Changjiang Mainstream Research Articles

From the water sources of the Tibetan Plateau to the ocean: State of nutrients in the Changjiang linked to land use changes and climate variability

Anthropogenic activity is an important driver of changes in the chemistry of nutrients (N, P, and Si) over watersheds at the sub-continental scale (e.g., 106 km2) and can markedly modify their seaward fluxes to the global ocean. In the present study, we reviewed the current status of nutrient chemistry in Changjiang (Yangtze River) based on data collected through 11 expeditions along a river course spanning 4,500 km and 15–20 major tributaries during 1997–2016 as well as monthly monitoring at the river mouth since 1980. The data were analyzed together with published results in the literature to synthesize the recent developments and current state of nutrients in the Changjiang. Previously published results from the Qinghai-Tibetan Plateau head waters were included to realize the systematics of nutrients for the whole drainage basin. Here, we showed that tributaries of the upper reaches of watersheds collectively determine the regime with high concentration and skewed species ratio of nutrients in the Changjiang mainstream, producing profound effects over a water course of 2,000–2,500 km further downstream and until the river mouth. Moreover, using data across the Three Gorges Reservoir (TGR) during 2003–2016, we evaluated the trapping and/or amplifying effects of the Three Gorges Dam (TGD) on nutrient chemistry. Tide-influenced river delta contributed an additional 20% dissolved inorganic phosphorus and 5–10% dissolved inorganic nitrogen and dissolved silicates to the seaward flux, dramatically affecting the stoichiometry of nutrients at the river mouth. Next, based on compiled data on supply and export, legacy nutrients were evaluated. Both nitrogen and phosphorus are in the accumulation phase over the watersheds, and the legacy nutrient fluxes are much higher than the annual riverine seaward fluxes. Finally, we demonstrated that the seaward fluxes of anthropogenic nutrients from the Changjiang exceed those from other top 10 largest rivers on this planet, which can be attributed to land use changes in the China over the last three to four decades.

Investigation of inner‐basin variation: Impact of large reservoirs on water regimes of downstream water bodies

AbstractLarge dams and reservoirs alter not only the natural flow regimes of streams and rivers but also their flooding cycles and flood magnitudes. Although the effect of dams and reservoirs has been reported for some vulnerable locations, the understanding of the inner‐basin variation with respect to the effects remains limited. In this study, we analyse the Three Gorges Dam (TGD) built on the Changjiang mainstream (Yangtze River) to investigate the dam effect variations in the system of interconnected water bodies located downstream. We investigated the effect of flow alterations along the downstream river network using discharge time series at different gauging stations. The river–lake interactions (referring to the interactions between the Changjiang mainstream and its tributary lakes i.e. the Dongting and Poyang lakes) and their roles in modifying the TGD effect intensity were also investigated in the large‐scale river–lake system. The results show that the water storage of the tributary lakes decreased after the activation of the TGD. Severe droughts occurred in the lakes, weakening their ability to recharge the Changjiang mainstream. As a consequence, the effect of the TGD on the Changjiang flow increase during the dry season diminished quickly downstream of the dam, whereas its impact on the flow decrease during the wet season gradually exacerbated along the mainstream, especially at sites located downstream of the lake outlets. Therefore, when assessing dam‐induced hydrological changes, special attention should be paid to the changes in the storage of tributary lakes and the associated effects in the mainstream. This is of high importance for managing the water resource trade‐offs between different water bodies in dam‐affected riverine systems.

Seasonal variability of stable isotopes in the Changjiang (Yangtze) river water and its implications for natural climate and anthropogenic impacts

BackgroundSeasonal variations of stable hydrogen and oxygen isotopes in river water can be used to indicate hydrological processes and the water cycle in the river basin. This study presents a high-resolution time series of stable oxygen isotopic record (δ18O) in the lower Changjiang (Yangtze) River mainstream and aims to investigate the regional water cycle concerning natural climate changes and anthropogenic activities.ResultsThe river water δ18O shows apparent seasonal variations, which is overall higher during the dry season and lower during the flood season. This basin-scale seasonal variation of δ18O in precipitation over the Changjiang catchment is calculated based on the long-term meteorological and hydrological data. The seasonal variation of δ18O in the lower Changjiang river water is largely constrained by monsoon-induced precipitation, but is more directly controlled by the mixing of waters from the upper reaches and the tributary lakes in the middle valley.ConclusionsThe relative contributions of waters from the upstream and tributary lakes to the lower Changjiang mainstream have been considerably altered by the construction of Three Gorges Dam via the regulation of river discharges. A comparison of river water isotopic records with water discharges from the lakes suggests that it takes about 2 weeks (~ 17 days) for the Changjiang river water to circumvent the mid-lower reaches of the catchment before traveling to the sea. This study suggests that the stable hydrogen and oxygen isotopes can be harnessed as sensitive indicators for water cycling within a large catchment that is driven primarily by natural forcing though subject to substantial human impacts.

