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Larger Asian rivers: Shorter and longer term hydro-climate changes in humid and arid environments
Quaternary International 336 (2014) 1e5
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Quaternary International journal homepage: www.elsevier.com/locate/quaint
Guest Editorial
Larger Asian rivers: Shorter and longer term hydro-climate changes in humid and arid environments
Climate change and human activities affect river ecosystems world-wide. Larger Asian rivers, especially those originating from the Himalayas and the Tibet Plateau (Fig. 1), are likely to be most affected by climate change and human activities, due to their fragile environments. Major large Asian rivers have been experiencing drastic changes in the water flow and in particular in sediment loads over the past decades (Fig. 2). These larger Asian river basins have very high population densities and will be at risk due to intensified flood and drought events, degradation of river ecosystems, and shortage of water supply. Chinese rivers in particular have been experiencing drastic changes in the water flow and sediment loads (c.f. Lu et al., 2013). In the worst cases, many inland waterbodies have been dried up according to the Bulletin of First National Census for Water (2013). China used to have around 50,000 rivers, each with a drainage basin area of at least 100 km2. According to the 2013 census, there were 22,909 rivers as of 2011, i.e. about 28,000 rivers fewer than government's previous estimates. It is believed that the disappearance of Chinese rivers is partially due to precipitation reduction, dam construction, and overconsumption and withdrawal of water. The extreme events such as recent droughts in Southwest China and the Yangtze River basin, and the flooding events in Sichuan and Northeast China, have had higher impacts on the populations and the various socio-economic sectors (e.g. agriculture, industry, and drinking water) than was observed before. Apart from extreme climate events, the anthropogenic activities such as reservoir construction, land cover changes, water diversion, and extraction were largely responsible for these extreme events. A series of special volumes have been published in Quaternary International to exchange various studies on larger Asian rivers. The previous 6 volumes include Volumes 186 (Lu and Chen, 2008), Volume 208 (Lu and Jiang, 2009), Volume 226 (Jiang et al., 2010), Volume 244 (Jiang et al., 2011), Volume 282 (Jiang et al., 2012) and Volume 304 (Jiang et al., 2013). This current volume is the 7th issue on larger Asian rivers. The theme of this current volume is changes in climatic, hydrological and sedimentary environments in larger Asian river basins. We organized 18 papers in total according to the topics into (1) Shorter term hydro-climate changes in humid regions; (2) Shorter term hydro-climate changes in arid regions; (3) Longer term hydro-climate changes. The collection of this volume highlights the most urgent concerns of today's freshwater problems. To obtain a quick overview of the research findings collected in this Seventh Special Issue on larger Asian rivers, each paper is briefly described below. http://dx.doi.org/10.1016/j.quaint.2014.05.039 1040-6182/© 2014 Elsevier Ltd and INQUA.
1. Shorter term hydro-climate changes in humid regions Potential evapotranspiration (ETo) is an integral component of the hydrological cycle and the global energy balance, and its longterm variation is of much concern in climate change studies. Li ZX et al. examined trends of temporal and spatial patterns in potential evapotranspiration and related driving factors for the period 1961e2009 over southwestern China. They concluded that southwestern China was experiencing a decrease of ETo at a rate of 5.0 mm/10 y during 1961e2009. This decrease mainly started in the 1980s when accelerated warming occurred, which reflected the existence of the “evaporation paradox”. The authors attributed the decrease to factors including wind and sunshine. (1) The decreased wind speed was the main driving force for ETo decrease because the higher saturated vapor pressure restrained evaporation owing to the weak water vapor exchange under lower wind speed. (2) Increased sunshine hours led to stronger evaporation. This influence mainly occurred in the Xizang plateaueHengduan Mountains and YunnaneGuizhou plateau. (3) The weakly increased precipitation was also linked to the decreased ETo because the precipitation increase partially increased ETo. (4) The decreased relative humidity caused a corresponding decline of saturated vapor pressure, leading to the rise of ETo. (5) Climate warming was not the crucial driving factor for ETo inter-annual variation, but the accelerated warming had some influence on summer and autumn ETo, especially during the period of 1991e2009. In addition, the topography also had some influence. Dam operations have had significant impact on the natural flow regimes in most of the world large rivers. The Mekong River in Southeast Asia was one of the few remaining rather intact large river basins. The natural flow regime, however, is changing due to the recent hydropower development of many large dams in both mainstream and tributaries. Lu XX et al. investigated the impacts of dams operation on the natural flows and the recent droughts, in particular in Chiang Saen, Thailand, close to the Chinese dams on the Mekong River mainstream. They used the Indicators of Hydrological Alteration (IHA) to examine the impacts of dams operation on water discharge. The water discharge was lower in the dry seasons (1-, 3-, and 7-day minima) in the postdam period (1992e2010) than in the pre-dam period (1960e1991). The water discharge in the wet seasons (1-, 3-, and 7-day maxima) was marginally lower in the post-dam period. The monthly mean value averaged over the entire post-dam period was higher in July (an increase of 15%), but lower in August (a decrease of 9%) than in the pre-dam period, suggesting the
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Guest Editorial / Quaternary International 336 (2014) 1e5
Fig. 1. Major large Asian rivers from Himalayas and Tibet Plataea: Yellow River, Yangtze River, Pearl River, Red River, Mekong River, Chao Phraya, Irrawaddy, Salween, Brahmaputra, Ganges, Indus, Armu Darya and Tarim.
reservoirs related more water for hydropower generation in July, but started to store water in August. The early monsoon withdrawal and low monsoon rainfall in 2009, as well as very low rainfall in the dry season of 2010, resulted in the 2010 severe drought in Yunnan and the lower Mekong basin. The authors argued that climate change, glacial melting in the river origin area, and more importantly, recent extreme weather events, could put the Chinese reservoirs at the center of debate.
Fig. 2. Cumulative sediment fluxes of pre-1980s and post-1990s to oceans from major large Asian rivers (Darya and Tarim are not included) (modified from Lu et al., 2011).
