Surging compound drought-heatwaves underrated in global soils.

Compound drought-heatwaves (CDHWs) accelerate the warming and drying of soils, triggering soil compound drought-heatwaves (SCDHWs) that jeopardize the health of soil ecosystems. Nevertheless, the behavior of these events worldwide and their responses to climatic warming are underexplored. Here, we show a global escalation in the frequency, duration, peak intensity, and severity of SCDHWs, as well as an increase in affected land area, from 1980 to 2023. The increasing trends, which are particularly prominent since the early 2000 s, and projected to persist throughout this century, are dominated by summertime SCDHWs and enhanced by El Niño. Intensive soil warming as well as climatologically lower soil temperatures compared to air temperatures lead to localized hotspots of escalating SCDHW severity in northern high latitudes, while prolonged duration causes such hotspots in northern South America. Transformation of natural ecosystems, particularly forests and wetlands, to cropland as well as forest degradation substantially enhance the strength of SCDHWs. Global SCDHWs consistently exhibit higher frequencies, longer durations, greater severities, and faster growth rates than CDHWs in all aspects from 1980 to 2023. They are undergoing a critical transition, with droughts replacing heatwaves as the primary constraint. We emphasize the significant intensification of SCDHWs in northern high latitudes as well as the prolonged duration of SCDHWs in the Southern Hemisphere, posing an underrated threat to achieving carbon neutrality and food security goals.

Centennial dynamics of floodplain wetland in the largest freshwater lake in China: Implications on floodplain lake restoration.

Long-term mapping of floodplain wetland dynamics is fundamental for wetland protection and restoration, but it is restricted to decadal scales using satellite observations owing to scarcity of spatial data over long-term scales. The present study concentrates on the centennial dynamics of floodplain wetland in Poyang Lake, the largest freshwater lake in China. Historical topographic maps and Landsat imagery were combined to reconstruct the centennial floodplain wetland map series. A robust random forest algorithm for the land cover classification was used to investigate the conversion of the floodplain wetland to other land cover types and quantify the magnitude of the influence of hydrological disconnection over the past century. Results show that the Poyang Lake floodplain wetland experienced a net loss of 35.7 %, from 5024.3 km2 in the 1920s-1940s to 3232.1 km2 in the 2020s, with the floodplain wetland loss occurring mostly from the 1950s to the 1970s. In addition, agricultural encroachment was identified as the predominant driver of floodplain wetland loss, with a total area of 931.0 km2 of the floodplain wetland converted into cropland. Furthermore, approximately 600 km2 of sub-lakes (larger than 1 km2) became isolated from the floodplain and thus unaffected by seasonal flood pulses, which highlights the need to account for the impact of hydrological disconnection on floodplain wetland dynamics. This study indicated the combination of historical maps and satellite observations as an effective tool to track long-term wetland changes. The resultant dataset provides an extended baseline and could shed some light on floodplain wetland conservation and restoration.

Reassessment of the declines in the largest freshwater lake in China (Poyang Lake): uneven trends, risks and underlying causes.

Water level decline in the largest freshwater lake in China (Poyang Lake) has raised grave concerns and continuous debates regarding the proposal of a water control project. Previous hydrologic investigations on the water level decline in Poyang Lake were conducted mostly during recession season and typical dry years and hence lacked the comprehensive view of associated risks and possible spatial heterogeneity of trend during low water level periods. The present study reassessed the long-term trend and regime shift of low water level variations and their associated risks based on hydrological data across multiple stations in Poyang Lake during 1952-2021. The underlying causes of the trends of water level decline were further investigated. Results showed uneven trends and risks of water level variations in different seasons and across different lake regions. The water level of all five hydrological stations in Poyang Lake significantly dropped in the recession season, and the risks of water level decline evidently increased since 2003, which could be largely attributed to the water level drop in the Yangtze River. In terms of the dry season, clear spatial differences of long-term trend of water level were found, with water level in the central and southern lake regions significantly dropping, which was probably caused by dramatic bathymetry undercutting in the central and northern lake regions. In addition, the impacts of topographic changes became significant when water level of Hukou fell below 13.8 and 11.8m for the northern and southern lake regions, respectively. By contrast, water level in the northern lake region showed increasing trends in the dry season. In addition, only the occurrence time of water level under moderate risk significantly advanced for all stations except for Hukou. The present study could provide a full picture of low water level trends, associated risks across different lake regions and underlying causes in Poyang Lake, thus providing insights into adaptive water resources management.

