River-lake System Research Articles

A framework based on spectral similarity to estimate hydrological connectivity in Juruá River floodplain lakes using 3-m PlanetScope data

Hydrological connectivity based on water surface connectivity controls the water exchange between large rivers and their floodplain lakes, which occurs by channelized flow through floodplain areas and overbank flow, and it is relevant to sustain the ecosystem’s health and biodiversity of floodplain waterbodies. Given the climate change impacts on floodplain aquatic habitats, further studies are needed to understand and quantify the river–lake connectivity and its temporal dynamics. However, few studies are dedicated to objectively estimating hydrological connectivity, and new commercial satellite datasets and machine learning approaches can advance the understanding of this important topic. This paper proposes a new framework for computing the hydrological connectivity of small floodplain lakes (river-Lake CONNECTivity or L-CONNECT) in the Amazon Juruá River. The L-CONNECT framework consists of spectral similarity analysis with machine learning in the river–lake system and three steps were implemented: (i) sampling process based on independent satellite-imagery visual interpretation; (ii) automated similarity features extraction from river-lake system; and (iii) training and validation of machine learning algorithm. A total of 552 3-m PlanetScope SuperDove imagery were acquired in 2020 and 2021 to perform our approach. In general, the L-CONNECT framework achieved 88% overall accuracy. However, not-connected lakes were not easily estimated. We found that the L-CONNECT framework managed to perform accurately over all lakes investigated independent on their distance to Juruá River channel (average accuracy of ∼ 86%), and that there was a low discordance (less than 30%) between the lakes’ hydrological connectivity responses acquired by Sentinel-2 against PlanetScope data. The mapping results showed that lakes were more connected to the Juruá River during the 2021 (not-connected lakes rate of 28%) compared to 2020, which was explained by higher cumulative precipitation during January, February, and March of 2021. Finally, the new L-CONNECT framework proposed here can support hydrological connectivity mapping of small floodplain lakes by considering assumptions relative to the similarity between river and water spectra as a proxy for that.

Can water release from local reservoirs cope with the droughts of downstream lake in a large river-lake system?

Lake is an important component of the Earth’s surface water bodies. Frequent and serious drought events in lakes have affected the security of regional water resources and caused the degradation of the lake ecosystem. Poyang Lake, the largest freshwater lake in China, has been suffering from increasingly severe hydrological droughts. This study simulated and investigated the effects of water release from local reservoirs in dry season on hydrological droughts in Poyang Lake based on a hydrodynamic model MIKE 21, and quantified the effectiveness of the joint operation of local reservoirs and the Three Gorges Reservoir (TGR). Major results showed that the water level of Poyang Lake during the dry period was significantly raised by the water release from local reservoirs, especially in the northern lake area. The average and the lowest lake level increased from 10.99 m to 11.18–11.59 m and from 9.97 m to 10.14–10.50 m at Xingzi station, respectively. Meanwhile, both the duration of low water and the drought severity/intensity (DS/DI) decreased significantly, with the duration decreasing from 128 days to 105–115 days for typical threshold lake level of 12.0 m, and the DS and DI decreasing from 176.6 m and 1.38 m/day to 117.6–155.3 m and 1.12–1.35 m/day, respectively. Moreover, the release oflocal reservoirs shortened the exposure duration of beach wetland, from nearly 3 months to about 1.5–2.5 months at the northern channel of Poyang Lake. The area with long exposure duration (>120 days) was also significantly reduced. In addition, the hydrodynamic simulations revealed that the joint operation of local reservoirs and the TGR further raised the average and the lowest lake level and decreased the duration of low water and drought severity and intensity. Outcomes of the study provided valuable references for mitigating lake droughts in complex river-lake-catchment system.

Fecal-related anthropogenic sources are key determinants of lake microbiomes through microbial source tracking

