The Yellow River Basin is a typical arid and semi-arid area, which is very sensitive to climate change. In recent years, it has become the area with the greatest shortage of water resources in China. In this study, a new two-way coupling model of land surface and hydrology has been explored to analyze the impacts of climate change and human activities on the runoff. It is of great theoretical and practical significance for making better management countermeasures and strategies to cope with climate change in the Yellow River Basin. The results showed that: (1) the annual average precipitation in the basin was 470.1 mm, which was higher in the lower reaches than in the middle and upper reaches. The annual average temperature is 5.8 °C. The entire basin showed a remarkable warming speed. The annual average pan evaporation is 1067.3 mm showing a downward trend throughout the basin; (2) from 1987 to 2009, the contribution rate of climate change to runoff change has not fluctuated by more than 5%. Since 2010, the precipitation caused by climate factors has increased runoff by 12~15%. The impact of land use change on runoff has been increasing annually. The influence of projects on runoff change was the leading factor of runoff reduction in the Yellow River Basin, with the contribution rate around 50%; and (3) for every 10% decrease in precipitation, the runoff decreases by 13~15.7%. When the temperature rises by 1.0 °C, the runoff decreases by 2.1~4.2%. The runoff in the upper reaches of the Yellow River was most sensitive to precipitation and temperature changes. This showed that the runoff in the plateau and mountainous areas were highly sensitive to climate change.

In the recent decades, the issue of water-resource security of the Daqing River Basin, which is one of the five major rivers in the Haihe River Basin, has become increasingly serious affected by climate change and human activities. In this paper, a dynamic simulation and early warning model of water quantity and quality in this basin based on the SWAT model was constructed to promote the implementation of water environment quality and safety bottom line in the Beijing-Tianjin-Hebei region. The results of the study are as follows: (1) When encountering a once-in-a-century rainstorm, the flood pressure of Zijingguan in the flood season is the highest, with the highest water level reaching 521.23 m, and the overall maximum runoff follows the order of Zijingguan > Fuping > Zhangfang. (2) When the NH₃-N emissions are reduced by 37.64~85.10% in each month (based on the level in 2017), the water quality at the outlet of the basin can reach the standard, and the upper limit of NH₃-N emissions is 504.5 t/m. (3) The regulation and control scheme seeking to “ensure the base flow with standard water quality” and “optimize NH₃-N annual emission” is proposed in this paper. The NH₃-N concentration at the outlet of all watersheds can reach the standard when the basic runoff of each sub-basin reaches 0.01 to 10.32 m³/s. In addition, concentrating the emission in July, August, and September and reducing the emission intensity of NH₃-N in proportion can significantly reduce the monthly average NH₃-N concentration (<1.99 mg/L) at the outlet section of the basin.

Assessing the impacts of land use and cover (LUCC) change and climate change on discharge and sediment load is beneficial for the regional management of water resources and the water environment. The Mann-Kendall test and soil and water assessment tool (SWAT) model were applied to analyze the change trends of meteorological and hydrological variables and to quantitatively assess the response of discharge and sediment load to LUCC and climate change, respectively, in the Hunhe River Basin (HRB). The results showed that LUCC changed little during 1980–2009, mainly from forestland (−0.36%) to other land use types, such as urban land (+0.13%) and grassland (+0.13%). Temperature increased significantly (p < 0.01), and precipitation showed a non-significant decreasing trend. Discharge showed a non-significant decreasing trend, and sediment load significantly (p < 0.05) decreased. The Nash–Sutcliffe efficiency (NSE), coefficient of determination (R²), and percent bias (PBIAS) during the calibration and validation periods indicated that good performance was achieved for the discharge simulation (NSE: 0.77–0.79; R²: 0.79–0.84; PBIAS: −9.61–1.48%) and satisfactory performance for the sediment load simulation (NSE: 0.65–0.77; R²: 0.65–0.78; PBIAS: −2.56–8.31%). The calibrated SWAT model was successfully utilized to assess the impacts of LUCC and climate change on discharge and sediment load, finding that the combined impact of LUCC and climate change decreased the annual discharge and sediment load by 22.65% and 31.51%, respectively. Minor changes in annual discharge (+0.08%) and sediment load (−1.33%) were caused by LUCC, and climate change led to a pronounced decrease in annual discharge (−22.69%) and sediment load (−30.61%). These findings indicate that climate change rather than LUCC dominated the hydrological alterations in the HRB. This study provides important information for decision makers to identify the reasons for changes in hydrological alterations and to design adaptive measures.

In recent years, the eutrophication in the typical tributaries of the Three Gorges Reservoir has attracted widespread attention. In the present paper, three typical eutrophic tributaries in the Three Gorges Reservoir area, that is, Xiaojiang River (Pengxi River), Daning River and Xiangxi River, were selected for research, and a two-dimensional hydrodynamic and water quality coupling model that is for the area from Zhutuo to dam site was constructed by using MIKE 21 software. On this basis, the temporal and spatial distribution of pollutants in three tributaries in 2021 was further simulated and analyzed. The results showed that: (1) The coupling model after calibration and validation had good simulation accuracy for hydrodynamic and water quality indexes in the reservoir area, and the simulation effect was in good agreement with the measured results; (2) The concentration of each pollutant was closely related to the change of water level and water quantity. From the highstand period to the pre-discharge stage (January-June), the concentrations of total nitrogen and total phosphorus increased, which were very high from the post-discharge period to the lowstand period (July-September); (3) The concentration of pollutants in the confluence of the main stream and tributaries was high due to the backward flow of the main stream. To sum up, the higher concentration of pollutants and the higher water temperature in spring and summer were conducive to the rapid reproduction of algae and other plankton, resulting in eutrophication, and the algal bloom was easy to break out in the reservoir area. During the water storage period and highstand period (October-December), the concentrations of total nitrogen and total phosphorus showed a downward trend, mainly due to the self-purification of water and the degradation of pollutants. The research conclusion can provide a reference for water quality evaluation, water supply safety and pollutant treatment in the Three Gorges Reservoir area.