Analysis of changes in river connectivity during the urbanization process: a case study of the southeastern plain of Yinzhou, China.

Affected by human activities, the naturally occurring river network in the southeastern plain of Yinzhou has gradually evolved into a natural-artificial composite water system, and changes in river connectivity due to changes in river network systems have caused water security problems, including urban flooding. To clarify the river connectivity change and its relationship with the urbanization process, this paper discusses an evaluation method for river connectivity based on complex networks and cellular automata (CA) from the perspective of complex systems, quantitatively analyzes the spatial-temporal characteristics of the structural and functional river connectivity in the study area during the 1990s-2020s, and reveals the impact of river nodes and chains on the connectivity level under the disturbance of natural or human factors. The results contained the following revelations: ① River connectivity showed a decreasing trend in the initial and rapid development stages of urbanization from the 1990s to the 2010s and a limited increasing trend in the optimization and upgrading stages from the 2010s to the 2020s. ② River network degradation and ongoing connectivity decline are found in the northeastern part of the study area. The highest river connectivity exists in Dongqianhukaifaqu. ③ The number of river nodes and chains should be maintained at approximately 80% for normal river connectivity. The nodes of high degree in the inflow area are listed in the key protection areas. ④ Changes in river connectivity are significantly correlated with the urbanization process. Changes in the functional connectivity level affect the magnitude of a flood. This study provides a theoretical basis for river network connectivity improvement and flood prevention in plain areas.

Identifying changes in flood characteristics and their causes from an event-based perspective in the Central Taihu Basin.

Increasing rainstorms induced by climate change and modification in the land surface due to urbanization have greatly altered floods at different spatio-temporal scales. However, investigating flood events in urbanized plains is challenging as anthropogenic behaviors can change river flow without rainfall. In addition, while the frequency and magnitude of floods have been well examined, knowledge about variations in the rate of flood change is still limited. To fill these gaps, we proposed a scheme that focused on flood responses to rainfall to detect changes in flood characteristics in the Central Taihu Basin, a highly urbanized region in the Yangtze River Delta of China. Four characteristic metrics were adopted to summarize the flood hydrograph, including the peak, increment, rising rate, and falling rate. We then examined trends of these metrics based on the selected rainfall-flood events from ten hydrological stations during 1970-2020. Subsequently, the reduction method was used to separate the impacts of regional climate change and human activities on flood characteristics alterations. Furthermore, the importance of fifteen factors was quantified by the random forest model. We found that there is a significant upward trend in the evolution of flood characteristics, except for the increment of floods. Flood characteristics exhibit higher values when rainfall accumulates, indicating stronger responses of floods to a large amount of rainfall. The results also show that human activities dominate and impact the peak, rising rate, and falling rate of floods more than climate change. Meanwhile, although cumulative precipitation is the most important factor, flood characteristics are also susceptible to anthropogenic factors, such as land use change and hydraulic engineering construction. Our findings, which provide insights into flood event identification and enhance the understanding of regional flood changes, will serve as a reference for water resource management and flood mitigation in urbanized areas.

Network structure and stability of the river connectivity in a rapidly urbanizing region.

River connectivity, which is key to the function of the river network, is deteriorating in highly urbanized areas. While previous studies have identified changes in the structure and connectivity of river networks, few studies have described the network structure of river connectivity and revealed the effects of urbanization on network structure. To this end, we detected the network structure and stability of river connectivity from a novel perspective of complex network theory. Taking the Taihu Plain as an example, we found that 1) the node degree of the river network was moderate (2.9), and the hub and connectivity were low (0.21 and 0.19, respectively). The hub and connectivity of the river network in the Yang-Cheng-Dian-Mao (YCDM) region were better than that in Wu-Cheng-Xi-Yu (WCXY) and Hang-Jia-Hu (HJH) regions. 2) The destruction of important nodes led to a dramatic decrease in the stability of river network connectivity. The river network structures on the Taihu Plain, HJH, WCXY, and YCDM were severely damaged when the removal rate of river network nodes exceeded 5 %, 7 %, 16 %, and 22 %, respectively. 3) The spatial response of the river network to urbanization was mainly negative, especially in highly urbanized regions. The effects of urbanization on network characteristics could be sorted as node degree (45 %), hub (21 %), and connectivity (18 %). Our results would provide theoretical support for the recognition, protection, and restoration of the river network in rapidly urbanizing regions.

