Research Of Water System Changes In The Pearl River Delta Under Intensive Urbanization In Recent 40 Years

The Pearl River Delta is a densely populated and rapidly growing region in China.With the proposal and implementation of the reform and opening-up policy in 1978,significant changes have occurred in land use and urbanization in this area,leading to major environmental impacts,particularly on the water system.This study utilized Landsat imagery from 1984 to 2019 to analyze the structural evolution of the river network system in the Pearl River Delta over the past 40 years.For the methodology of this research,the support vector machine image classification algorithm was used to classify satellite imagery to obtain river network data for the Pearl River Delta,which was divided into two parts based on morphological features: rivers and ponds.For river networks,a unified water system structure index system was used to quantitatively analyze and compare its structural changes based on six indicators: river density,water surface rate,mainstream area-length ratio,shape index,tributary development coefficient,and fractal dimension.For ponds,several quantitative indicators,including standard deviation,coefficient of variation,and the Theil coefficient,were used to compare their developmental stages based on spatial differentiation.The change rate was quantified by the intensity of water body changes and classified into four evolving types: strong expansion,weak expansion,relatively stable,and contraction,thereby dividing the evolutionary stages.Moreover,this study referred to Anderson JR’s land use classification system to classify land cover other than water bodies in ten periods of imagery from the Greater Bay Area,extracted land cover data,and calculated single land use dynamics,comprehensive land use dynamics,and transition matrices as indicators reflecting changes in the extent,velocity,and direction of land types such as river networks.Results showed that the river network system in the Pearl River Delta underwent significant changes over the past 40 years.The overall density of river networks decreased,while water surface rate and tributary development coefficient increased rapidly before 2006,with an intensive average growth rate of about 0.071%.The growth rate slowed from 2006 to 2013,and since 2013,it has been decreasing rapidly at an annual rate of approximately-0.073%.Shape index and fractal dimension generally declined,indicating that the complexity of the rivers has decreased,gradually moving from their original natural winding shape to canalization under artificial intervention.The study also found that the total area of ponds in the Pearl River Delta significantly increased from 1986 to 2013,reaching its peak in 2013 with a total increase of 84.63%;thereafter,the total area showed a downward trend with a total decrease of approximately 31.34%.2013 was identified as a turning point,demonstrating that human activities have continuously affected the spatial distribution and scale of ponds in the past 40 years.Ponds have transformed from random distribution with different sizes in the early stage to artificial distribution and shapes.Changes in land use,industrial transfer,government guidance,and financial motives are the main causes of this transformation.Furthermore,this study explored the changes in land use types in the Pearl River Delta region,showing that the trajectories of land use change are synchronized with those of rivers and ponds in terms of the time points at which changes occur.Built-up areas have had a significant increase in recent years,yet the meanwhile cultivated land and forest land have decreased.Single land use dynamics indicate that forests,shrubs,wasteland,and built-up areas are the most active types of change,among which builtup areas are the only type with sustained positive growth in the last 40 years.Comprehensive land use dynamics indicate that before 1991,wasteland and shrubs were the largest proportion of types transferred out,with a significant amount of wasteland and shrubs transformed into built-up areas.After 1991,grassland and wasteland replaced them as the main types of area transfer.The changes in the water area are most closely related to two types of land use that are most sensitive to urbanization: cultivated land and built-up areas.Among them,the largest area of water conversion to other land use types is cultivated land,followed by built-up areas.The largest area of land use conversion to water is also cultivated land,followed by builtup areas.Both show a trend of increasing before 2006 and decreasing after 2009,synchronizing with the rate of urbanization.By analyzing the changes in these two indicators over time,it is found that the period between 2006 and 2009 was the most active period of land use change,as well as the active period of changes in rivers and ponds.This can provide a basis for discussing the response of river network spatialtemporal changes to the process of urbanization.