Dong Mei Sun

This study employed a coupled water‐air two‐phase flow and salt water transport model to analyze the behaviors of generated airflow in unsaturated zones and the fluctuations of salinity at the salt–fresh water interface in a two‐layered unconfined aquifer with a sloping beach surface subjected to tidal oscillations. The simulation results show that as the new dynamic steady state including effects of tidal fluctuations is reached through multiple tidal cycles, the dispersion zone in the lower salt water wedge is broadened because fresh water/salt water therein flows continuously landward or seaward during tidal cycles. The upper salt–fresh water interface exhibits more vulnerable to the tidal fluctuations, and the variation of salinity therein is periodic, which is irrelevant to the hydraulic head but is influenced by the direction and velocity of surrounding water‐flow. With the tidal level fluctuating, airflow is mainly concentrated in the lower permeable layer due to the restraint of the upper semi‐permeable layer, and the time‐lag between the pore‐air pressure and the tidal level increases with distance from the coastline. The effect of airflow in unsaturated zones can be transmitted downward, causing both the magnitude of salinity and its amplitude in the upper salt–fresh water interface to be smaller for the case with airflow than without airflow due to the resistance of airflow to water‐flow. Sensitivity analysis reveal that distributions of airflow in unsaturated zones are affected by the permeability of the upper/lower layer and the van Genuchten parameter of the lower layer, not by the van Genuchten parameter of the upper layer, whereas the salinity fluctuations in the salt–fresh water interface are affected only by soil parameters of the lower layer.

To improve water quality in the Haihe River, 6 cycle routes and 16 water diversion schemes were simulated, and their influence on water quality was analyzed. First, a one-dimensional hydrodynamic and water quality model of the Haihe River was established. Then, according to an orthogonal experiment, 16 cycle schemes were proposed, and an orthogonal experimental table, including 3 factors and 4 levels, was used to optimize the 16 schemes. Average pollutant concentration from Sanchakou to the Erdao Sluice served as the index for orthogonal experiments. Index values were obtained from the model and the influence of three factors on chemical oxygen demand, total nitrogen, total phosphorus, and nitrogen ammonia was evaluated by range analysis. In addition, optimal schemes to reduce pollutants in each route were also obtained. After comparing the simulated results of the optimal schemes in each route, an optimal route was obtained. A combination of the one-dimensional hydrodynamic and water quality models with orthogonal experimental design was found to provide an efficient method to simulate and optimize the schemes, thereby improving water quality by water flow regulation in plain river network regions by taking into account multiple factors such as water cycle route, the operation of sluice, and the water diversion project.

In this study, a water‐air two‐phase flow model was employed to investigate the formation, extension, and dissipation of groundwater ridging induced by recharge events in a hypothetical hillslope‐riparian zone, considering interactions between the liquid and gas phases in soil voids. The simulation results show that, after a rain begins, the groundwater table near the stream is elevated instantaneously and significantly, thereby generating a pressure gradient driving water toward both the stream (the discharge of groundwater to the stream) and upslope (the extension of groundwater ridging into upslope). Meanwhile, the airflow upslope triggered by the advancing wetting front moves downward gradually. Therefore, the extension of groundwater ridging into upslope and the downward airflow interact within a certain region. After the rain stops, groundwater ridging near the stream declines quickly while the airflow in the lower part of upslope is still moving into the hillslope. Thus, the airflow upslope mitigates the dissipation of groundwater ridging. Additionally, the development of groundwater ridging under different conditions, including rain intensity, intrinsic permeability, capillary fringe height, and initial groundwater table, was analyzed. Changes in intrinsic permeability affect the magnitude of groundwater ridging near the stream, as well as the downward speed of airflow, thereby generating highly complex responses. The capillary fringe is not a controlling factor but an influence factor on the formation of groundwater ridging, which is mainly related to the antecedent moisture. It was demonstrated that groundwater ridging also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.

The Yuqiao Reservoir is an important drinking water source for the city of Tianjin. In recent years, the water quality of this reservoir has been seriously threatened by eutrophication resulting from increased nonpoint source pollution loads. In this study, we used a Soil and Water Assessment Tool (SWAT) model to calculate pollution loads (total nitrogen [TN] and total phosphorus [TP]) in the area surrounding the Yuqiao Reservoir. We then established a coupled two-dimensional hydrodynamic and water quality model that was used to simulate temporal and spatial distributions of TN and TP from June to September 2008. SWAT model results provided water quality boundary conditions for the coupled two-dimensional model. Results indicated that Liu Xiangying and Wu Baihu were the main TN inlets, and the average TN concentration in the reservoir was highest in September (3.74 mg/L). Meanwhile, the TP concentration in the eastern portion of the Yuqiao Reservoir was higher than that in the west, and the average TP concentration peaked in August (1.56 mg/L). The combination of the SWAT model and two-dimensional hydrodynamic and water quality model provides an effective method to simulate water pollution with consideration of various factors.