Lake microbiomes are essential indicators of lake health and are strongly influenced by allochthonous microbial communities from various sources within the watershed. However, quantifying the contributions of multiple inputs to lake microbiomes is challenging because of the complex nature of river‒lake systems and the presence of many untraceable sources. Here, Jianhu Lake‒‒a geographically simple and closed plateau lake in southwestern China, was surveyed to disentangle the contributions of five distinct sources (three input rivers that receive town sewage, stormwater runoff, and creek spring water, as well as two nonpoint sources, duck ponds and dry farmland) to the overall lake microbiomes. We found that feces-loading sources, namely town sewage and duck aquaculture, accounted for 48.7% of the total variations in lake microbiomes. In contrast, the combined contribution of the remaining three sources amounted to 13.21%, despite these less-influential sources (e.g., stormwater runoff) may introduce an even larger volume of allochthonous materials into the lake. In addition, approximately 38.1% of the variations in the lake microbiomes were attributed to unknown sources. Sewage effluents also caused a significant loss of lake microbial diversity, and there was a tendency for large-scale microbial homogeneity in lake sediments that resembled those from duck ponds. We then used a targeted approach to track host-specific fecal pollution, and found that human feces were the primary source, followed by ruminant and chicken/duck feces, all of which can be successfully traced back to the feces-loading sources. In our further modelling of sediment transport from three rivers into the whole lake, we observed a significant relationship between sediment accumulation and adsorbed microorganisms only for the sewage-receiving river. Together, lines of evidence indicate that both point and nonpoint fecal-related anthropogenic sources possess discriminatory power for shaping microbial geographic patterns of the lake, posing threats to the survival of local indigenous lake microbiomes.

Asymmetry of blue and green water changes in the Yangtze river basin, China, examined by multi-water-variable calibrated SWAT model

Renewable freshwater is a fundamental natural resource for sustaining human social development and the health of terrestrial ecosystems. While blue water (BW), which can be directly utilized by human beings, has received significant attention, the importance of green water (GW), crucial for ecosystem safety, has often been overlooked in water resource assessments. To date, there has been a lack of integrated research incorporating the detection and attribution of changes in both BW and GW in the Yangtze River basin (YRB), China’s largest river basin, resulting in a deficiency of comparative and interactive analyses between them. In this study, we examined the historical trends and long-term memory of BW and GW flows across 272 sub-basins of the YRB using a multi-water-variable calibrated SWAT model, which was validated by streamflow, evapotranspiration, and terrestrial water storage change. We determined the individual contributions of climate variability and land use changes to BW and GW changes. Additionally, we identified the responses characteristics of BW and GW changes in four different ecosystems primarily characterized by alpine grasslands, cropland, forests, and river–lake systems, respectively. The results indicate a non-significant decreasing trend in precipitation and BW in most regions, while an increasing trend is observed in GW, which is expected to persist in the future. Climate variability played a predominant role in BW and GW changes, while land use change also appeared to have significant impact on monthly GW changes. GW showed a positive correlation with precipitation/humidity index in relatively dry and moderately humid areas, but a negative correlation in highly humid areas. Moreover, a stronger correlation was found between the gross primary production (GPP) of terrestrial ecosystems and GW compared to GPP and BW. The findings contribute to guiding the harmonious utilization of water resources between humans and ecosystems in the YRB, as well as optimizing water allocation.

Nonstationary frequency analysis and uncertainty quantification for extreme low lake levels in a large river-lake-catchment system

Extreme hydrological events have become increasingly frequent on a global scale. The middle Yangtze River also faces a substantial challenge in dealing with extreme flooding and drought. However, the long-term characteristics of the extreme hydrological regime have not yet been adequately recognized. Moreover, there is uncertainty in the extreme value estimation, and this uncertainty needs to be distinguished and quantified. In this study, we investigated the nonstationary frequency characteristics of extreme low lake levels (ELLLs), taking the Poyang Lake as an example. Daily lake levels from 1960 to 2022 were utilized to estimate the return level using the generalized Pareto distribution (GPD). The uncertainty from three sources, i.e., the parameter estimator, threshold selection, and covariate, was quantified via variance decomposition. The results indicate that (1) the parameter estimator is the predominant source of uncertainty, with a contribution rate of approximately 87 %. The total uncertainty of the covariate, threshold, and interaction term is only 13 %. (2) Two indexes, namely the annual minimum water level (WLmin) and the days with peak over the 90 % threshold per year (DPOT90), decreased (0.01–0.03 m/year) and increased (0.17–1.39 days/year), respectively, indicating a progressively severe drought trend for Poyang Lake. (3) The return level with return period of 5 to 100 years significantly decreased after the early 21st century. A large spatial heterogeneity was identified for the variation in the return level, and the change rate of the return level with a 100-year return period ranged from 5 % to 40 % for the whole lake. (4) The ELLLs had a stronger correlation with the catchment discharge than with the Yangtze River discharge and the large-scale atmospheric circulation indices. This study provides a methodology with reduced uncertainty for nonstationary frequency analysis (NFA) of ELLLs exemplified in large river–lake systems.