Spatially non-stationary relationships between urbanization and the characteristics and storage-regulation capacities of river systems in the Tai Lake Plain, China.

Given environmental or hydrological functions influenced by changing river networks in the development of rapid urbanization, a clear understanding of the relationships between comprehensive urbanization (CUB) and river network characteristics (RNC), storage capacity (RSC), and regulation capacity (RRC) is urgently needed. In the rapidly urbanized Tai Lake Plain (TLP), China, various methods and multisource data were integrated to estimate the dynamics of RNC, RSC, and RRC as well as their interactions with urbanization. The bivariate Moran's I methods were applied to detect and visualize the spatial dependency of RNC, RSC, or RRC on urbanization. Geographically weighted regression (GWR) model was set up to characterize spatial heterogeneity of urbanization influences on RNC, RSC and RRC. Our results indicated that RNC, RSC and RRC variables each showed an overall decreasing trend across space from 1960s to 2010s, particularly in those of tributary rivers. RNC, RSC, or RRC had globally negative correlations with CUB, respectively, but looking at local scale the spatial correlations between each pair were categorized as four types: high-high, high-low, low-low, and low-high. GWR was identified to accurately predict the response of most RNC, RSC, or RRC variables to CUB (R2: 0.6-0.8). The predictive ability of GWR was spatially non-stationary. The obtained relationships presented different directions and strength in space. All variables except for the water surface ratio (Wp) were more positively affected by CUB in the middle eastern parts of TLP. Drainage density, RSC and RRC variables were more negatively influenced by CUB in the northeast compared to other parts. The quantitative results of spatial relationships between urbanization and RNC, RSC or RRC can provide location-specific guidance for river environment protection and regional flood risk management.

[Analysis of Rainfall Runoff Pollution and Pollution Load Estimation for Urban Communities in a Highly Urbanized Region].

The Taihu Lake plain is a highly urbanized region in China with many water-related environmental problems. Although point-source pollution has been effectively controlled by government legislation, urban surface runoff pollution is still a major issue. Different types of urban communities were selected for rainfall runoff experiments. According to the monitoring data of rainfall events, multiple methods were used to analyze the characteristics of surface runoff pollution and estimate the pollution load for different types of communities. The results indicated that surface runoff from urban communities reduced the river water quality. Certain degrees of the 'first flush' effect occurred in different types of urban communities. The surface runoff pollution in the commercial residential community was weaker than that in commercial and private residential communities; however, the first flush occurred more frequently in the commercial residential community. Holding back 30% of the surface runoff could effectively improve the runoff water quality in commercial and private residential communities as well as the commercial residential community with restaurants. In the commercial residential community, 25% of surface runoff should be held to improve runoff water quality effectively. The loads of pollutants, especially nitrogen and phosphorus, in urban communities in the Taihu Lake basin were higher than those in other regions in China. This research can assist with the reduction of surface runoff pollution in highly urbanized communities.

Assessing the impacts of climatic and anthropogenic factors on water level variation in the Taihu Plain based on non-stationary statistical models.

Water level (a vital indicator for flood warnings and water management in floodplains) has been changed notably due to climatic and anthropogenic forces; however, very little is known about the relative effects of these agents. In this study, we take the Taihu Plain as an example to investigate potential factors driving changes in water level components through quantiles from 1954 to 2014. To quantify the extent of water level component changes attributable to climate variability and human activity, several non-stationary models considering rainfall, tide, evaporation, and hydraulic regulation as covariates are established based on generalized additive models for location, scale, and shape. The results indicate that most water level components increased over time and changed abruptly around the mid-1980s. As for climatic factors, the variability of rainfall, tide and evaporation significantly affected water level variation based on most quantiles from 1954 to 2014. Among several kinds of human activities, hydraulic regulation was a key factor influencing water level based on a high correlation coefficient. Positive effects were identified from hydraulic regulation regarding the association between rainfall and water level components; these effects depend on water level quantiles and the amount of rainfall occurrence. Our study has broad implications, providing a better understanding of water level variation and regional flood management.

Effects of industry structures on water quality in different urbanized regions using an improved entropy-weighted matter-elementmethodology.