Optimizing payment for ecosystem services in a drinking water source watershed by quantifying the supply and demand of soil retention service.

Payment for ecosystem services (PES) plays a vital role in coordinating the relationship between ecosystem services supply and demand sides in watersheds. The upstream soil retention service brings significant off-site benefits to the downstream stakeholders. To fill gaps in the supply and demand of soil retention services for PES, we developed an approach that combined long-term observation data, hydrological model, and cost-benefit analysis. We applied and demonstrated the approach in a typical drinking water source watershed. By constructing the relationship between water clarity and the demanded trophic state, we identified the demand for soil retention as the suspended sediment concentration ≤4.4 mg L-1 at a transboundary station. Then, a well-calibrated hydrological model was applied to simulate the downstream sediment reduction under 36 upstream reforestation scenarios. Results showed that cropland reforestation effectively reduced downstream sediment loads by up to 37.8%. However, the efficiency of cropland reforestation for soil retention supply was influenced by its area, slope, and location. The cost-benefit analysis revealed that the feasible sediment reduction was 11,000 t per year, and the market-equilibrium price was 5800 CNY (Chinese Yuan, 7 CNY equaled 1 USD in 2020) per ton. The downstream side should pay 64 million CNY annually for soil retention provided by reforesting at upstream sloping cropland of 8° or above. This study suggested that the approach was helpful for integrating soil retention service supply and demand at a watershed scale to support PES decision-making.

Combining historical maps and landsat images to delineate the centennial-scale changes of lake wetlands in Taihu Lake Basin, China.

Lake wetlands (LWs) are essential components of the ecosystem and play an irreplaceable role in flood regulation, carbon fixation, and biodiversity maintenance. Continuous monitoring of LWs' change is necessary in the context of increased human disturbance and climate change, particularly in Taihu Lake Basin, China, an area exposed to early human exploitation. Yet, long-time series of LWs detection in this region is still unavailable due to the data limitation. To quantify the spatiotemporal dynamics of LWs and the associated driving forces, we combined 236 historical topographic maps and thousands of Landsat satellite images from the 1910s to 2021 to delineate the centennial-scale changes of lake wetlands for the first time in this region. We also applied land use transitions and statistical analyses to quantitively explore the climatic and anthropogenic factors behind LWs variations. Our results document a dramatic decline in the area and number of LWs in the Taihu Lake Basin over the last century and a shift in the 2000s: Taihu Lake Basin has seen a total of 89.15% loss in lake littoral wetlands and a decrease of 14.5% in the whole lake wetlands area, with a net reduction of 68 (from 156 in the 1910s to 88 in the 2021) lakes. This decrease has been especially predominant during the 1910s-2000s, because of the policy initiatives for reclamation and aquacultural industries. The area and number of LWs have gradually been recovered since the 2000s as the country strengthened concern on the ecological restoration and sustainable development. The statistical results suggested that human activities played a dominant role in the LWs changes, with GDP and population explained 80.74% of the changes, coupled with climatic contribution of only around 20%. This long-term investigation will provide baseline information for future lake wetlands monitoring. Our findings could also provide a guidance for decision makers regarding water resources management, environmental protection and land-use planning in urban areas.

Impacts of hydrological alteration on ecosystem services changes of a large river-connected lake (Poyang Lake), China.