Projections of future water resources in large river basins are of the utmost importance. However, it is difficult to predict hydrological processes by applying physically based hydrological models. Su BD et al. used artificial neural networks to project streamflow in the Songhuajiang River basin, one of the important agricultural areas in China. Multi-layer Perceptron Networks were applied to construct relationships between climate factors and hydrological variables in this study. Possible changes of precipitation and streamflow in 2011e2050 in the Songhuajiang River basin were obtained by considering the climate conditions of the ECHAM5/MPI-OM under three SRES scenarios and the ensemble mean of the CMIP5 models under three RCP scenarios. Their results indicate that inter-annual and inter-decadal changes of precipitation and streamflow (at the Jiamusi hydrological station) did not show obvious trends in 2011e2050. However, seasonal precipitation and streamflow showed various tendencies under different emission scenarios, and their inner annual distribution patterns tended to change as well. Furthermore, precipitation and streamflow had more obvious increasing tendencies under the RCP scenarios than under the SRES scenarios. Rivers have many service functions, and play an important role in people's livelihood and agricultural production. During the last decades, intensive human activities have been threatening river ecosystem. For shipping, hydraulic engineering facilities (e.g. spur dikes) have been built along river bank to increase water depth. Natural habitat conditions (i.e. substrate, flow regime, water physico-chemical properties) have been
Guest Editorial / Quaternary International 336 (2014) 1e5
altered. Macroinvertebrates are often considered as good indicators of long-term changes in environments due to their confinement to the bottom, long life cycles and limited abilities of movement. Pan BZ et al. conducted field investigations of macroinvertebrates in the West River in November 2009 (at low water level) and May 2010 (at high water level). Altogether, 70 macroinvertebrate taxa belonging to 30 families and 59 genera were identified. Among them were 16 annelids, 21 molluscs, 32 arthropods, and 1 miscellaneous animal. The average density and biomass of total macroinvertebrates were 140 ind m2 and 0.23 g dry weight m2, respectively. Canonical Correspondence Analysis (CCA) revealed that major factors structuring macroinvertebrate assemblages were flow velocity, water depth, conductivity, total nitrogen and substrates (clay, silt, cobbles and bedrock). Density and diversity of macroinvertebrates peaked in the cobbles, while biomass reached to the maximum in the bedrock. Stable and heterogeneous habitats were beneficial to development of benthic assemblages. To conserve these habitats, type and construction position of spur dikes should receive attention. Yan YX et al. examined the scale effects of Area-Specific sediment yield (Ys) in Zhujiang basin. There was no apparent relation between Ys and contributing area (Ad) for stations when Ad is <100,000 km2, while there was a negative relation of Ys and Ad for the stations with Ad >100,000 km2 located on the middle and lower reaches. For the four sub-basins of Zhujiang basin, three kinds of relations between Ys and Ad were established: (1) Ys increased with Ad for the sub-basin of the upper reach; (2) Ys increased with Ad then declines for the two sub-basins of the middle reaches; (3) Ys decreased with Ad for the sub-basin of the lower reaches. The YseAd relation was further investigated using the spatial variation of hill slopes, Land Use and Land Cover (LULC), and channel bed slopes. Finally, to remove the effect of Ad and to make Ys comparable across scales, the scaled equations both linear and non-linear were established between Ys and Ad, and then Ys was scaled to the standard units of 1000 km2 and 10,000 km2. Using a Kriging method, Ys maps were created and spatial patterns of Ys were discussed. Mao LJ et al. studied grain size, and trace and rare earth elements (REEs) of floodplain sediments taken from the Xiangjiang River, a tributary of the Yangtze River. The results were used to characterize the sediments REE composition and distribution and to identify the sediment sources and the effects of industrial pollution. The total REE (SREE) concentrations of the river sediments had wide variations, ranging from 37.5 to 237.4 mg g1, with an average of 145.6 mg g1. The chondrite-normalized REE pattern was differentiated into two groups, whereas the average upper continental crust (UCC)-normalized REE was classified into four groups. SREE had strong positive correlations with clay (%) (r ¼ 0.91), and <30 mm (%) (r ¼ 0.89) and strong negative correlations with sand (%) (r ¼ 0.91), implying that the grain size was an important factor in controlling the REE abundance because REEs were preferentially enriched in clay minerals in fine sediments and diluted by quartz and carbonate minerals in coarse sediments. SREE yields strong positive correlations with Mn (r ¼ 0.78) and Fe (r ¼ 0.97), suggesting that the REE composition of Xiangjiang river sediments was controlled by clay mineral compositions. Fe- and Mn- oxyhydroxides accumulated in fine-grained sediments and control the REE abundance. Two stations (XJ08 and XJ14) showed different UCC- and chondrite-normalized REE distribution patterns with relative light rare earth element (LREE) depletion and heavy rare earth element (HREE) enrichment compared with other stations, suggesting they were polluted by industrial effluents. Therefore, REEs could be effectively used to trace sediment provenance and pollution sources.
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2. Shorter term hydro-climate changes in arid regions Feng J et al. quantified the contribution of climatic factors to potential evapotranspiration (ET0) under different drought periods, making use of the FAO PenmaneMonteith method, linear regression analysis, ManneKendall test, and the drought classification derived from the Standardized Precipitation Index (SPI). ET0 and meteorological data from 93 North China weather stations from 1961 to 2010 located in a semi-arid climate zone were analyzed. The annual ET0 values showed a significantly increasing trend with the aggravation of drought. They indicated that in drought periods, temperature and sunshine hours had positive effects on the variation of ET0, while other key climatic factors (i.e. effective precipitation, vapor pressure, wind speed, relative humidity) had negative effects on ET0. ET0 was most sensitive to effective precipitation and mean daily temperature, with sensitivity rates of 4.163 and 1.368, respectively. The sensitivities of ET0 decreased over time in the drought period, which was caused by the joint effects of the climatic factors. The decreasing sensitivity of wind speed and relative humidity increased ET0, and the increase of daily temperature produced the majority of rise in ET0 rates. The decrease of wind speed and relative humidity resulted in positive contributions to ET0. The increase in the sensitivity of precipitation and vapor pressure resulted in strongly decreased ET0, and the decrease of sunshine hours resulted in little change in ET0 rates. The variability of temperature extremes has been widely discussed in the past few decades. Liang K et al. used the daily maximum and minimum air temperature from 1961 to 2011 at 62 meteorological stations in the Yellow River basin to investigate the changes in 13 indices of temperature extremes recommended by the ETCCDI. Both cold and warm extremes showed a warming trend, with a large proportion of stations having statistically significant trends for most temperature indices. Because of a faster increase in minimum temperature than maximum temperature, diurnal