Comprehensive evaluation of the hydrological health evolution and its driving forces in the river-lake system

The complex pattern of the river-lake system with lakes at its core is disturbed by external stressors that imbalance the river-lake relationship and potentially threaten the health of aquatic ecosystems. This study aimed to investigate the hydrological health of the Yangtze River-Dongting Lake system from a systemic evolutionary perspective and quantify the impact of external driving forces. Based on the cross-wavelet transform method, the dynamic changes in the hydrological health of the river-lake system during the study period (1969–2019) were assessed, and the response relationship was described via multifractal analysis. Finally, a long short-term memory (LSTM) model was constructed to quantify the impact of external driving force changes on hydrological health. The results are described as follows: (1) the hydrological health of the river-lake system has declined, but the lake conditions are stable; (2) the Yangtze River-Dongting Lake system has higher synchronous changes during high-water season but lower during low-water season; (3) a positive correlation exists between the hydrological health changes of the Yangtze River and Dongting Lake, and the regulation of Dongting Lake is critical to the hydrological health of the Yangtze River, with only a minor impact of the Yangtze River on Dongting Lake; (4) the multifractal spectrum shows that the impact of large hydrological fluctuations at Zhicheng station has an advantage over small fluctuations on Luoshan station, and the response of hydrological health at the Luoshan station to lake is stronger than to upstream water, regardless of the long-short time scale; (5) climate change and non-climatic factors have jointly mediated a decline in hydrological health of Dongting Lake, with non-climatic factors being the main driving force, and an impact weight of 88.6% at the annual scale. On a monthly scale, the most favourable and unfavorable impact of non-climatic factors was reported in June (4.69) and October (−25.37), respectively.

Influences of increasing water release of the Three Gorges Reservoir during dry season on water regimes of downstream lake

AbstractThe severe and frequent drought in Poyang Lake, the largest freshwater lake in China, has seriously affected the local water resources security and wetland ecological security. Especially, the impoundment of the Three Gorges Reservoir (TGR) in the Yangtze River further aggravated the drought in lake. To explore the approaches to alleviate drought in Poyang Lake, this study simulated and quantified the influences of increasing water release of the TGR during the dry periods on water regimes of Poyang Lake, including the lake water level, duration of low water, flow velocity, flow field of lake, as well as potential suitable habitat area of migratory birds, based on a hydrodynamic model. Major results showed that the increase of water release of the TGR raised the lake water level, especially in the northern lake area, with the average lake level increase of 0.2–1.08 m and the lowest lake level increase of 0.16–0.68 m. Meanwhile, the duration of low water was significantly shortened, from 127 days to 87–116 days (threshold lake level of 12.0 m) at Xingzi station. In addition, the hydrodynamic simulations revealed that the increasing water release of the TGR reduced the lake flow velocity and discharge of Poyang Lake, which decreased the outflow of lake by 1.66–17.87 × 108 m3, accounting for about 0.47%–5.11% of the total outflow in the dry season. Finally, the potential suitable habitat area with 0.2–0.6 m water depth increased by 1.7–27.9 km2 in December–February, and showed a significant expansion trend in the central area of Poyang Lake. Outcomes of the study are useful for understanding the influences of water release of the TGR on water regimes of downstream lakes, and can provide valuable references for mitigating lake droughts in complex river–lake system.

Geographic characteristics and environmental variables determine the diversities and assembly of the algal communities in interconnected river-lake system

Algal blooms in lakes are a major hazard worldwide. Although various geographical and environmental patterns affect algal communities during river-lake transit, a thorough understanding of what patterns shape the algal communities is still rarely researched, particularly in complex interconnected river-lake systems. In this study, focusing on the most typical interconnected river-lake system in China, the Dongting Lake, we collected paired water and sediment samples in summer, when algal biomass and growth rate are at high levels. Based on 23S rRNA gene sequencing, we investigated the heterogeneity and the differences in assembly mechanisms of planktonic and benthic algae in Dongting Lake. Planktonic algae contained more Cyanobacteria and Cryptophyta, while sediment harbored higher proportions of Bacillariophyta and Chlorophyta. For planktonic algae, stochastic dispersal dominated the assembly of the communities. Upstream rivers and confluences were important sources of planktonic algae in lakes. Meanwhile, for benthic algae, deterministic environmental filtering shaped the communities, and the proportion of benthic algae exploded with increasing N:P ratio and Cu concentration until reaching thresholds of 1.5 and 0.013 g/kg respectively, and then started falling, showing non-linear responses. This study revealed the variability of different aspects of algal communities in different habitats, traced the main sources of planktonic algae, and identified the thresholds for benthic algal shifts in response to environmental filters. Hence, upstream and downstream monitoring as well as thresholds of environmental factors should be considered in further aquatic ecological monitoring or regulatory programs of harmful algal blooms in these complex systems.