Large lakes provide various types of ecosystem services (ESs), of which stocks and variations induced by hydrological alterations are largely unquantified. The present study investigates the long-term changes of five key ESs (i.e., flood regulation, water supply, fish production, nutrient retention and biodiversity conservation) in a large river connected lake (Poyang Lake), with special attention to impacts of hydrological alteration induced by the Three Gorges Dam (TGD). Hydrological data series, hydrodynamic model, the nutrient balance, fishery statistics, and wetland winter waterbird survey data from 1980 to 2016 were employed. Results showed that Poyang Lake provide significant ESs, with long-term average flood regulation, water supply and nutrient retention services of 167.7 × 108 m3, 31.53 × 108 m3, and 15.12% of total phosphorus load, respectively. The fish production service ranged from 1.74 × 104t to 7.19 × 104t, with an average value of 3.12 × 104t. All five key ESs exhibited a downward trend since the 2000s, especially for water supply, fish production and nutrient retention services (p < 0.05), which might be largely attributed to the hydrological condition changes induced by TGD operation. Nevertheless, more detailed monitoring data and biophysical models are required to further acknowledge the changes in biodiversity conservation and fish production services and their linkages with the TGD. The present study sheds light on long-term ES changes in large lakes and their possible linkages with human influences through hydropower projects.

Differences in the responses of flow and nutrient load to isolated and coupled future climate and land use changes.

Understanding the differences in the responses of river hydrology and water quality to climate and land use changes is particularly crucial for the development and management of water resources in the future. This study was carried out to assess the isolated and coupled effects of future climate change and land use change on the flow and nutrient load of the Xitiaoxi watershed in southeast China by applying the calibrated Hydrological Simulation Program Fortran model. Four representative concentration pathways released by the Intergovernmental Panel on Climate Change and two projected land use change scenarios were used to simulate future conditions. The results indicate that climate change would result in flow increased with an average variation of 25.2% in the future, and the increased flow would be mainly concentrated on the high flow part of the total flow duration curve. Climate change would also induce seasonal shifts to nutrient load. The effects of land use change showed that nutrient load was more sensitive than flow, made Orthophosphate load increase by 2.8%-154.7%, and flow increase by 7.2%-15.1%. The results for coupled climate and land use changes indicate that flow and nutrient load would be more affected by climate change than by land use change. Climate and land use changes may amplify or weaken each other's effects on flow and nutrient load, which suggests that both should be incorporated into hydrologic models when studying the future conditions. The results of this study can help decision-makers guide management practices that aim to minimize flow and nutrient load.

Water quality control plan with BATHTUB model for lake inflow rivers - a case study of total phosphorus in Northwest Lake Taihu, China.

Total phosphorus (TP) standards for lakes differ from those for rivers in GB3838-2002. This disjunction may lead to the failure of lake-water quality improvement plans that control nutrient inputs from inflow rivers. With monthly monitoring data for the period 2009-2015, Northwest Lake Taihu was regarded as a case study and the BATHTUB model was utilised to simulate the correspondence between concentrations of TP in the lake and in its inflow rivers. A control plan for TP in Northwest Lake Taihu's inflow rivers is proposed. To guarantee the fulfillment of the control goals of TP in the lake, concentration of TP in the inflow rivers of North Zone, Zhushan Bay, Meiliang Bay and Gonghu Bay should be reduced by 50%, 58%, 18% and 11%, respectively, and TP flux loads should be maintained under 227.35, 173.39, 113.69 and 90.62 ta-1, respectively. Meanwhile, total TP influxes from Northwest Lake Taihu should be maintained under 604.63 ta-1. A control plan that is more restrictive than GB3838-2002 should be proposed to address the TP pollution of the lake. This research provides the foundation for quantifying reduction of the nutrient loading from the catchment and for maintaining Lake Taihu and other typical eutrophic lakes.

Inferring land use and land cover impact on stream water quality using a Bayesian hierarchical modeling approach in the Xitiaoxi River Watershed, China.

Lake eutrophication has become a very serious environmental problem in China. If water pollution is to be controlled and ultimately eliminated, it is essential to understand how human activities affect surface water quality. A recently developed technique using the Bayesian hierarchical linear regression model revealed the effects of land use and land cover (LULC) on stream water quality at a watershed scale. Six LULC categories combined with watershed characteristics, including size, slope, and permeability were the variables that were studied. The pollutants of concern were nutrient concentrations of total nitrogen (TN) and total phosphorus (TP), common pollutants found in eutrophication. The monthly monitoring data at 41 sites in the Xitiaoxi Watershed, China during 2009-2010 were used for model demonstration. The results showed that the relationships between LULC and stream water quality are so complicated that the effects are varied over large areas. The models suggested that urban and agricultural land are important sources of TN and TP concentrations, while rural residential land is one of the major sources of TN. Certain agricultural practices (excessive fertilizer application) result in greater concentrations of nutrients in paddy fields, artificial grasslands, and artificial woodlands. This study suggests that Bayesian hierarchical modeling is a powerful tool for examining the complicated relationships between land use and water quality on different scales, and for developing land use and water management policies.

