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Hydrologic alterations in the rapidly changing environment in the Pearl River Delta, South China

Under the support of a GRF grant funded by Hong Kong RGC, Professor Chen Yongqin and his team have completed a study on the hydrologic alterations in the Pearl River Delta (PRD) region. Hydrologic regimes fluctuate in response to climatic variations and human influences. Over the past three decades, PRD has been the fastest developing region in China. Human activities mainly associated with urbanization and industrialization in the region have been rapidly changing the environment, making the PRD crisscross river network an ideal area for studying human-induced hydrologic alterations.

In this study, researchers first used linear regression method to systematically evaluate the long-term variations of extreme water levels defined as exceeding/falling below certain thresholds (mean±std) across the delta. It was found that the low water level has been significantly dropping in the upper PRD while the opposite situation has occurred in the lower PRD. Secondly, they used Mann-Kendall trend test and Kriging interpolation method to detect the spatial and temporal patterns of the trends of the extreme high and low water levels related to different magnitudes of streamflow. The results indicate that the total flow of Xijiang and Beijiang has not changed significantly over the past five decades. However, some discharge of Xijiang has been transferred to Beijiang because of the lowering of Beijiang water level as a result of intensive in-channel sand dredging. Due to extensive but uneven sand dredging and other human activities, water level has dropped down significantly in the upper PRD and become highly variable in the middle PRD as reflected by a mixture of both increasing and decreasing trends of different water level characteristics. Nevertheless, increasing trends in water level can be clearly identified in the coastal areas due to two main reasons. Finally, they applied Beyes model and Lepage test to detect change point(s) of water level time series and analyzed the associated statistical properties of high/low water levels in summer (Jun, Jul, Aug) and winter (Dec, Jan, Feb). With respect to winter mean low water level, 14 out of 19 stations had two change points. The first abrupt change occurred around 1980 and the second abrupt change in the early 1990s. As for summer mean high water level, 12 out of 19 stations have only one change point. Identification and analysis of the causes for the hydrologic alterations in terms of water level will help us understand the impacts of human activities and thus provide important scientific evidence for managing the water environment in the PRD region.

PRD stream network and gauging stations Mean water level in PRD Sand dredging from river channel Dropping stage at Sanshui station