Ichthyoplankton recruitment from mainstream of the Changjiang River into the Dongting Lake, the second largest freshwater lake in China

Floodplains are important and distinctive ecosystems around the world, and the recruitment of ichthyoplankton from river to floodplain lakes is crucial to maintain this unique ecosystem. However, this process has not been well documented. In this study, ichthyoplankton were sampled to investigate the details of ichthyoplankton recruitment from the Changjiang (Yangtze) River to the Dongting Lake via a floodway channel, the Songzi River, from May to July in 2013 and 2014. During the study period, 41 species of eggs and larvae were sampled. Among the samples, 16 were river-lake migratory species (RL), representing 23.5% of the species in the Dongting Lake. In 2013, an estimated 130 million eggs and 3 180 million larvae drifted through the sampling section, and in 2014, an estimated 1 060 million eggs and 1 040 million larvae drifted through the sampling section. The amount of eggs and larvae of RL reached 3 210 million in 2013 and 1 850 million in 2014, respectively. These results demonstrated the importance of ichthyoplankton recruitment from the river to the lake, as species diversity will decrease sharply without this recruitment. Canonical correspondence analysis (CCA) showed that water temperature and water flow are the two most important factors influencing the spawning activities of fish. To maintain the high fish diversity in the Changjiang floodplain, we suggest to irrigate the channel to increase water discharge and increase the transport of ichthyoplankton from the Changjiang mainstream into the Dongting Lake.

Satellite-derived particulate organic carbon flux in the Changjiang River through different stages of the Three Gorges Dam

As the largest Asian river and fourth world's largest river by water flow, the Changjiang River transports a considerable amount of terrigenous particulate organic carbon (POC) into ocean, which has experienced significant pressure from human activities. We conducted monthly sampling (from May 2015 to May 2016) at Datong, the most downstream non-tidal hydrological station along the Changjiang River. To monitor long-term POC variations, we developed a two-step POC algorithm for Landsat satellite data and calculated monthly POC flux during 2000–2016. Monthly POC flux ranged from a minimum of 1.4 × 104 t C in February 2016 to a maximum of 52.04 × 104 t C in May 2002, with a mean of 13.04 × 104 t C/month. At Datong, monthly POC flux was positively exponentially related to water flow (N = 118, R = 0.69, p < 0.01). In the wet season (May to October) with high water flow, a large amount of terrigenous POC was flushed into the Changjiang River. After the Three Gorges Dam (TGD), both POC concentration (R = 0.46, p < 0.01) and POC flux (R = 0.33, p < 0.01) significantly exponentially decreased during 2000–2016. After 2011 with regular operation of the TGD, POC transport at Datong showed relative stability and its seasonal difference became smaller. POC concentration from the TGD to Datong in the Changjiang mainstream (~1000 km) was also derived from Landsat data, which was spatially varied with water flow, slope gradient, and watercourse width. This study improved our understanding of the spatiotemporal variations of POC transport in the Changjiang mainstream, especially the influences of the TGD construction.

Effect of the Three Gorges Dam Project on flood control in the Dongting Lake area, China, in a 1998-type flood

To evaluate the protective effect of the Three Gorges Dam Project (TGP) against flooding in the middle region of the Changjiang River basin, we applied an integrated watershed hydrological model using gauged daily precipitation data for 1998, when the second largest flood of the last century occurred in the basin. From the results simulated by applying the daily average discharge rate controlled by each of the upper limit values of 40,000, 45,000, 50,000, and 55,000 m 3/s as the safety discharge in the downstream reach, we were unable to find any clear reduction effect on the water level during the flood period, not only in Dongting Lake, but also in the Changjiang mainstream, when the upper discharge limit exceeded 50,000 m 3/s. When an upper discharge limit of 40,000 m 3/s was used, although a flood protection effect was clearly exhibited in both the mainstream and the lake, the simulated storage volume of the TGP reservoir markedly exceeded the total flood control volume (221.5 × 10 8 m 3) at the inflow peak. These results suggest that the TGP would not work efficiently for decreasing flood damage in the Dongting Lake area in the event of another 1998-type flood without other countermeasures to control the increased water level in the lake.