Centennial loss of lake wetlands in the Yangtze Plain, China: Impacts of land use changes accompanied by hydrological connectivity loss.

Humans have played a fundamental role in altering lake wetland ecosystems, necessitating the use of diverse data types to accurately quantify long-term changes, identify potential drivers, and establish a baseline status. We complied high-resolution historical topographic maps and Landsat imagery to assess the dynamics of the lake wetlands in the Yangtze Plain over the past century, with special attention to land use and hydrological connectivity changes. Results showed an overall loss of 45.6 % (∼11,859.5 km2) of the lake wetlands over the past century. The number of lakes larger than 10 km2 decreased from 149 to 100 due to lake dispersion, vanishing, and shrinkage. The extent of lake wetland loss was 3.8 times larger during the 1930s-1970s than that in the 1970s-1990s. Thereafter, the lake wetland area remained relatively stable, and a net increase was observed during the 2010s-2020s in the Yangtze Plain. The significant loss of lake wetland was predominately driven by agricultural activities and urban land expansion, accounting for 81.1 % and 4.9 % of the total losses, respectively. In addition, the changes in longitudinal and lateral hydrological connectivity further exacerbated the lake wetland changes across the Yangtze Plain through isolation between lakes and the Yangtze River and within the lakes. A total of 130 lakes have been isolated from the Yangtze River due to the construction of sluices and dykes throughout the Yangtze Plain, resulting in the decrease in the proportion of floodplain marsh from 28.3 % in the 1930s to 8.0 % in the 2020s. Furthermore, over 260 sub-lakes larger than 1 km2 (with a total area of 1276.4 km2) are experiencing a loss of connectivity with their parent lakes currently. This study could provide an improved historical baseline of lake wetland changes to guide the conservation planning to wetland protection and prioritization area in the Yangtze Plain.

Chlorophyll a variations and responses to environmental stressors along hydrological connectivity gradients: Insights from a large floodplain lake.

Understanding the key drivers of eutrophication in floodplain lakes has long been a challenge. In this study, the Chlorophyll a (Chla) variations and associated relationships with environmental stressors along the temporal hydrological connectivity gradient were investigated using a 11-year dataset in a large floodplain lake (Poyang Lake). A geostatistical method was firstly used to calculate the hydrological connectivity curves for each sampling campaign that was further classified by K-means technique. Linear mixed effect (LME) models were developed through the inclusion of the site as a random effect to identify the limiting factors of Chla variations. The results identified three clear hydrological connectivity variation patterns with remarkable connecting water area changes in Poyang Lake. Furthermore, hydrological connectivity changes exerted a great influence on environmental variables in Poyang Lake, with a decrease in nutrient concentrations as the hydrological connectivity enhanced. The Chla exhibited contrast variations with nutrient variables along the temporal hydrological connectivity gradient and generally depended on WT, DO, EC and TP, for the entire study period. Nevertheless, the relative roles of nutrient and non-nutrient variables in phytoplankton growth varied with different degrees of hydrological connectivity as confirmed by the LME models. In the low hydrological connectivity phase, the Chla dynamics were controlled only by water temperature with sufficient nutrients available. In the high hydrological connectivity phase, the synergistic influences of both nutrient and physical variables jointly limited the Chla dynamics. In addition, a significant increasing trend was observed for Chla variations from 2008 to 2018 in the HHC phase, which could largely be attributed to the elevated nutrient concentrations. This study confirmed the strong influences of hydrological connectivity on the nutrient and non-nutrient limitation of phytoplankton growth in floodplain lakes. The present study could provide new insights on the driving mechanisms underlying phytoplankton growth in floodplain lakes.

Assessing the inundation dynamics and its impacts on habitat suitability in Poyang Lake based on integrating Landsat and MODIS observations.