temperature range (DTR) decreased by 0.07 C /decade. For cold extremes indices, i.e. coldest night (TNn), coldest day (TXn), cool nights (TN10p), cool days (TX10p), frost day (FD) and icing day (ID), the trends during the period of 1961e2011 were 0.23 and 0.22 C /decade, and 5.23, 3.31, 2.61 and 3.16 days/ decade, respectively. For warm extremes, i.e. warmest night (TNx), warmest day (TXx), warm nights (TN90p), warm days (TX90p), warm spell duration index (WSDI) and growing season length (GSL), the trends were 0.20 and 0.21 C /decade, and 3.80, 3.26, 2.93 and 4.81 days/decade, respectively. In general, the indices had the largest change magnitudes in the arid and semi-arid subregions, followed by the high-elevation sub-region, and the indices had the smallest changes in the eastern monsoon sub-region. Relationships between the changes in the temperature extremes and the changes in the large-scale atmospheric circulations were also studied. The changes in the temperature extremes were consistent with the changes in the area and strength of Northern Hemisphere Subtropical High (NHSH) and Northern Hemisphere Polar Vortex (NHPV). Hydrological prediction of ungauged basins remains a major challenge in hydrology. A key issue in the application of hydrological models to ungauged areas is regional parameterization. Wang GQ et al. presented a regional calibration approach to constructing regional relationships between the parameters of the Snowmeltbased Water Balance Model (SWBM) and catchment characteristics. Data from 38 well-gauged catchments were used to establish regional regression equations of model parameters, and data from additional four independent catchments were used to test and validate the proposed regional calibration approach. They indicated that the SWBM performed well in monthly discharge simulations for the 38 well-gauged catchments. The regional relationships of
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the SWBM parameters derived from the regional calibration approach were found to be highly correlated to catchment characteristics. In addition, the SWBM performed well in discharge simulations for the additional four test sites using only the constructed regional equations to estimate model parameters. Trends and abrupt detection of temperature and precipitation extremes are important in climate change research. Chen YN et al. analyzed the trend and abrupt change in temperature and precipitation extremes using the meteorological data from 68 stations in the arid region of Northwest China (ARNC) from 1961 to 2010. Results showed that abrupt change in both temperature and precipitation extremes in Northwest China occurred in around 1986. Interestingly, an abrupt change in Index B of the Tibetan Plateau (TPI_B) was detected in 1985. The temperature and precipitation extremes had strong and significant associations with TPI_B over the period of 1961e2010 (R ¼ 0.685, p < 0.01, and R ¼ 0.441, p < 0.01, respectively). They behaved consistently, with a weakening and decreasing trend from 1961 to 1984 and a strengthening and increasing trend from 1985 to 2010. Thus, TPI_B was probably an important factor in the abrupt change in both temperature and precipitation extremes in the ARNC. Ye ZX et al. analyzed the characteristics and relationship between runoff, sediment and climate over the past 50 years at the Aral Hydrological Station of Tarim River. Temporal analysis showed that both water and sediment at the Aral Hydrological Station decreased in the past 50 years, although these trends were not significant. The monthly distribution analysis showed that sediment load was more concentrated and more unstable compared to runoff during the flood season. The correlation analysis showed that annual sediment load was more related to annual maximum peak discharge followed by precipitation. This study considered the impact factors of the mainstream sediment changes as a foundation for subsequent research such as analysis of sediment sources and relationship with climate change. The Aksu River is the most important recharge source of the Tarim River in northwest China, representing 73.2% of the total volume to the Tarim River in the dry season. There are three types of recharge: glacier-snow meltwater, precipitation, and groundwater. Precipitation and baseflow in the basin play a decisive role. Fan YT et al. examined the dynamics of precipitation and baseflow in the last past 50 years. The digital filtering method was used to separate baseflow from surface flow. Relationships between precipitation and baseflow were identified by correlation and wavelet analysis. They found: 1) There was a significant increasing trend of both precipitation and baseflow In the past 50 years. The 1990s was the wettest decade of precipitation, and baseflow in the late 1990s was also the wettest. 2) The baseflow showed an increasing trend in all seasons, and precipitation exhibited a monotonic increasing trend in summer and autumn, and decreasing trend in spring and winter. 3) The responses of baseflow to precipitation were different by season. The baseflow had a 0e3 months lag behind precipitation. Understanding the impact of climate change on water resources is essential for sustainable development. Kong YL and Pang ZH evaluated the impact of climate change on discharges of Urumqi River, a medium-sized headwater catchment in the Tianshan mountain, where impacts of human activities on water were negligible and the influence of glacier melt was minor as far as the mountainous outlet was concerned. Time series of temperature, precipitation and river discharge of the hydrological station at the mountainous outlet from 1959 to 2006 were analyzed using different statistical methods, including linear regression, ManneKendall test and wavelet analysis. Although both temperature and precipitation showed a significant upward trend with a gradient of 0.02 C /y and 2.08 mm/y, respectively, there was no significant rising trend
in the stream discharge. The reasons were attributed to the hysteresis and buffering effects of groundwater in conveying the change from precipitation to stream discharge. Precipitation had a stronger influence on stream discharge than air temperature throughout the 48 years of instrumental records in the Urumqi River catchment. Gao JG et al. assessed the spatial patterns of alpine grassland changes (expansion and contraction) and their responses to climate changes in the context of global change in recent years on the southern Tibetan Plateau, Mt. Qomomagma National Natural Preserve (QNNP), covering an area of 36,594 km2 and located on the southern Tibetan Plateau e the world's “Third Pole”, was selected. Alpine grassland cover was obtained by image reclassification of more than five pairs of remote sensing images taken in 1976e2006. Expansion and contraction were determined by cover changes in alpine grassland, and were expressed as binary variables. Logistic regression was then used to analyze the impacts of climate-, neighborhood- and topographic-related variables on alpine grsssland changes. The cover of alpine grassland was increased by 84.3 km2. The expansion rate was significantly higher on the lower slopes, with a higher mean annual temperature (MAT), higher inter-annual variability trend of temperature (IVT), and inter-annual variability trend of precipitation (IVP), close to lakes and distant from roads. While most contraction was found on the lower slopes and in the areas close to streams, lakes, and roads. A significant impact was also driven by MAT. MAT significantly affected alpine grassland expansion and contraction in the QNNP, while mean annual precipitation (MAP) did not. IVT and IVP had significant effects on alpine grassland expansion but not on contraction. They suggested that climate changes had different effects on alpine grassland changes in different areas of the Tibetan Plateau. 