maximum monthly sediment (July) and the minimum monthly sediment (January) of the Yangtze River was 42:1 before the impoundment of the Three Gorges reservoir in 2003, but ratio of the maximum monthly sediment (July) and the minimum monthly sediment (February) was 11:1 after 2003. The ratio of the maximum monthly sediment (July) and the minimum monthly sediment (December) of the Pearl River was 109:1 in 1956-1959 (the natural period), but ratio of the maximum monthly sediment (June) and the minimum monthly sediment (February) was only 54:1 after the impoundment of Longtan reservoir in 2006.In summary, compared to the Yangtze River and the Pearl River, the water and sediment of the Yellow River was more sensitive to climate change and human activities. The impact of human activities on runoff and sediment was more obvious in the north river basins than in the south basins. |
PDF Full Text Request