Poyang Lake, the largest freshwater lake in China, serves critical ecosystem function for water regulation and biodiversity conservation. However, it experienced dramatic changes in lake inundation due to recent climate change and human activities, causing ecological and economic problems. Here, we applied a multiple-index water detection rule to integrated Landsat and MODIS products to reconstruct surface water series at 30-m and 8-day resolutions and quantified the spatio-temporal inundation dynamics in Poyang Lake over the past 20 years (2000-2019). Furthermore, their influences on habitat suitability for herbivorous birds were also assessed from the perspectives of hydroperiod and vegetation growth threshold. The significant declining trend (-26.66 km2 yr-1, p < 0.001) for the annual minimum water areas throughout the 20 years implied that Poyang Lake was undergoing a continuous shrinkage in the low-water season. On the monthly scale, inundation frequency (IF) decreases were more significant in September-January, most of which occurred in the alluvial delta zones near the lake center. The altered water regime after the Three Gorges Dam (TGD) might be the dominant contributor responsible for the continuous lake shrinkage during the recent low-water period. The sub-lakes suffered from spring drought rather than winter drought, triggering vegetation successions regarding the reversed trend of the well-documented xerophilization in Poyang Lake. The recent earlier and prolonged dry seasons caused an increase of suitable habitat for herbivorous birds (13.92 km2 year-1, p < 0.1), but triggered a potential risk of food quality degradation when the migratory waterbirds peaked in December. These results provide a clear reference for optimizing the hydrologic management and biodiversity conservation of Poyang Lake.

Toward ecological function zoning and comparison to the Ecological Redline Policy: a case study in the Poyang Lake Region, China.

Faced with huge environmental problems of ecosystem degradation, "Ecological Redline Policy (ERP)" in China is a new key national-level policy to manage different land use functions in accordance with development and environmental limits. As the water-land complex ecosystem with the largest freshwater lake, wetland natural reserves and ecological importance in China, Poyang Lake Region (PLR) is selected to quantify and map multiple ecosystem services, investigate the ecological function zoning as part of research on ecological zoning control and major ecological source areas to illustrate and address the implementation of this strategy based on the importance and vulnerability analysis of ecosystem services. According to ecological function zoning results, extremely important, highly important, medium important and important zones respectively account for 26.1%, 28.1%, 17.4% and 28.4% of the total area. With an area of 5422.2 km2, the extremely important zone is 1010.6 km2 larger than the ERP. Moreover, 81.6% of the ERP is located in the extremely important zone. By discussing the implications and applications of ecological management, this study contributes to the ecological protection of Poyang Lake and provides a foundation for research on ecological function zoning at the regional scale.

Role of Flooding Patterns in the Biomass Production of Vegetation in a Typical Herbaceous Wetland, Poyang Lake Wetland, China.

Flooding is an important factor influencing the biomass production of vegetation in natural wetland ecosystems. However, how biomass production is linked to flooding patterns in wetland areas remains unclear. We utilized gauging station data, a digital elevation model, vegetation survey data, and a Landsat 8 image to study the effects of average inundation depth (AID) and inundation duration (IDU) of flooding on end-of-season biomass of vegetation in Poyang Lake wetland, in particular, after operation of Three Gorges Dam. The end-of-season biomass of wetland vegetation showed Gaussian distributions along both the AID and IDU gradients. The most favorable flooding conditions for biomass production of vegetation in the wetland had an AID ranging from 3.9 to 4.0 m and an IDU ranging from 39% to 41%. For sites with a lower AID (<3.9 m; IDU < 39%), the end-of-season biomass values were positively related, whereas for sites with a higher AID (4.0 m; IDU > 41%), the end-of-season biomass values were negatively related. After the operation of the Three Gorges Dam, flooding patterns characterized by AID and IDU of the Poyang Lake wetland were significantly alleviated, resulting in a mixed changing trend of vegetation biomass across the wetland. Compared with 1980-2002, the increase of end-of-season biomass in lower surfaces caused by the alleviated flooding pattern far exceeded the decrease of end-of-season biomass in higher surfaces, resulting in an end-of-season biomass increase of 1.0%-6.7% since 2003. These results improved our understanding of the production trends of vegetation in the wetland and provided additional scientific guidance for vegetation restoration and wetland management in similar wetlands.