3. Longer term hydro-climate changes Li F et al. used time sequence of the JZ-2010 profile derived from AMS 14C dating and content of the sediment elements Ti, Rb, and Sr, the Rb/Sr values as well as the granularity and the magnetic susceptibility to reconstruct the environmental humidity changes in the Jianghan Plain since 12,700 cal. BP. The comprehensive analyses of multiple alternative indicators indicated that the regional moisture turned from relatively dry to wet after the Late-Glacial period and reached its optimum until the mid-Holocene. However, the trend was punctuated by several decreased humidity phases. Beginning with the severe dry event approximately 4400e4100 cal. BP, the environment was dry as a whole, with modest humidity from approximately 3900 cal. BP. Humidity changes in the study area were dominated by the East Asian monsoon system, which was under the influence of the gradual southward migration of the ITCZ, driven by the summer solar insolation changes in the Northern Hemisphere due to orbital forcing. As the first attempt to apply Rb/Sr values and Ti concentrations in tracing moisture evolution in a region with typical subtropical humid climate, this study highlighted the different effects and consequences of the proxies throughout the palaeoenvironmental change process, and the impacts of regional topography that sustained tectonic subsidence since the Cretaceous as well as the southeast tilt that opened the plain to the summer monsoon. The fluctuation of magnetic susceptibility was associated with the granularity feature, the reductioneoxidation conditions, and the impact of human factors on the natural sedimentary environments during the historic period in the Jianghan Plain. Liu R et al. collected a 45.3-m long core from the Hemudu civilization site (Luojiang county, Zhejiang Province; 29 59.0640 N, 12121.7540 E), located in the Ningshao plain on the southern margin of Hangzhou Bay. The core, named the HMD Core, was
Guest Editorial / Quaternary International 336 (2014) 1e5
divided into seven lithological layers with clay and silt as the main lithological components. The sporopollen assemblages of this core combined with TOC/TN and stable organic carbon isotope (d13CTOC) analysis revealed the sedimentary environmental changes since the later part of the Late Pleistocene in the Ningshao plain. The study area changed from terrestrial fluvial-flood plain faces before Marine Isotope Stage (MIS) 3 to shore plain during transgression in MIS3, and became a freshwater lake during regression at the time of the Last Glacial Maximum (LGM). The area started to become a semi-salt lake influenced by sea water during 15,900e5000 cal. BP, when the lake was deeper and the climate became gradually warmer. After the seawater withdrew later, the lake became desalinated, and then swampy with accumulation of peat until 1000 cal. BP. After that, the area became dry land. This study also indicated that the sediments of the core were of endogenous organic origin and the d13CTOC value was controlled by change in the concentration of atmospheric CO2, due to global changes. Dai L et al. analyzed 45 seafloor surface sediment samples from the SCS and nine surface sediment samples from the Pearl River mouth to improve the understanding of pollen transportation and deposition mechanisms in northern South China Sea (SCS). The spatial distributions of pollen and spore taxa suggested various transport mechanisms and production at the source areas. The dominant pollen Pinus, Poaceae, Cyperaceae and fern spores were most abundant in the river mouth and decreased with increasing distance from land, suggesting they were mainly derived from the Pearl River basin and strongly dispersed by river plume and ocean currents, which reflected a dominant pollen transportation pattern in the northern SCS. Podocarpus and temperate broadleaved components pollen such as evergreen Quercus and Castanopsis were common in marine samples and scattered in the Pearl River samples, reflecting their extensive source areas and dispersal in more than one route. However, some pollen types of conifers were mostly deposited close to the shore, such as Dacrydium, Picea and Abies, which were restricted near Hainan Island and Taiwan. Excluding evergreen Quercus and Castanopsis pollen, the total tropical and subtropical components poorly reflected their parent plants, and fewer such pollen were transported over long distances. Spatially varying ratios of spores to pollen indicated that spores were relatively easily transported by water over longer distances compared to pollen grains. This study provided a sound basis for interpreting Quaternary pollen deposits from the northern SCS, and for further evaluation of pollen dispersal and deposition in marine and fluvial deposits. Seabirds migrate between marine and island ecosystems, and their subfossil remains have been widely used in palaeoecological studies. Xu LQ et al. collected bone subfossils of seabirds from Ganquan Island of the Xisha archipelago, South China Sea. Stable isotopes (d15N and d13C) in well-preserved ancient collagen samples were analyzed to investigate seabird palaeodiets over the past
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2200 years. Stable isotope-based palaeodietary analysis indicated a significant change in seabirds' foraging behavior at 1200 BP. A long-term connection between seabird population and their trophic levels was identified. High-trophic prey contributed more to the diet of birds during the period of relatively high seabird abundance at 1200e200 BP. Both collagen d15N and d13C indicated that intra/inter-specific competitions might force seabirds to dive deeper or travel further to catch prey. Acknowledgments We are thankful for Ministry of Education, Singapore (MOE2011T2-1-101), National Basic Research Program of China (973 Program) (No. 2012CB4955903), the National Natural Science Foundation of China (41171406). We would also like to thank Professor Norm Catto for his consistent help in compiling this special volume. References Jiang, T., Fischer, T., Lu, X.X., 2013. Larger Asian rivers: changes in hydro-climate and water environments. Quaternary International 304, 1e4. Jiang, T., Fischer, T., Lu, X.X., 2012. Larger Asian rivers: climate, water discharge, water and sediment quality. Quaternary International 282, 1e4. Jiang, T., Fischer, T., Lu, X.X., 2011. Larger Asian rivers: climate, hydrology and ecosystems. Quaternary International 244, 127e129. Jiang, T., Fischer, T., Lu, X.X., 2010. Larger Asian Rivers: climate change, river flow, and watershed management. Quaternary International 226, 1e3. Lu, X.X., Jiang, T., 2009. Larger Asian rivers: climate change, river flow and sediment flux. Quaternary International 208, 1e3. Lu, X.X., Chen, X., 2008. Large Asian rivers and their interactions with estuaries and coasts. Quaternary International 186, 1e3. Lu, X.X., Zhang, S.R., Xu, J.C., Merz, J., 2011. The changing sediment loads of the Hindu Kush-Himalayan rivers: an overview. In: Sediment Problems and Sediment Management in Asian River Basins (Proceedings of the Workshop Held at Hyderabad, India, September 2009). IAHS Publ. 349. Lu, X.X., Ran, L.S., Liu, S., Jiang, T., Zhang, S.R., Wang, J.J., 2013. Sediment loads' response to climate change: a preliminary study of eight large Chinese rivers. International Journal of Sediment Research 28 (1), 1e14. Ministry of Water Resources, P.R.China, 2013. Bulletin of First National Census for Water. China Waterpub Press.
Xixi Lu* Department of Geography, National University of Singapore, Singapore Tong Jiang National Climate Center, China Meteorological Administration, Beijing, China Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China * Corresponding author. E-mail address: [email protected] (X. Lu).