Modulation of sediment load recovery downstream of Three Gorges Dam in the Yangtze River

The sediment load in the Yangtze River downstream of the Three Gorges Dam (TGD) has substantially declined in recent decades. The decrease is more profound below the TGD, e.g., a 97% decrease at Yichang, compared with that at the delta apex, 1200 km downstream, e.g., a 75% decrease, implying along-river sediment recovery. Two large river-connected lakes, i.e., Dongting and Poyang Lakes, may play a role in the re-establishment of the river’s morphodynamic equilibrium, but a quantitative data-based understanding of this interaction is not yet available. In this work, we collected a series of field data to quantify the sediment gain and loss in the river-lake system in the middle-lower Yangtze River, and evaluate the lake’s response to the reduction in riverine sediment supply. We find that Dongting Lake and Poyang Lake shifted from net sedimentation to erosion in 2006 and 2000, and back to a sedimentation regime again after 2017 and 2018, respectively. Natural morphodynamic adaptation and sand mining play an important role in the regime changes in the Dongting Lake whereas sand mining dominates the abrupt changes in the Poyang Lake. The Dongting and Poyang Lake contributed maximum by 38% (2015) and 17% (2006) (respectively) to the sediment recovery in the erosion regime, whereas the riverbed erosion dominates the main sediment source. These changes in the relative contribution of sediment sources also indicates a response time of ~ 20 years in the lakes towards a new equilibrium state. It is noteworthy that the lakes’ buffer effects may be overestimated as the supplied sediment from the lakes is rather small compared to the significant dam trapping in the upstream basin and sediment source from downstream degradation. The results imply that river management and restoration should take into account of the river-lake interactions and feedback impact at decadal time scales.

Transfer of the zp3a gene results in changes in egg adhesiveness and buoyancy in transgenic zebrafish.

Reproductive strategies and spawning habits play key roles in the evolution of endemic East Asian cyprinids. However, the molecular mechanisms underlying the regulation of spawning habits are not well understood. We recently identified zona pellucida (Zp) as the top differentially expressed protein between East Asian cyprinids that produce adhesive and semi-buoyant eggs, suggesting that Zp protein may play important roles in the regulation of egg type. In this work, we generated transgenic zebrafish in which oocyte-specific expression of zp genes from rare minnow ( Gobiocypris rarus), an East Asian cyprinid laying adhesive eggs, was driven by a zebrafish zp3.2 gene promoter. We found that the transgenic eggs obtained partial adhesiveness and exhibited alteration in hydration and buoyancy. Abnormal metabolism of vitellogenin (VTG) may contribute to enhanced hydration and/or buoyancy. Our work shows that expression of the exogenous zp3a gene from an adhesive-egg producing fish is sufficient to induce changes in both egg adhesiveness and buoyancy in zebrafish, emphasizing the important role of zp genes in the regulation of spawning habits. Our results thus provide new insights into how endemic East Asian cyprinids may have adapted to the Yangtze river-lake system via changes in spawning habits.

Data Stream Approach for Exploration of Droughts and Floods Driving Forces in the Dongting Lake Wetland