Multidecadal water quality deterioration in the largest freshwater lake in China (Poyang Lake): Implications on eutrophication management.

Poyang Lake is the largest freshwater lake in China and a globally important wetland with various functions. Exploring the multidecadal trend of water quality and hydroclimatic conditions is important for understanding the adaption of the lake system under the pressure from multiple anthropogenic and meteorological stressors. The present study applied the Mann-Kendall trend analysis and Pettitt test to detect the trend and breakpoints of hydroclimatic, and water quality parameters (from the 1980s to 2018) and the trend of monthly-seasonal ammonia (NH4-N) and total phosphorus (TP)concentrations (from 2002 to 2018) in Poyang Lake. Results showed that Poyang Lake had undergone a highly significant warming trend from 1980 to 2018, with a warming rate of 0.44 °C/decade in terms of annual daily mean air temperature. The wind speed and water level of the lake presented a highly significant decreasing trend, whereas no notable trend was detected for precipitation variations. The annual mean total nitrogen (TN), NH4-N, TP, and permanganate index (CODMn) concentrations showed significant upward trends from the 1980s to 2018. Remarkable abrupt shifts were detected for TN, NH4-N, and CODMn in around 2003. They were in accordance with the water level breakpoint of the lake, thus implying the important role of hydrological conditions in water quality variations in floodplain lakes. A significant increasing trend has been detected for Chl-a variations during wet season from 2008 to 2018, which could be attributed to the increasing trend of nutrient concentration during the nutrient-limited phase of Poyang Lake. These hydroclimatic and water quality trends suggest a high risk of increasing phytoplankton growth in Poyang Lake. This study thus emphasizes the need for adaptive lake eutrophication management for floodplain lakes, particularly the consideration of the strong trade-off and synergies between hydroclimatic conditions and water quality variations.

Monitoring the spatio-temporal dynamics of the wetland vegetation in Poyang Lake by Landsat and MODIS observations.

Poyang Lake, the largest freshwater lake of China, provides critical ecological functions for water circulation and biodiversity conservation as a dynamic wetland system. However, recent climate change and human activities exerted strong pressures on this ecosystem. In this paper, we applied object-based image analysis (OBIA) and Radom Forests (RF) classifier to ten Landsat images to examine the land cover composition and its change during 1987-2017 low water season at Poyang Lake. NDVI time series (2000-2017) derived from MODIS imagery was used to document the changes of vegetation growth status. To investigate the potential driving mechanism of the inundation patterns, we differentiated the spatial-temporal changes of vegetation coverage and NDVI accumulation on eight elevation bands. Major result indicates that the vegetation area increased by 15.5% of the lake area during 1987-2017. A much faster-increasing rate (58.0 km2 year-1) can be observed during 2001-2009 as compared to that of the overall study period (18.4 km2 year-1). Analysis of NDVI accumulation showed that 42.1% of the lake's area displayed a significant increasing trend during 2000-2017. Spatially, the increase of vegetation area and NDVI accumulation mainly took place in the 11-12 m elevation band in the lower lake center. Early dry season and prolonged exposure period after the operation of Three Gorges Dam (TGD) was the major reason for the spatio-temporal evolution of the wetland vegetation in Poyang Lake. The Lake's water level started to fall below 12 m before 9th November might cause a boost of vegetation growth in the low lake center, and in turn, triggering xerophilization for the vegetation in the highlands and a shift in foraging patterns of waterbirds due to phenology variations. The findings of this study provide a clear reference for sustaining the inter-annual stability of the ecosystem by controlling the depth of water in the lake.

Mapping Aboveground Biomass of Four Typical Vegetation Types in the Poyang Lake Wetlands Based on Random Forest Modelling and Landsat Images.