Contents lists available at ScienceDirect
Quaternary International journal homepage: www.elsevier.com/locate/quaint
Guest Editorial
Larger Asian rivers: Shorter and longer term hydro-climate changes in humid and arid environments
Climate change and human activities affect river ecosystems world-wide. Larger Asian rivers, especially those originating from the Himalayas and the Tibet Plateau (Fig. 1), are likely to be most affected by climate change and human activities, due to their fragile environments. Major large Asian rivers have been experiencing drastic changes in the water flow and in particular in sediment loads over the past decades (Fig. 2). These larger Asian river basins have very high population densities and will be at risk due to intensified flood and drought events, degradation of river ecosystems, and shortage of water supply. Chinese rivers in particular have been experiencing drastic changes in the water flow and sediment loads (c.f. Lu et al., 2013). In the worst cases, many inland waterbodies have been dried up according to the Bulletin of First National Census for Water (2013). China used to have around 50,000 rivers, each with a drainage basin area of at least 100 km2. According to the 2013 census, there were 22,909 rivers as of 2011, i.e. about 28,000 rivers fewer than government's previous estimates. It is believed that the disappearance of Chinese rivers is partially due to precipitation reduction, dam construction, and overconsumption and withdrawal of water. The extreme events such as recent droughts in Southwest China and the Yangtze River basin, and the flooding events in Sichuan and Northeast China, have had higher impacts on the populations and the various socio-economic sectors (e.g. agriculture, industry, and drinking water) than was observed before. Apart from extreme climate events, the anthropogenic activities such as reservoir construction, land cover changes, water diversion, and extraction were largely responsible for these extreme events. A series of special volumes have been published in Quaternary International to exchange various studies on larger Asian rivers. The previous 6 volumes include Volumes 186 (Lu and Chen, 2008), Volume 208 (Lu and Jiang, 2009), Volume 226 (Jiang et al., 2010), Volume 244 (Jiang et al., 2011), Volume 282 (Jiang et al., 2012) and Volume 304 (Jiang et al., 2013). This current volume is the 7th issue on larger Asian rivers. The theme of this current volume is changes in climatic, hydrological and sedimentary environments in larger Asian river basins. We organized 18 papers in total according to the topics into (1) Shorter term hydro-climate changes in humid regions; (2) Shorter term hydro-climate changes in arid regions; (3) Longer term hydro-climate changes. The collection of this volume highlights the most urgent concerns of today's freshwater problems. To obtain a quick overview of the research findings collected in this Seventh Special Issue on larger Asian rivers, each paper is briefly described below. http://dx.doi.org/10.1016/j.quaint.2014.05.039 1040-6182/© 2014 Elsevier Ltd and INQUA.
1. Shorter term hydro-climate changes in humid regions Potential evapotranspiration (ETo) is an integral component of the hydrological cycle and the global energy balance, and its longterm variation is of much concern in climate change studies. Li ZX et al. examined trends of temporal and spatial patterns in potential evapotranspiration and related driving factors for the period 1961e2009 over southwestern China. They concluded that southwestern China was experiencing a decrease of ETo at a rate of 5.0 mm/10 y during 1961e2009. This decrease mainly started in the 1980s when accelerated warming occurred, which reflected the existence of the “evaporation paradox”. The authors attributed the decrease to factors including wind and sunshine. (1) The decreased wind speed was the main driving force for ETo decrease because the higher saturated vapor pressure restrained evaporation owing to the weak water vapor exchange under lower wind speed. (2) Increased sunshine hours led to stronger evaporation. This influence mainly occurred in the Xizang plateaueHengduan Mountains and YunnaneGuizhou plateau. (3) The weakly increased precipitation was also linked to the decreased ETo because the precipitation increase partially increased ETo. (4) The decreased relative humidity caused a corresponding decline of saturated vapor pressure, leading to the rise of ETo. (5) Climate warming was not the crucial driving factor for ETo inter-annual variation, but the accelerated warming had some influence on summer and autumn ETo, especially during the period of 1991e2009. In addition, the topography also had some influence. Dam operations have had significant impact on the natural flow regimes in most of the world large rivers. The Mekong River in Southeast Asia was one of the few remaining rather intact large river basins. The natural flow regime, however, is changing due to the recent hydropower development of many large dams in both mainstream and tributaries. Lu XX et al. investigated the impacts of dams operation on the natural flows and the recent droughts, in particular in Chiang Saen, Thailand, close to the Chinese dams on the Mekong River mainstream. They used the Indicators of Hydrological Alteration (IHA) to examine the impacts of dams operation on water discharge. The water discharge was lower in the dry seasons (1-, 3-, and 7-day minima) in the postdam period (1992e2010) than in the pre-dam period (1960e1991). The water discharge in the wet seasons (1-, 3-, and 7-day maxima) was marginally lower in the post-dam period. The monthly mean value averaged over the entire post-dam period was higher in July (an increase of 15%), but lower in August (a decrease of 9%) than in the pre-dam period, suggesting the
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Guest Editorial / Quaternary International 336 (2014) 1e5
Fig. 1. Major large Asian rivers from Himalayas and Tibet Plataea: Yellow River, Yangtze River, Pearl River, Red River, Mekong River, Chao Phraya, Irrawaddy, Salween, Brahmaputra, Ganges, Indus, Armu Darya and Tarim.
reservoirs related more water for hydropower generation in July, but started to store water in August. The early monsoon withdrawal and low monsoon rainfall in 2009, as well as very low rainfall in the dry season of 2010, resulted in the 2010 severe drought in Yunnan and the lower Mekong basin. The authors argued that climate change, glacial melting in the river origin area, and more importantly, recent extreme weather events, could put the Chinese reservoirs at the center of debate.
Fig. 2. Cumulative sediment fluxes of pre-1980s and post-1990s to oceans from major large Asian rivers (Darya and Tarim are not included) (modified from Lu et al., 2011).