Wetlands are important environmental resources that are vulnerable to droughts and floods. Studying drought-flood events and their driving factors is essential for wetland resource planning and management. However, climate change and human activities present dynamic challenges that traditional approaches are unable to simulate dynamically in a rapidly changing environment. This makes quantitative analysis difficult. Our research focused on the innovative use of the data stream model, namely online bagging of Hoeffding adaptive trees, to quantify drought and flood drivers in response to climate change and human activity. The proposed approach was applied to a river-lake system, the Dongting Lake wetland. The frequency and duration characteristics of drought-flood events were analyzed. In addition, the cyclical changes of droughts and floods were analyzed by wavelet analysis. Then, drought-flood indicators as well as climatic and hydrological factors were entered into a dynamic data stream model for quantitative calculations. The results showed that the water conservancy projects largely reduced flood events while aggravating droughts. The frequency of floods decreased by 4.91% and the frequency of droughts increased by 6.81% following the construction of the Gezhouba Hydro-project and the Three Gorges Dam. Precipitation and Sankou streamflow were two dominant factors in the Dongting Lake drought and flood events, both of which had a feature importance value of approximately 0.3. This research showed how the data stream model can be used in a changing environment and the applicability of the conclusions reached through real-world instances. Moreover, these quantitative outputs can help in the sustainable utilization of Dongting Lake wetland resources.

River–Lake System Connectivity Effectively Reduced the Salinity of Lake Water in Bosten Lake, Northwest China

High salinity in water constitutes a serious problem for the aquatic environment management of Bosten Lake. Weak water exchange and water movement are the essential factors for the high total dissolved solids (TDS) content of lake water. To improve the water quality of Bosten Lake, a river–lake system connectivity project (water diversion) was introduced starting at the end of 2018, which diverted fresh water from the Kaidu River and the Huangshuigou River to Bosten Lake. In this study, the effect and its mechanism of water diversion on the TDS content of Bosten Lake were evaluated using continuous-field monitoring data. The results showed that the water diversion effectively reduced the TDS content of the lake water and changed Bosten Lake from a brackish lake back to a freshwater lake. Water diversion also improved the spatial distribution of TDS content. One year, two years, and three years after the implementation of the water diversion project, the TDS content of northern, southern, and eastern lake water significantly decreased by more than 20%, 25%, and 30%, respectively. Our study demonstrated that water diversion significantly increased the annual endogenous TDS pollutant amount discharged from the lake and reduced and homogenized the TDS content of the whole lake. TDS content reduction of the lake was realized by water diversion accelerating water movement and water exchange in the lake, especially for the northern, southern, and eastern waters. Therefore, water diversion could be used as an effective measure for water quality improvement in semi-closed inland lakes in arid areas.

Editorial: The impact of Three Gorges Dam on downstream river-lake system

Editorial: The impact of Three Gorges Dam on downstream river-lake system

Dry-Season Water Level Shift Induced by Channel Change of the River–Lake System in China’s Largest Freshwater Lake, Poyang Lake

As a large open lake that connects to the middle reaches of Yangtze River, Poyang Lake, China, has experienced dramatic water level changes, especially during the dry periods (November–March) in recent years. For better understanding the shift of dry-season lake level, we quantified the variations induced by channel changes of the river–lake system (CCORLS). Attempts were achieved through establishing two Long Short-Term Memory (LSTM) based water level prediction models, combined hydrological data in the periods of 1956–2000 and 2006–2016. The results indicated that: (1) With reference to earlier (1956–2000) dry periods, the recent (2006–2016) mean water levels had an overall decreasing trend, with values of 0.04–1.26 m across the lake. (2) The response of water level to CCORLS was spatially heterogeneous, triggering a sharp reduction in the central to northern lake, an obvious reduction in the lake outlet, and a relatively smaller reduction in the southern lake. (3) The bottom topography alteration of Poyang Lake made a dominating contribution to decline the lake water levels, while the Yangtze River’s mainstream channel near the lake played a minor role in the decrease of water level. Results shed light on the impact of systematic watercourse changes on the water level of a lake connected a river and enhanced understanding of the characteristics and mechanisms behind the hydrological changes of large freshwater lakes.

Characterization of Water and Total Nitrogen Contributions from the Inflow Rivers to Lake: A Study of West Dongting Lake in China

The input of pollutants into the lake has ainfluence on the water quality of the lake. Detailing the contribution of water and nutrients from the inflow rivers are essential for lake water management. Major ions Na+ and Cl− were applied to trace contributions of water and total nitrogen (TN) from the inflow rivers to West Dongting Lake. By using the two-source mixing model and the two-component TN mixing model, the results showed that water and TN in West Dongting Lake were mainly contributed by local rivers rather than the three outfalls, in which the contribution of Yuan River was larger than that of Li River. However, the contribution rates of water and TN of the three outfalls to West Dongting Lake during the wet season reached above 20% and 30%, respectively, indicating that the contribution of the three outfalls to West Dongting Lake could not be ignored. Among them, the contributions of Songzi outfall were higher than those from Taiping and Ouchi outfalls. Therefore, we suggest that the relevant departments should identify key river management targets during different water periods, implement refined management over water pollution in West Dongting Lake, and prioritize the pollutant input of Yuran River and Songzi outfall (especially during the wet season). This study demonstrates that major ion tracing is suitable for estimating the contribution rates of different nutrient sources in the river-lake system, which will provide valuable information for protecting the water quality of West Dongting Lake in the future.