Urbanization and industrialization significantly impact water quality, and detecting the specific factors which influence water quality change would greatly improve urban water environment management. In this study, an improved entropy-weighted matter-element method is used to assess the variations of water quality in two regions with different levels of urbanization in the Yangtze River Delta. Redundancy analysis was used to detect the effects of different industries on water quality. Results show that (1) an improved entropy weight-based matter-element method measures weights of pollutants and water quality levels more reliably and accurately; (2) the improvement rate of water quality in highly urbanized regions is 42.9% during 2005-2014 which is 17.2% higher than that in regions with low urbanization; (3) a decreasing concentration of total phosphorus is the main reason for changes of water quality in both regions, with decreasing concentrations of permanganate index and ammonium nitrogen having a strong influence on changes of water quality in the highly urbanized regions; (4) the decreasing proportion of fishery and heavy industries and the increasing proportion of the tertiary industries significantly influence water quality in highly urbanized regions while the decreasing proportion of animal husbandry is the most important factor influencing the changes of water quality in lowly urbanized regions.

Stream loss in an urbanized and agricultural watershed in China.

Stream losses are extensively observed due to human activities in the world, and the patterns of stream loss vary in different land use types. However, relationship between stream loss pattern and land use covers is poorly understood. We select the lower Taihu watershed (LTWS) within Yangtze River Delta (YRD), which is dominated by agricultural and urban covers and a typical case of most urbanized watersheds in China. In this study, we measured the stream loss of LTWS from 1960s to 2010s and investigated its relation to different land use covers and impervious area percentage (IAP) in order to figure out the main factor of stream loss in this area. The results show that urban area has tripled with fractional contribution from 10.3% to 33.18% in the form of conversion from agriculture to urban area during 1990-2015. 12.5% of all the streams are lost and 1st-order streams contribute most (91.8%) to the total stream loss. Urban cover contribute most (76%) to total streams loss compared to other land use types. We find that 1st-order streams have highest stream loss intensity, which is mainly caused by urban expansion, but preferred protections are given to highest-order streams. The linear model of correlation of pixel-level streams loss and IAP shows that the streams loss is statistically significant positive with IAP of cells (R2 = 0.91). Tradeoffs between city expansion and river network make small channels sacrifice for the urbanization. Urgent measures including legislation must be taken to protect small streams during urbanization nowadays and in future.

Spatiotemporal variation of vegetation coverage and its associated influence factor analysis in the Yangtze River Delta, eastern China.

Vegetation is a natural tie that connects the atmosphere, hydrosphere, biosphere, and pedosphere. Quantitatively evaluating the variability of vegetation coverage and exploring its associated influence factors are essential for ecological security and sustainable economic development. In this paper, the spatiotemporal variation of vegetation coverage and its response to climatic factors and land use change were investigated in the Yangtze River Delta (YRD) from 2001 to 2015, based on normalized difference vegetation index (NDVI) data, vegetation type data, climate data, and land use/cover change (LUCC) data. The results indicated that the annual mean vegetation coverage revealed a nonsignificant decreasing trend over the whole YRD. Areas characterized by significant decreasing (P < 0.05) trends were mainly concentrated on the central and northern part of the YRD, and significant increasing (P < 0.05) trends were mainly located in the southern part of the study area. Except for grassland and cultivated crops, vegetation coverage of the other types of vegetation was all exhibiting increasing trends. Temperature has a more pronounced impact on vegetation growth than precipitation at both the annual and monthly scales. Furthermore, vegetation growth exhibited a time lag effect for 1~2 months in response to precipitation, while there was no such phenomenon with temperature. Land use change caused by urbanization is an important driving factor for the decrease of vegetation coverage in the YRD, and the effect of land use change on the vegetation dynamic should not be overlook.

Effect of urbanisation on extreme precipitation based on nonstationary models in the Yangtze River Delta metropolitan region.

Urban expansion has led to a significant increase in the proportion of areas with impervious surfaces, thereby affecting the weather system and changing local precipitation. Four nonstationary generalised extreme value (GEV) models were constructed to assess the impact of urbanisation on the annual maximum 1-day precipitation (Rx1day) and annual maximum consecutive 5-day precipitation (Rx5day). Among these models, the one that modelled the location parameter as a function of local factors, urban factors, suburban factors, and Pacific Decadal Oscillation (namely, the M4USP model), exhibited a better performance at fitting the Rx1day and Rx5day than the stationary GEV model for urban and rural stations, as well as for highly urbanised suburban stations. Comparing the M4USP model with a nonstationary model varying with Pacific Decadal Oscillation (namely, the M1P model), it is found that the urban expansion could increase the magnitudes of extreme precipitation and its recurrence levels under different return periods. Specifically, the recurrence levels of Rx1day and Rx5day increased by 25.9% and 59.1% for highly urbanised stations, 34.2% and 36.9% for lowly urbanised stations, and 30.7% and 61.5% for rural stations, respectively. The decision makers should strike a balance between urbanisation and extreme precipitation to adapt to a changing environment.