Projections of future water resources in large river basins are of the utmost importance. However, it is difficult to predict hydrological processes by applying physically based hydrological models. Su BD et al. used artificial neural networks to project streamflow in the Songhuajiang River basin, one of the important agricultural areas in China. Multi-layer Perceptron Networks were applied to construct relationships between climate factors and hydrological variables in this study. Possible changes of precipitation and streamflow in 2011e2050 in the Songhuajiang River basin were obtained by considering the climate conditions of the ECHAM5/MPI-OM under three SRES scenarios and the ensemble mean of the CMIP5 models under three RCP scenarios. Their results indicate that inter-annual and inter-decadal changes of precipitation and streamflow (at the Jiamusi hydrological station) did not show obvious trends in 2011e2050. However, seasonal precipitation and streamflow showed various tendencies under different emission scenarios, and their inner annual distribution patterns tended to change as well. Furthermore, precipitation and streamflow had more obvious increasing tendencies under the RCP scenarios than under the SRES scenarios. Rivers have many service functions, and play an important role in people's livelihood and agricultural production. During the last decades, intensive human activities have been threatening river ecosystem. For shipping, hydraulic engineering facilities (e.g. spur dikes) have been built along river bank to increase water depth. Natural habitat conditions (i.e. substrate, flow regime, water physico-chemical properties) have been
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altered. Macroinvertebrates are often considered as good indicators of long-term changes in environments due to their confinement to the bottom, long life cycles and limited abilities of movement. Pan BZ et al. conducted field investigations of macroinvertebrates in the West River in November 2009 (at low water level) and May 2010 (at high water level). Altogether, 70 macroinvertebrate taxa belonging to 30 families and 59 genera were identified. Among them were 16 annelids, 21 molluscs, 32 arthropods, and 1 miscellaneous animal. The average density and biomass of total macroinvertebrates were 140 ind m2 and 0.23 g dry weight m2, respectively. Canonical Correspondence Analysis (CCA) revealed that major factors structuring macroinvertebrate assemblages were flow velocity, water depth, conductivity, total nitrogen and substrates (clay, silt, cobbles and bedrock). Density and diversity of macroinvertebrates peaked in the cobbles, while biomass reached to the maximum in the bedrock. Stable and heterogeneous habitats were beneficial to development of benthic assemblages. To conserve these habitats, type and construction position of spur dikes should receive attention. Yan YX et al. examined the scale effects of Area-Specific sediment yield (Ys) in Zhujiang basin. There was no apparent relation between Ys and contributing area (Ad) for stations when Ad is <100,000 km2, while there was a negative relation of Ys and Ad for the stations with Ad >100,000 km2 located on the middle and lower reaches. For the four sub-basins of Zhujiang basin, three kinds of relations between Ys and Ad were established: (1) Ys increased with Ad for the sub-basin of the upper reach; (2) Ys increased with Ad then declines for the two sub-basins of the middle reaches; (3) Ys decreased with Ad for the sub-basin of the lower reaches. The YseAd relation was further investigated using the spatial variation of hill slopes, Land Use and Land Cover (LULC), and channel bed slopes. Finally, to remove the effect of Ad and to make Ys comparable across scales, the scaled equations both linear and non-linear were established between Ys and Ad, and then Ys was scaled to the standard units of 1000 km2 and 10,000 km2. Using a Kriging method, Ys maps were created and spatial patterns of Ys were discussed. Mao LJ et al. studied grain size, and trace and rare earth elements (REEs) of floodplain sediments taken from the Xiangjiang River, a tributary of the Yangtze River. The results were used to characterize the sediments REE composition and distribution and to identify the sediment sources and the effects of industrial pollution. The total REE (SREE) concentrations of the river sediments had wide variations, ranging from 37.5 to 237.4 mg g1, with an average of 145.6 mg g1. The chondrite-normalized REE pattern was differentiated into two groups, whereas the average upper continental crust (UCC)-normalized REE was classified into four groups. SREE had strong positive correlations with clay (%) (r ¼ 0.91), and <30 mm (%) (r ¼ 0.89) and strong negative correlations with sand (%) (r ¼ 0.91), implying that the grain size was an important factor in controlling the REE abundance because REEs were preferentially enriched in clay minerals in fine sediments and diluted by quartz and carbonate minerals in coarse sediments. SREE yields strong positive correlations with Mn (r ¼ 0.78) and Fe (r ¼ 0.97), suggesting that the REE composition of Xiangjiang river sediments was controlled by clay mineral compositions. Fe- and Mn- oxyhydroxides accumulated in fine-grained sediments and control the REE abundance. Two stations (XJ08 and XJ14) showed different UCC- and chondrite-normalized REE distribution patterns with relative light rare earth element (LREE) depletion and heavy rare earth element (HREE) enrichment compared with other stations, suggesting they were polluted by industrial effluents. Therefore, REEs could be effectively used to trace sediment provenance and pollution sources.
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2. Shorter term hydro-climate changes in arid regions Feng J et al. quantified the contribution of climatic factors to potential evapotranspiration (ET0) under different drought periods, making use of the FAO PenmaneMonteith method, linear regression analysis, ManneKendall test, and the drought classification derived from the Standardized Precipitation Index (SPI). ET0 and meteorological data from 93 North China weather stations from 1961 to 2010 located in a semi-arid climate zone were analyzed. The annual ET0 values showed a significantly increasing trend with the aggravation of drought. They indicated that in drought periods, temperature and sunshine hours had positive effects on the variation of ET0, while other key climatic factors (i.e. effective precipitation, vapor pressure, wind speed, relative humidity) had negative effects on ET0. ET0 was most sensitive to effective precipitation and mean daily temperature, with sensitivity rates of 4.163 and 1.368, respectively. The sensitivities of ET0 decreased over time in the drought period, which was caused by the joint effects of the climatic factors. The decreasing sensitivity of wind speed and relative humidity increased ET0, and the increase of daily temperature produced the majority of rise in ET0 rates. The decrease of wind speed and relative humidity resulted in positive contributions to ET0. The increase in the sensitivity of precipitation and vapor pressure resulted in strongly decreased ET0, and the decrease of sunshine hours resulted in little change in ET0 rates. The variability of temperature extremes has been widely discussed in the past few decades. Liang K et al. used the daily maximum and minimum air temperature from 1961 to 2011 at 62 meteorological stations in the Yellow River basin to investigate the changes in 13 indices of temperature extremes recommended by the ETCCDI. Both cold and warm extremes showed a warming trend, with a large proportion of stations having statistically significant trends for most temperature indices. Because of a faster increase in minimum temperature than maximum temperature, diurnal temperature