Anthropogenic-driven chronological increase of sediment organic carbon burial in a river-lake system

Total organic carbon (TOC) in lake sediments from upstream catchments is deposited and buried in substrate, recording historical environmental changes. However, the linkage among natural variability, anthropogenic activity, and TOC burial for has not yet been clarified. This study examined the lake sediments of five 200-cm-deep dated depositional cores in west Dongting lake, China to quantify the magnitude, allocation, and amplitude of TOC burial. 44.47–59.36% of TOC burial flux was buried at 100–200 cm, suggesting lake sediments at deep layers stored considerable carbon. TOC burial rate (BRTOC) decreased along the lake entrance to its body, which was explained by the geochemical differences. Since 1900, BRTOC presented an increasing with a 4–7 times uptrend, showing three sedimentary stages with the increased human disturbance, such as deforestation, hydroelectric facilities. Moreover, the coefficient of variation of BRTOC in the third stage was lower than that in the second stage for the implementation of watershed reforestation and reservoir construction. Our findings stressed that natural variations of lake sedimentation background induced the change of TOC burial among the depositional sites, and enhanced that anthropogenic perturbation drove its chronological increases. This research unveiled the linkage between TOC burial, natural variability, and human disturbance from the perspective of burial evolutions in a lacustrine sedimentary environment.

Understanding groundwater behaviors and exchange dynamics in a linked catchment-floodplain-lake system

Groundwater and surface water are hydrologically interconnected systems that exhibit dynamic water, heat and mass exchanges. In this study, a conceptual framework was used to investigate groundwater behaviors and associated hydrological exchanges by combining field measurements, digital filtering and analytical approaches, exemplified by a linked catchment-floodplain-lake system (Poyang Lake, China). The results show that the hydrological regime for both groundwater and surface water exhibit a seasonal variability in the lake catchment. Topographically, the lake catchment can be divided into the mountainous baseflow, ungauged lateral groundwater and floodplain groundwater that contribute to the lake storage changes. Although groundwater flow is generally from the mountainous catchment to the lake floodplain areas due to topographic effects, precipitation provides an additional input for the shallow groundwater and is expected to enhance the groundwater dynamics in terms of spatially heterogeneous responses. The estimation indicates that about 40 % of the catchment river discharge may be coming from the mountainous baseflow (~290 × 108 m3/yr) and discharged into the lake through a surface flow pathway. The ungauged groundwater-lake interaction shows the annual discharge volume is up to 10 × 108 m3/yr and associated exchange fluxes tend to be stronger during spring-summer months (23–45 m3/s) than those of autumn-winter months (9–22 m3/s). Additionally, the floodplain groundwater-lake exchange (~9.5 × 108 m3/yr) indicates that groundwater generally receives the lake water during summer months (mean flux = 110 m3/s) and discharges into the lake during other months (90 m3/s) through a subsurface pathway. This study highlights the importance of groundwater's contributions to the surface river-lake system in terms of the flux variability and different transport pathways. The outcomes of this work will benefit future water resources management and applications by providing a methodology for predicting the groundwater hydrology of large lake-catchment systems.

Spatio-temporal variations in water quality of a river–lake system during restoration treatments

To fill the knowledge gap about the functioning of the lake–river system subjected to restoration treatments, two tributaries, a shallow, restored lake and its outflow, were examined. The quality of water inflows, lake and outflow was compared before (BR), during sustainable (SR, deep water aeration, phosphorus inactivation and biomanipulation for 3 years) and limited lake restoration (LR, only aeration for 2 years). Physico-chemical parameters were analysed monthly at five stations. The nutrient concentrations at the inflows decreased over the years due to the improvement of water and sewage management in the catchment (in Mielcuch from 18.0 to 8.0 mgN L−1 and 1.0 to 0.6 mgP L−1). The decline at the outflow was the result of a better quality of water at the tributaries and SR in the lake. During LR, decrease of phosphorus concentration still occurred (0.11 mgP L−1), but nitrogen concentration slightly increased (3.9 mgN L−1). Although the outflowing waters still transported a high content of chlorophyll a and suspended solids during SR, their amount was lower (34.5 μg L−1 and 17 mg L−1, respectively) than that during BR and LR. During restoration, it is significant to monitor the water quality not only in the lake but also at the outflow. The slow deterioration of water quality at the outflow indicated that introducing changes in the applied restoration methods must be done carefully because the previously achieved effect may be lost. Hence, restoration of the upstream lake and good quality of its tributaries are of great importance for water bodies located downstream.