Degrading flood regulation function of river systems in the urbanization process.

This study developed the potential regulation capacity (PRC) and actual regulation capacity (ARC) to characterize the flood regulation function of river systems in the delta plains. Spatial autocorrelation and the grey relational analysis were then employed to investigate the spatial-temporal change of the flood regulation function in the urbanization process, of which the results revealed its spatial coupling and quantitative relation with the urbanization process and river systems changes in the Taihu Plain. The results indicated that: (1) the PRC exhibited a 20.3%-decrease during the 1960s to the 2010s, though its change exhibited significant structural and hierarchical differences. The global distributions of the PRCs all presented insignificant clustered characteristics, and the local distribution of smaller PRC regions with a high flood risk was stable; (2) the ARC exhibited a 33.2%-reduction in the recent 50years, and its decrement also exhibited an accelerating trend. The global ARC distribution changed from significant clusters to insignificant aggregations, and the local distribution range of the smaller ARC regions with a high flood risk gradually increased; (3) the spatial coupling degree between the change in the PRC and the urbanization process was greater than that between the change in the ARC and the urbanization process, and the effect of urbanization on the change in the PRC was also larger than the change in the ARC; (4) the spatial coupling degree between the changes in the ARC and the river systems was greater than that between the changes in the PRC and the river systems, though the impacts of the river systems changes on the change in the PRC were larger than that on the change in the ARC. Therefore, the developed indicators based on limited and available information can serve as references for planning effective flood control strategies in the delta plains.

[Characteristics of Stable Isotopes in Precipitation and Their Moisture Sources in Mengzi Region, Southern Yunnan].

The δD and δ¹8O values in precpitation have disciplinary variations, and they have close connections with meteorological parameters and moisture sources. Based on the continuously collected precipitation samples in Mengzi from Jan. 2009 to Dec. 2011, the reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the National Centre for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR), and the Hybrid Single-Particle Lagrangian Integrated Trajectory for 4.8 (HYSPLIT_4.8) model, we analyzed the variations of δD and δ¹8O in precipitation at synoptic scale in Mengzi, and the relations between δ¹8O in precipitation and air temperature, δ¹8O in precipitation and amount, δ¹8O in precipitation and wind speed, δ¹8O in precipitation and moisture sources. The results showed that the variations of δD and δ¹8O values in precipitation exhibited remarkable seasonal variability. The stable isotopic values in precipitation were higher during dry season than those during wet season. The relations between δ¹8O in precipitation and air temperature, δ¹8O in precipitation and amount indicated significant negative correlations. However, it exhibited significant positive correlation between precipitation δ¹8O and wind speed at different pressure levels (300 hPa, 500 hPa, 700 hPa, and 800 hPa), and this result indicated that the wind speed was an important influencing factor for the variations of precipitation δ¹8O. With the increasing rainfall levels, the intercept and slope of meteoric water line also increased, and this phenomenon suggested that there was a secondary evaporation effect under sub-cloud in stable isotopes of precipitation. The backward trajectory model showed that the main moisture sources during dry season came from the westerly and the inland. However, during wet season, the remote ocean vapor was the main moisture source in Mengzi, and the δ¹8O values in precipitation were much lower during typhoon period.

Dynamic simulation of water resources in an urban wetland based on coupled water quantity and water quality models.

Water quality in wetlands plays a huge role in maintaining the health of the wetland ecosystem. Water quality should be controlled by an appropriate water allocation policy for the protection of the wetlands. In this paper, models of rainfall/runoff, non-point source pollution load, water quantity/quality, and dynamic pollutant-carrying capacity were established to simulate the water quantity/quality of Xixi-wetland river network (in the Taihu basin, China). The simulation results showed a satisfactory agreement with field observations. Furthermore, a 'node-river-node' algorithm that adjusts to the 'Three Steps Method' was adopted to improve the dynamic pollutant-carrying capacity model and simulate the pollutant-carrying capacity in benchmark years. The simulation result shows that the water quality of the river network could reach class III stably all year round if the anthropogenic pollution is reduced to one-third of the current annual amount. Further investigation estimated the minimum amount of water diversion in benchmark years under the reasonable water quantity-regulating rule to keep water quality as class III. With comparison of the designed scale, the water diversion can be reduced by 184 million m3 for a dry year, 191 million m3 for a normal year, and 198 million m3 for a wet year.