range (DTR) decreased by 0.07 C /decade. For cold extremes indices, i.e. coldest night (TNn), coldest day (TXn), cool nights (TN10p), cool days (TX10p), frost day (FD) and icing day (ID), the trends during the period of 1961e2011 were 0.23 and 0.22 C /decade, and 5.23, 3.31, 2.61 and 3.16 days/ decade, respectively. For warm extremes, i.e. warmest night (TNx), warmest day (TXx), warm nights (TN90p), warm days (TX90p), warm spell duration index (WSDI) and growing season length (GSL), the trends were 0.20 and 0.21 C /decade, and 3.80, 3.26, 2.93 and 4.81 days/decade, respectively. In general, the indices had the largest change magnitudes in the arid and semi-arid subregions, followed by the high-elevation sub-region, and the indices had the smallest changes in the eastern monsoon sub-region. Relationships between the changes in the temperature extremes and the changes in the large-scale atmospheric circulations were also studied. The changes in the temperature extremes were consistent with the changes in the area and strength of Northern Hemisphere Subtropical High (NHSH) and Northern Hemisphere Polar Vortex (NHPV). Hydrological prediction of ungauged basins remains a major challenge in hydrology. A key issue in the application of hydrological models to ungauged areas is regional parameterization. Wang GQ et al. presented a regional calibration approach to constructing regional relationships between the parameters of the Snowmeltbased Water Balance Model (SWBM) and catchment characteristics. Data from 38 well-gauged catchments were used to establish regional regression equations of model parameters, and data from additional four independent catchments were used to test and validate the proposed regional calibration approach. They indicated that the SWBM performed well in monthly discharge simulations for the 38 well-gauged catchments. The regional relationships of
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the SWBM parameters derived from the regional calibration approach were found to be highly correlated to catchment characteristics. In addition, the SWBM performed well in discharge simulations for the additional four test sites using only the constructed regional equations to estimate model parameters. Trends and abrupt detection of temperature and precipitation extremes are important in climate change research. Chen YN et al. analyzed the trend and abrupt change in temperature and precipitation extremes using the meteorological data from 68 stations in the arid region of Northwest China (ARNC) from 1961 to 2010. Results showed that abrupt change in both temperature and precipitation extremes in Northwest China occurred in around 1986. Interestingly, an abrupt change in Index B of the Tibetan Plateau (TPI_B) was detected in 1985. The temperature and precipitation extremes had strong and significant associations with TPI_B over the period of 1961e2010 (R ¼ 0.685, p < 0.01, and R ¼ 0.441, p < 0.01, respectively). They behaved consistently, with a weakening and decreasing trend from 1961 to 1984 and a strengthening and increasing trend from 1985 to 2010. Thus, TPI_B was probably an important factor in the abrupt change in both temperature and precipitation extremes in the ARNC. Ye ZX et al. analyzed the characteristics and relationship between runoff, sediment and climate over the past 50 years at the Aral Hydrological Station of Tarim River. Temporal analysis showed that both water and sediment at the Aral Hydrological Station decreased in the past 50 years, although these trends were not significant. The monthly distribution analysis showed that sediment load was more concentrated and more unstable compared to runoff during the flood season. The correlation analysis showed that annual sediment load was more related to annual maximum peak discharge followed by precipitation. This study considered the impact factors of the mainstream sediment changes as a foundation for subsequent research such as analysis of sediment sources and relationship with climate change. The Aksu River is the most important recharge source of the Tarim River in northwest China, representing 73.2% of the total volume to the Tarim River in the dry season. There are three types of recharge: glacier-snow meltwater, precipitation, and groundwater. Precipitation and baseflow in the basin play a decisive role. Fan YT et al. examined the dynamics of precipitation and baseflow in the last past 50 years. The digital filtering method was used to separate baseflow from surface flow. Relationships between precipitation and baseflow were identified by correlation and wavelet analysis. They found: 1) There was a significant increasing trend of both precipitation and baseflow In the past 50 years. The 1990s was the wettest decade of precipitation, and baseflow in the late 1990s was also the wettest. 2) The baseflow showed an increasing trend in all seasons, and precipitation exhibited a monotonic increasing trend in summer and autumn, and decreasing trend in spring and winter. 3) The responses of baseflow to precipitation were different by season. The baseflow had a 0e3 months lag behind precipitation. Understanding the impact of climate change on water resources is essential for sustainable development. Kong YL and Pang ZH evaluated the impact of climate change on discharges of Urumqi River, a medium-sized headwater catchment in the Tianshan mountain, where impacts of human activities on water were negligible and the influence of glacier melt was minor as far as the mountainous outlet was concerned. Time series of temperature, precipitation and river discharge of the hydrological station at the mountainous outlet from 1959 to 2006 were analyzed using different statistical methods, including linear regression, ManneKendall test and wavelet analysis. Although both temperature and precipitation showed a significant upward trend with a gradient of 0.02 C /y and 2.08 mm/y, respectively, there was no significant rising trend
in the stream discharge. The reasons were attributed to the hysteresis and buffering effects of groundwater in conveying the change from precipitation to stream discharge. Precipitation had a stronger influence on stream discharge than air temperature throughout the 48 years of instrumental records in the Urumqi River catchment. Gao JG et al. assessed the spatial patterns of alpine grassland changes (expansion and contraction) and their responses to climate changes in the context of global change in recent years on the southern Tibetan Plateau, Mt. Qomomagma National Natural Preserve (QNNP), covering an area of 36,594 km2 and located on the southern Tibetan Plateau e the world's “Third Pole”, was selected. Alpine grassland cover was obtained by image reclassification of more than five pairs of remote sensing images taken in 1976e2006. Expansion and contraction were determined by cover changes in alpine grassland, and were expressed as binary variables. Logistic regression was then used to analyze the impacts of climate-, neighborhood- and topographic-related variables on alpine grsssland changes. The cover of alpine grassland was increased by 84.3 km2. The expansion rate was significantly higher on the lower slopes, with a higher mean annual temperature (MAT), higher inter-annual variability trend of temperature (IVT), and inter-annual variability trend of precipitation (IVP), close to lakes and distant from roads. While most contraction was found on the lower slopes and in the areas close to streams, lakes, and roads. A significant impact was also driven by MAT. MAT significantly affected alpine grassland expansion and contraction in the QNNP, while mean annual precipitation (MAP) did not. IVT and IVP had significant effects on alpine grassland expansion but not on contraction. They suggested that climate changes had different effects on alpine grassland changes in different areas of the Tibetan Plateau. 