Detailed attribution of long-term water exchange in the Yangtze-Poyang system at multiple time scales

The driving factors of water exchange in the Yangtze–Poyang river–lake system (YPRLS) which is vital to the water resource management of YPRLS still remain unclear. In this study, the impacts of three major factors on the water exchange in the YPRLS from 1989 to 2016 were distinguished. Our results revealed that the catchment inflow into Poyang Lake (CIPL) dominates the variations in Qh (the discharge of Poyang Lake’s outflow), contributing nearly 100 % of the annual variation and evenly 120 % of the seasonal variations. The forcing of the Yangtze River, including the effect of operation of the Three Gorges Dam (TGD) and effects of other Yangtze-related factors in addition to it, stably explains approximately over 76 % of the annual and seasonal variations in Zh (water level at the junction of the river and lake). The sensitivity coefficient of the seasonal Qh to the CIPL and that of the seasonal Zh to the forcing of the Yangtze River, varying with the seasons, are 0.63 and 0.41, respectively. Furthermore, the CIPL-induced and the Yangtze-induced frequency of the backflow from the river to the lake (BRL) is 26 times and 7 times, respectively. Specifically, the frequency of BRL events with backflow discharge of <3000 m3/s and >3000 m3/s is mainly affected by the CIPL and the forcing of the Yangtze River, respectively. The variations in the seasonal Zh are speculated to be influenced by the water regime and the stage–discharge relationships, which may explain the stable dominate of the forcing of the Yangtze River on the seasonal Zh, and may affect the timing of autumn drought in the Poyang Lake. Our results may help to capture the main causes of complex hydrological changes in the YPRLS.

Assessing the performance of the Tiangong-2 wide-swath imaging altimeter observations for water level monitoring over complex and shallow lakes

As a significant hydrological parameter, the water level of complex and shallow river-lake wetland has important implications for a number of hydrological, biological, physical and chemical processes. Current in-situ and satellite-based methods both have disadvantages in capturing water levels over smaller surface water bodies with spatial heterogeneity or complexity. Interferometric imaging radar altimeter (InIRA) is a latest generation of Ku-band radar altimeter onboard Tiangong-2 (TG-2), the Chinese space laboratory, launched in 2016. Similar to the design idea of NASA’s SWOT, it is expected to improve this situation through wide-swath altimetry. However, the capacity of the TG-2 InIRA to measure water levels within small surface water bodies remains unexamined. To explore these performances, particularly their potential impact on hydrology research, an assessment and comparison between in-situ and TG-2 derived measurements was performed over the largest river-lake wetland in China. The results showed that the TG-2 InIRA, with a broad range of coverage, was able to effectively capture most of the surface water at its elevation. For the focused shallow depressions, a relatively good correlation was observed between the in-situ and InIRA-derived water surface elevations (WSEs), with an R2 of 0.97, RMSE of 0.29 m, and absolute difference ranging from 3 to 55 cm. The accuracy of InIRA-derived WSEs was highly controlled by the depression sizes, and the absolute biases broadly increased in a power law fashion as the size of the depression decreased. In addition, surface water profiles along the Ganjiang and Xiushui River channels indicated that the TG-2 InIRA could also measure river WSEs, with absolute differences below 0.43 m. Although differences were observed, the results of the river gradient distribution were consistent with the practical situation and showed overall slopes of 18.72 cm/km and 12.43 cm/km for the Xiushui and Ganjiang Rivers, respectively. Such results demonstrated that TG-2 InIRA measurements can satisfactorily capture spatial patterns of the surface water gradient, which is useful for accurately estimating river discharge. This study will not only bridge the gap between in-situ measurements and forthcoming SWOT products but also contribute to water resource management and hydrologic service assessments over data-sparse regions of river-lake systems within the study area.