Daily Runoff Simulation by an Integrated Catchment Model in the Middle and Lower Regions of the Changjiang Basin, China

To simulate daily runoff in the middle and lower regions of the Changjiang (Yangtze River) basin, we used an integrated, spatially distributed model consisting of the Hydrological Simulation Program—FORTRAN (HSPF) and two newly developed submodels, the paddy runoff model (PRM), simulating the runoff from paddy fields, and the lake discharge model (LDM), simulating the hydraulic effect of the Changjiang mainstream on the discharge from neighboring lakes. In the PRM, the ratios of the simulated monthly evapotranspiration relative to the observed potential evaporation were close to or exceeded 1.0 in both of the test subcatchments in the full ponding period with plentiful irrigation water, corresponding to many previous observations of evapotranspiraion rates from paddy fields. Whereas the original HSPF could not consider the backwater effect of the water level in the mainstream, the daily discharge from the lakes simulated by the LDM was dependent on the water level in the mainstream and corresponded to the...

Nutrients in the Changjiang River

N, P and SiO3-Si in the Changjiang mainstream and its major tributaries and lakes were investigated in the dry season from November to December, 1997, and in the flood season in August and October, 1998. An even distribution of SiO3-Si was found along the Changjiang River. However, the concentrations of total nitrogen, total dissolved nitrogen, dissolved inorganic nitrogen, nitrate and total phosphorus, total particulate phosphorus increased notably in the upper reaches, which reflected an increasing impact from human activities. Those concentrations in the middle and lower reaches of the Changjiang River were relatively constant. Dissolved N was the major form of N and the particulate P was the major form of P in the Changjiang River. The molar ratio of dissolved N to dissolved P was extremely high (192.5-317.5), while that of the particulate form was low (5.6-37.7). High N/P ratio reflected a significant input of anthropogenic N such as N from precipitation and N lost from water and soil etc. Dissolved N and P was in a quasi-equilibrium state in the process from precipitate to the river. In the turbid river water, light limitation, rather than P limitation, seemed more likely to be a controlling factor for the growth of phytoplankton. A positive linear correlationship between the concentration of dissolved N and the river's runoff was found, mainly in the upper reaches, which was related to the non-point sources of N. Over the past decades, N concentration has greatly increased, but the change of P concentration was not as significant as N. The nutrient fluxes of the Changjiang mainstream and tributaries were estimated, and the result showed that the nutrient fluxes were mainly controlled by the runoff, of which more than a half came from the tributaries. These investigations carried out before water storage of the Three Gorges Dam will supply a scientific base for studying the influences of the Three Gorges Dam on the ecology and environment of the Changjiang River and its estuary.

EFFECT OF “LAND TO LAKE” POLICY AROUND THE DONGTING LAKE ON THE FLOOD PROTECTION IN THE MIDDLE REGION OF THE CHANGJIANG RIVER BASIN, CHINA

To evaluate the flood protection effect of the conversion of polders to retarding basin around the Dongting Lake in the middle region of the Changjiang River, China, we applied the integrated watershed hydrological model using gauged daily precipitation data of 1998 when the second largest flood occurred in the basin in the last century. While the estimated storage capacity of the Dongting Lake increased 11.5×109 m3 by the conversion of polders in comparison with the estimated capacity at the flood period in 1998 at the average water level of 35.0 m, the simulated daily average water levels of the lake showed that the protection of the flood was impossible by only increasing capacity in the confluence between the Changjiang mainstream and the Dongting Lake in case of 1998 flood. The model also simulated that the flood protection was fully achieved at the confluence by making the volume diverging from the Changjiang mainstream to the Dongting lake increase with the diverging ratio at 1950s under the appropriate discharge control by the Three Gorges Dam. These results suggest that “return land to lake” policy around the Dongting Lake probably brings the best flood protection effect when the increased capacity of the lake uses for the control of the water level in the Jinjiang section of the Changjiang mainstream.

EFFECT OF THREE GORGES DAM ON FLOOD PROTECTION IN THE MIDDLE REGION OF THE CHANGJIANG RIVER BASIN IN CASE OF THE OCCURRENCE OF 1998 TYPE FLOOD

To evaluate the protection effect of the Three Gorges Dam Project (TGP) on the flood in the middle region of the Changjiang River basin, we applied the integrated watershed hydrological model using gauged daily precipitation data of 1998 when the second largest flood occurred in the basin in the last century. From the results simulated by applying the discharge volume controlled by each upper limitation value from 40, 000 m3/s to 60, 000 m3/s, we could not find the clear effect on the reduction of the water level during flood period not only in the Dongting Lake but also in the Changjiang mainstream in the cases that the upper limitation discharge value were over 50, 000 m3/s. In the case that the upper limitation value was 40, 000m3/s, although the flood protection effect was clearly exerted in both the mainstream and the lake, the simulated storage volume of TGP remarkably exceeded the total flood control volume (221.5×108 m3) at the peak of inflow to TGP. These results suggested that TGP flood control ability does not efficiently work for the decrease of flood damage in the middle region of the Changjiang basin in case of the occurrence of 1998 type flood phenomena.