3. Longer term hydro-climate changes Li F et al. used time sequence of the JZ-2010 profile derived from AMS 14C dating and content of the sediment elements Ti, Rb, and Sr, the Rb/Sr values as well as the granularity and the magnetic susceptibility to reconstruct the environmental humidity changes in the Jianghan Plain since 12,700 cal. BP. The comprehensive analyses of multiple alternative indicators indicated that the regional moisture turned from relatively dry to wet after the Late-Glacial period and reached its optimum until the mid-Holocene. However, the trend was punctuated by several decreased humidity phases. Beginning with the severe dry event approximately 4400e4100 cal. BP, the environment was dry as a whole, with modest humidity from approximately 3900 cal. BP. Humidity changes in the study area were dominated by the East Asian monsoon system, which was under the influence of the gradual southward migration of the ITCZ, driven by the summer solar insolation changes in the Northern Hemisphere due to orbital forcing. As the first attempt to apply Rb/Sr values and Ti concentrations in tracing moisture evolution in a region with typical subtropical humid climate, this study highlighted the different effects and consequences of the proxies throughout the palaeoenvironmental change process, and the impacts of regional topography that sustained tectonic subsidence since the Cretaceous as well as the southeast tilt that opened the plain to the summer monsoon. The fluctuation of magnetic susceptibility was associated with the granularity feature, the reductioneoxidation conditions, and the impact of human factors on the natural sedimentary environments during the historic period in the Jianghan Plain. Liu R et al. collected a 45.3-m long core from the Hemudu civilization site (Luojiang county, Zhejiang Province; 29 59.0640 N, 12121.7540 E), located in the Ningshao plain on the southern margin of Hangzhou Bay. The core, named the HMD Core, was
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divided into seven lithological layers with clay and silt as the main lithological components. The sporopollen assemblages of this core combined with TOC/TN and stable organic carbon isotope (d13CTOC) analysis revealed the sedimentary environmental changes since the later part of the Late Pleistocene in the Ningshao plain. The study area changed from terrestrial fluvial-flood plain faces before Marine Isotope Stage (MIS) 3 to shore plain during transgression in MIS3, and became a freshwater lake during regression at the time of the Last Glacial Maximum (LGM). The area started to become a semi-salt lake influenced by sea water during 15,900e5000 cal. BP, when the lake was deeper and the climate became gradually warmer. After the seawater withdrew later, the lake became desalinated, and then swampy with accumulation of peat until 1000 cal. BP. After that, the area became dry land. This study also indicated that the sediments of the core were of endogenous organic origin and the d13CTOC value was controlled by change in the concentration of atmospheric CO2, due to global changes. Dai L et al. analyzed 45 seafloor surface sediment samples from the SCS and nine surface sediment samples from the Pearl River mouth to improve the understanding of pollen transportation and deposition mechanisms in northern South China Sea (SCS). The spatial distributions of pollen and spore taxa suggested various transport mechanisms and production at the source areas. The dominant pollen Pinus, Poaceae, Cyperaceae and fern spores were most abundant in the river mouth and decreased with increasing distance from land, suggesting they were mainly derived from the Pearl River basin and strongly dispersed by river plume and ocean currents, which reflected a dominant pollen transportation pattern in the northern SCS. Podocarpus and temperate broadleaved components pollen such as evergreen Quercus and Castanopsis were common in marine samples and scattered in the Pearl River samples, reflecting their extensive source areas and dispersal in more than one route. However, some pollen types of conifers were mostly deposited close to the shore, such as Dacrydium, Picea and Abies, which were restricted near Hainan Island and Taiwan. Excluding evergreen Quercus and Castanopsis pollen, the total tropical and subtropical components poorly reflected their parent plants, and fewer such pollen were transported over long distances. Spatially varying ratios of spores to pollen indicated that spores were relatively easily transported by water over longer distances compared to pollen grains. This study provided a sound basis for interpreting Quaternary pollen deposits from the northern SCS, and for further evaluation of pollen dispersal and deposition in marine and fluvial deposits. Seabirds migrate between marine and island ecosystems, and their subfossil remains have been widely used in palaeoecological studies. Xu LQ et al. collected bone subfossils of seabirds from Ganquan Island of the Xisha archipelago, South China Sea. Stable isotopes (d15N and d13C) in well-preserved ancient collagen samples were analyzed to investigate seabird palaeodiets over the past
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2200 years. Stable isotope-based palaeodietary analysis indicated a significant change in seabirds' foraging behavior at 1200 BP. A long-term connection between seabird population and their trophic levels was identified. High-trophic prey contributed more to the diet of birds during the period of relatively high seabird abundance at 1200e200 BP. Both collagen d15N and d13C indicated that intra/inter-specific competitions might force seabirds to dive deeper or travel further to catch prey. Acknowledgments We are thankful for Ministry of Education, Singapore (MOE2011T2-1-101), National Basic Research Program of China (973 Program) (No. 2012CB4955903), the National Natural Science Foundation of China (41171406). We would also like to thank Professor Norm Catto for his consistent help in compiling this special volume. References Jiang, T., Fischer, T., Lu, X.X., 2013. Larger Asian rivers: changes in hydro-climate and water environments. Quaternary International 304, 1e4. Jiang, T., Fischer, T., Lu, X.X., 2012. Larger Asian rivers: climate, water discharge, water and sediment quality. Quaternary International 282, 1e4. Jiang, T., Fischer, T., Lu, X.X., 2011. Larger Asian rivers: climate, hydrology and ecosystems. Quaternary International 244, 127e129. Jiang, T., Fischer, T., Lu, X.X., 2010. Larger Asian Rivers: climate change, river flow, and watershed management. Quaternary International 226, 1e3. Lu, X.X., Jiang, T., 2009. Larger Asian rivers: climate change, river flow and sediment flux. Quaternary International 208, 1e3. Lu, X.X., Chen, X., 2008. Large Asian rivers and their interactions with estuaries and coasts. Quaternary International 186, 1e3. Lu, X.X., Zhang, S.R., Xu, J.C., Merz, J., 2011. The changing sediment loads of the Hindu Kush-Himalayan rivers: an overview. In: Sediment Problems and Sediment Management in Asian River Basins (Proceedings of the Workshop Held at Hyderabad, India, September 2009). IAHS Publ. 349. Lu, X.X., Ran, L.S., Liu, S., Jiang, T., Zhang, S.R., Wang, J.J., 2013. Sediment loads' response to climate change: a preliminary study of eight large Chinese rivers. International Journal of Sediment Research 28 (1), 1e14. Ministry of Water Resources, P.R.China, 2013. Bulletin of First National Census for Water. China Waterpub Press.
Xixi Lu* Department of Geography, National University of Singapore, Singapore Tong Jiang National Climate Center, China Meteorological Administration, Beijing, China Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing 210044, China * Corresponding author. E-mail address: [email protected] (X. Lu).