Spatial and temporal characteristics, influencing factors and prediction models of water quality and algae in early stage of Middle Route of South-North Water Diversion Project.

The Middle Route (MR) of the South-to-North Water Diversion Project (SNWDP) of China is one of the world's largest inter-basin water diversion projects. As an important source of drinking water in North China, its water quality safety determines the success or failure of a sizable water supply. At present, there is a lack of in-depth and systematic understanding of the interaction between hydrodynamics and the water environment as well as water ecological processes in the main canal at the early stages of operation. It is not currently possible to accurately predict water quality and algae status at the early stage of canal ecosystem succession. Change trends and distribution characteristics of the main water ecological environment elements in the main canal at the early MR stage are analyzed in this study. The main factors influencing algae are investigated by principal component analysis (PCA) to characterize the water quality and algae transport distribution in the main MR canal under the complex multi-sluice joint dispatching conditions. The relationship between environmental factors, hydrodynamic, water quality, and algae in the coupled canal-sluice system in the SNWDP MR is determined. Algae distributions under different water transmission conditions in a typical canal section are predicted accordingly. CODMn and algal density in the main canal are shown to increase from south to north along the canal. DO decreases from south to north; other water quality indexes do not significantly differ from north to south. Algal density along the canal differs to the greatest extent in summer, followed by spring and autumn, and is the weakest in winter. The predicted algae densities in the main canal under different water conveying conditions show that single sluice control and strong water flow from Taocha Head Section increase the flow velocity after passing through the sluice with a fixed opening. Algal density decreases flow rate increases under single sluice regulation conditions. The maximum rate of algal density change reaches 22.13% and 29.55% under double sluice and four sluice scheduling. Algae control effects grow significantly as the number of control sluices increases. The results of this work may provide technical support for water quality forecasting and algae risk warning in the SNWDP MR as well as a workable reference for similar projects.

Analysis of spatio-temporal variation in phytoplankton and its relationship with water quality parameters in the South-to-North Water Diversion Project of China.

The Middle Route of the South-to-North Water Diversion Project of China (MRSNWDPC), the longest trans-basin water diversion project in the world, has been in operation for over 6 years. The water quality of this mega hydro-project affects the safety of more than 60 million people and the health of an ecosystem over 160,000 km2. Abnormal algal proliferation can cause water quality deterioration, eutrophication, and hydro-project operation issues. However, few studies have investigated and reported planktonic algae and their relationship with the water quality of this trans-basin water diversion project. Here, spatio-temporal characteristics of algal cell density (ACD) and 11 water quality parameters, including water temperature (WT), pH, dissolved oxygen (DO), permanganate index (CODMn), 5-day biochemical oxygen demand (BOD5), fecal coliforms (F. coli), total phosphorus (TP), total nitrogen (TN), ammonia nitrogen (NH3-N), fluoride (F-), and sulfate (SO42-) in the MRSNWDPC from May 2015 to February 2019 were determined using multivariate statistical approaches. Consistent seasonal variation in ACD was observed each year, which grew in spring and then continuously decreased from summer to winter. Summer and winter are the seasons with the highest and lowest ACDs, with average values of 572.95 × 104 cell/L and 157.09 × 104 cell/L, respectively. The NH3-N was positively correlated with ACD growth in all seasons, with Pearson correlation coefficients ranging from 0.594 to 0.738 (P < 0.01). The results of the principal component analysis show that the sources affecting the water quality variation in this project are complex, and NH3-N was the most critical water quality parameter affecting ACD variation, which was linked to ACD in four seasons with strong positive loadings ranging from 0.754 to 0.882, followed by CODMn. The management department of the MRSNWDPC should focus on key periods of phytoplankton control in spring and summer; in addition, variation in the concentrations of NH3-N and CODMn merits special attention. This study provides a helpful reference for the water quality security and algae control strategy of the MRSNWDPC and similar projects in the world.