Comparative evaluation of river water quality and ecological changes at upstream and downstream sites of dams/sluices in different regulation scenarios

Abstract

The construction of dams and sluices (DSs) is conducive to human development, but affects river water quality (WQ) and water ecology (WE) seriously. While many studies have explored the DS-induced impact on hydrology, water quality and water ecology, the difference in DS regulations for different objectives and interests (such as flood control, power generation and ecological protection) are not taken into the account although their impacts on WQ and WE are different. Discharge is the key factor to measure different DS regulation and its variation directly affects the long-term WQ and WE in rivers. Thus, studying short-term multi-scenario DS regulation with different discharge is very important for providing insights to their long-term irregular management. Furthermore, the relationships between discharges and the change of WQ indicators are different in the upstream and downstream of DSs and at different distance to DSs in the downstream. However, it is not well understood how the WQ and WE indicators at upstream and downstream sites of DSs change under different regulation scenarios in long-term and short-term. Using the monitored WQ and WE indicators at twelve sampling sites (SPs) of six DSs in the Huai River Basin (HRB), this study investigates the impact of long-term irregular DS regulation on changes in WQ and WE indicators, i.e., the long-term regulation experiment (LTRE), and then the relationship between the alteration of DS discharge flow and changes in WQ indicators, i.e., the short-term regulation experiment (STRE) under four scenarios with different flow magnitudes at one of sluices (Huaidian Sluice). The results of long-term regulations show that the WQ at downstream sites of most DSs was worse than their corresponding upstream sites. The similarity of WE indicators at upstream and downstream sites of DSs was low, meaning the great impact of DS long-term regulation on WE. The results of short-term regulations show that small flow magnitude (20 m3/s in this study) helps release and reduce upstream pollution of DSs as it does not disturb downstream riverbed pollutions, resulting in an increase of downstream river pollution. Further increasing flow magnitude (40 m3/s in this study) helps reduce and release upstream pollution, but increases river pollution significantly at downstream due to the release of pollutions in the sediments of downstream riverbed, which may cause a great river ecological impact since pollutants cannot be diluted and degraded in time. A flow magnitude about 60 m3/s can help release and rapidly dilute and degrade sediment pollutions in downstream riverbed. However, a larger flow magnitude may also be inappropriate due to the strong downstream riverbed erosion except for special regulation requirements. This study can provide more insights for water resources management to develop long-term ecological DS regulation schemes for in HRB and other similar DS dominated basins.