Water sources carry chemicals that can have a significant impact on the water environment of a river network, and understanding the contribution of different water sources to the river network can help to manage the pollution of the river network at its source. Hydrological connectivity of a river network affects the self-purification capacity and flood prevention capacity of the river. Thus an isotope tracer approach was applied to figure out the contribution rate of different water bodies to a river network and hydrological connectivity was quantified by introducing retention rate. Changzhou city was selected as the study area because it is an urbanized city with the characteristics of plain river network and it is faced with poor hydrological connectivity due to artificial constructions (dams and pumps) and human activity (urbanization). River water, well water (shallow groundwater), lake water and rainfall were collected during the flood season and nonflood season, and hydrogen and oxygen isotopes were determined. The temporal and spatial variations in hydrogen and oxygen isotopes in different water bodies and the state of the water cycle in different water bodies were analyzed. IsoSource and MixSIAR models were established to analyze the contribution rate of river network water sources in the study area, and their effectiveness was compared. Results of MixSIAR model were selected to evaluate the hydrological connectivity of the river network in the study area, providing a method to quantify the hydrological connectivity of specific river of the river network in Changzhou. This method could also be applied to other urban plain river network area to study its river connectivity.

The Yellow River Basin is of great significance to China’s economic and social development and ecological security. The Yellow River Basin is not only an important ecological barrier but also an important economic zone. In this paper, natural hydrological conditions were taken as a reference, a habitat simulation model of the key sections of the Yellow River was constructed based on the MIKE 21 model, and an ecological water requirement assessment method for river ecological integrity combined with habitat simulation and features of the hydrological reference group was established, which takes into account the survival and reproduction of indicator species. The suitable flow rates for the spawning period of Silurus lanzhouensis in Lanzhou and Xiaheyan and Cyprinus carpio in Toudaoguai, Longmen and Huayuankou were 350-720 m3/s, 350-600 m3/s, 150-500 m³/s, 260-400 m3/s, and 100-500 m³/s, respectively. Therefore, high pulse flow with a low flow peak should be guaranteed in mid- to late April. The peak flow should be at least approximately 1,000 m3/s to ensure that fish receive spawning signals, with a high pulse flow process occurring 1-2 times in May to June. The annual ecological water requirement of the Lanzhou, Xiaheyan, Toudaoguai, Longmen and Huayuankou sections was 9.1-11 ×109 m³, 6.3-10.4×109 m³, 3.8-8.2×109 m³, 4.7-11.3×109 m³ and 7.9-15.4×109 m³, respectively. The model quantitatively simulates the changes in ecological water requirement of indicator fishes in key sections of the Yellow River, and an effective and more realistic tool for ecological water requirement accounting of the Yellow River has been provided.