Analysis of river health variation under the background of urbanization based on entropy weight and matter-element model: A case study in Huzhou City in the Yangtze River Delta, China.

Maintaining the health of the river ecosystem is an essential ecological and environmental guarantee for regional sustainable development and one of the basic objectives in water resource management. With the rapid development of urbanization, the river health situation is deteriorating, especially in urban areas. The river health evaluation is a complex process that involves various natural and social components; eight eco-hydrological indicators were selected to establish an evaluation system, and the variation of river health status under the background of urbanization was explored based on entropy weight and matter-element model. The comprehensive correlative degrees of urban river health of Huzhou City in 2001, 2006 and 2010 were then calculated. The results indicated that river health status of the study area was in the direction of pathological trend, and the impact of limiting factors (such as Shannon's diversity index and agroforestry output growth rate) played an important role in river health. The variation of maximum correlative degree could be classified into stationary status, deterioration status, deterioration-to-improvement status, and improvement-to-deterioration status. There was a severe deterioration situation of river health under the background of urbanization.

Scale-dependence effects of landscape on seasonal water quality in Xitiaoxi catchment of Taihu Basin, China.

Further understanding the mechanisms of landscape-water interactions is of great importance to water quality management in the Xitiaoxi catchment. Pearson's correlation analysis, stepwise multiple regression and redundancy analysis were adopted in this study to investigate the relation between water quality and landscape at the sub-catchment and 200 m riparian zone scales during dry and wet seasons. Landscape was characterized by natural environmental factors, land use patterns and four selected landscape configuration metrics. The obtained results indicated that land use categories of urban and forest were dominant landscape attributes, which influenced water quality. Natural environment and landscape configuration were overwhelmed due to land management activities and hydrologic conditions. In general, the landscape of the 200 m riparian zone appeared to have slightly greater influence on water than did the sub-catchment, and water quality was slightly better explained by all landscape attributes in the wet season than in the dry season. The results suggested that management efforts aimed at maintaining and restoring river water quality should currently focus on the protection of riparian zones and the development of an updated long-term continuous data set and higher resolution digital maps to discuss the minimum width of the riparian zone necessary to protect water quality.

Impacts of urbanization on river system structure: a case study on Qinhuai River Basin, Yangtze River Delta.

Stream structure is usually dominated by various human activities over a short term. An analysis of variation in stream structure from 1979 to 2009 in the Qinhuai River Basin, China, was performed based on remote sensing images and topographic maps by using ArcGIS. A series of river parameters derived from river geomorphology are listed to describe the status of river structure in the past and present. Results showed that urbanization caused a huge increase in the impervious area. The number of rivers in the study area has decreased and length of rivers has shortened. Over the 30 years, there was a 41.03% decrease in river length. Complexity and stability of streams have also changed and consequently the storage capacities of river channels in intensively urbanized areas are much lower than in moderately urbanized areas, indicating a greater risk of floods. Therefore, more attention should be paid to the urban disturbance to rivers.

Temporal and spatial variation of water level in urbanizing plain river network region.

As one of the most developed regions in China, the plain of East China is undergoing gradually increased flooding under the obvious urbanization process. This paper mainly analyses the trend of water level time series in the region during the past decades, and assesses the temporal and spatial variation of water level and indicators of hydrological alteration. The results show that there is a trend of increasing water level. Bigger slope and higher significant level can be observed in monthly minimum than in monthly maximum water level, in peri-urban than in urban areas. Meanwhile, it is observed that the mean monthly minimum and maximum water level increased in both urban and peri-urban regions, while decreased coefficients of variation (Cv) in urban and increased Cv in peri-urban regions were calculated. Most indicators of hydrologic alteration in urban stations are concentrated to the range of variability approach target, while most indicators are discrete in peri-urban stations. And the degree of hydrologic alteration is higher in peri-urban than in urban regions.