Wetland biomass is an important indicator of wetland ecosystem health. In this study, four dominant vegetation communities (Carex cinerascens, Phalaris arundinacea, Artemisia selengensis, and Miscanthus sacchariflorus) in the Poyang Lake wetland from 2010 to 2016 were classified from Landsat images using spectral information divergence (SID). We combined aboveground biomass (AGB) field measurements and remote sensing data to establish a suitable model for estimating wetland AGB in Poyang Lake, which is on the Ramsar Convention's list of Wetlands of International Importance. The results showed that (1) overall, the classification accuracy for vegetation pixels across 5 years ranged from 59.1% to 73.7% and (2) the inter-annual and spatial variations in the AGB of the four vegetation types were clear. C. cinerascens had an average AGB density value of 1.28 kg m-2 in Poyang Lake from 2010 to 2016; M. sacchariflorus had the highest AGB density with an average value of 1.39 kg m-2; A. selengensis had almost the same level at 1.26 kg m-2; and P. arundinacea had the lowest AGB density at 0.64 kg m-2. This study provides useful experience for estimating carbon sequestration of vegetation in freshwater wetlands.

Hydrodynamic and water quality modeling of a large floodplain lake (Poyang Lake) in China.

Floodplain lakes are valuable to humans because of their various functions and are characterized by dramatic hydrological condition variations. In this study, a two-dimensional coupled hydrodynamic and water quality model was applied in a large floodplain lake (i.e., Poyang Lake), to investigate spatial and temporal water quality variations. The model was established based on detailed data such as lake terrain, hydrological, and water quality. Observed lake water level and discharge and water quality parameters (TN, TP, CODMn, and NH4-N) were used to assess model performance. The hydrodynamic model results showed satisfactory results with R2 and MRE values ranging between 0.96 and 0.99 and between 2.45 and 6.14%, respectively, for lake water level simulations. The water quality model basically captured the temporal variations in water quality parameters with R2 of TN, TP, CODMn, and NH4-N simulation ranges of 0.56-0.91, 0.44-0.66, 0.64-0.67, and 0.44-0.57, respectively, with TP of Xingzi Station and CODMn of Duchang Station excluded, which may be further optimized with supplementation of sewage and industrial discharge data. The modeled average TN, TP, CODMn, and NH4-N concentrations across the lake were 1.36, 0.05, 1.99, and 0.48 mg/L, respectively. The modeled spatial variations of the lake showed that the main channel of the lake acted as a main pollutant passageway, and the east part of the lake suffered high level of pollution. In addition, consistent with previous water quality evaluations based on field investigations, water quality was the highest (average TN = 1.35 mg/L) during high water level periods and the poorest (average TN = 1.96 mg/L) during low water level periods. Scenario analysis showed that by decreasing discharge of upstream flow by 20% could result in the increase of TN and TP concentrations by 25.6% and 23.2% respectively. In summary, the model successfully reproduced the complex water and pollutant exchange processes in the systems involving upstream rivers, the Poyang Lake, and the Yangtze River. The model is beneficial for future modeling of the impact of different load reduction and other hydrological regime changes on water quality variation and provides a relevant example for floodplain lake management.

Dynamic water quality evaluation based on fuzzy matter-element model and functional data analysis, a case study in Poyang Lake.

Comprehensively evaluating water quality with a single method alone is challenging because water quality evaluation involves complex, uncertain, and fuzzy processes. Moreover, water quality evaluation is limited by finite water quality monitoring that can only represent water quality conditions at certain time points. Thus, the present study proposed a dynamic fuzzy matter-element model (D-FME) to comprehensively and continuously evaluate water quality status. D-FME was first constructed by introducing functional data analysis (FDA) theory into a fuzzy matter-element model and then validated using monthly water quality data for the Poyang Lake outlet (Hukou) from 2011 to 2012. Results showed that the finite water quality indicators were represented as dynamic functional curves despite missing values and irregular sampling time. The water quality rank feature curve was integrated by the D-FME model and revealed comprehensive and continuous variations in water quality. The water quality in Hukou showed remarkable seasonal variations, with the best water quality in summer and worst water quality in winter. These trends were significantly correlated with water level fluctuations (R = -0.71, p < 0.01). Moreover, the extension weight curves of key indicators indicated that total nitrogen and total phosphorus were the most important pollutants that influence the water quality of the Poyang Lake outlet. The proposed D-FME model can obtain scientific and intuitive results. Moreover, the D-FME model is not restricted to water quality evaluation and can be readily applied to other areas with similar problems.