| It is a long-lasting challenge in subsurface hydrologic modeling to develop numerically efficient algorithm for coupling unsaturated and saturated flow and contaminant transport, especially in regional-scale modeling. Fully three-dimensional (3-D) modeling approach is considered as the most accurate method to study the water flow and solute transport in the subsurface system. However, the3-D unsaturated water flow equation is highly nonlinear and very fine elements are required to obtain the accurate numerical results. The computational cost is heavy and is a great hamper for the application of the fully3-D numerical models. Accurate simulation of nitrogen transport in the subsurface system is more difficult due to its complex transformation processes and would consume more computational time. So it is necessary and important to improve the simulation efficiency for the unsaturated-saturated water flow and contaminant transport modeling in the regional subsurface system. This work reviews the progress in unsaturated-saturated water flow and contaminant transport theories and modeling methods. And the research progress in nitrogen transformation and transport are also included. The numerical approaches and module functions as well as the advantages and disadvantages of the most popular hydrological models are introduced in details.The major purpose of our work is to improve the simulation efficiency of regional unsaturated-saturated water flow and contaminant transport modeling. A simplified3-D unsaturated-saturated water flow model has been established. And a simplified3-D saturated solute transport model has been proposed based on the water flow model. Then the coupling methods of integrating the unsaturated one-dimensional (1-D) water flow module and the3-D groundwater module have been introduced in details. The fully coupling unsaturated-saturated solute transport model and the fully unsaturated-saturated nitrogen transformation and transport model are established with the similar integration method as the water flow model. All these models have been well-tested by comparing the simulation results with the popular models or softwares, such as Visual Modflow, Feflow, SWMS2D, Hydrus1D and HydroGeoSphere, and also with the analytical solutions and measured data. The limitations of the model are discussed as well. Moreover, the applicability of the unsaturated-saturated water flow and solute transport models to the simulation of the groundwater and salt transport in the regional irrigation district is presented. Two field experiments are run to test the applicability of the nitrogen model. The detailed studies are presented as follows.Firstly, the average head gradient plane of the irregular triangle prism is proposed to solve the simplified numerical simulation problem of water flow in irregular subsurface system. The items of water balance in the triangle prism are calculated based on the average head gradient plane. The vertical flow flux between upper and lower layers is computed by the finite difference method. The modeling approach is verified and the computational efficiency is evaluated in several hypothetical examples by comparing the simulation results with those of widely used softwares, including HydruslD, SWMS2D, Visual Modflow and Feflow.Secondly, the simplified3-D saturated solute transport model is established based on the hybrid finite element method and the finite difference method. The net lateral solute mass to the control volume of the node is computed by the finite element method and the vertical solute mass is calculated by the finite difference method. Furthermore, upwind finite element scheme and upwind finite difference method are used respectively in the horizontal and vertical directions, which can minimize the numerical oscillation. The model is applied to the simulation of solute transport in different hypothetic conditions. Good agreements are shown from the comparison between the results of the proposed model and the analytic solutions.Thirdly, two schemes are developed for coupling numerical simulation of unsaturated-saturated water flow at the regional scale. The modeling domain is divided into sub-areas in horizon according to spatially distributed inputs, flow characteristics, and topography conditions. The unsaturated zone of each sub-area is represented by individual1-D soil column. The3-D saturated water flow equation is employed to formulate the3-D groundwater model. The unsaturated and saturated zones are implicitly or explicitly coupled in space and time through the vertical flow. Both two coupling schemes are verified and the computational efficiencies are evaluated in several hypothetical examples by comparing the simulation results with those of widely used softwares, including HydruslD, SWMS2D, FEFLOW and HydroGeoSphere. Moreover, due to the assumptions involved in the model development, the intrinsic limitations of the implicit coupling method are discussed.(Reference:Zhu, Yan, Shi, Liangsheng, Lin, Lin, Yang, Jinzhong, Ye, Ming. A Fully Coupled Numerical Modeling for Regional Unsaturated-Saturated Water Flow [J]. Journal of Hydrology.2012,475:188-203.)Moreover, an efficient integrated modeling approach is developed to simulate the contaminant transport in the subsurface system. The unsaturated zone is divided into a number of horizontal sub-areas. Solute migration through the unsaturated zone of each sub-area is assumed to be vertical and can be represented by the1-D advection-dispersion equation (ADE), which is then coupled to the3-D ADE representing the subsequent groundwater transport. The finite element method is adopted to discretize the vertical solute equation, while the hybrid finite element method and finite difference method is used to discretize the3-D saturated solute transport equation. The unsaturated and saturated solute transport equations are combined into a unified matrix by the mass balance analysis for the adjacent nodes located at the1-D soil column and at the water table. Two hypothetical cases and one field case are simulated to test the validity of the model with the results compared with those from HYDRUS-1D, SWMS2D and the measured data. The model limitations are analyzed as well. It shows that the model will lose accuracy when the lateral dispersion effect is dominant in the unsaturated zone.(Reference:Zhu, Yan, Shi, Liangsheng, Yang, Jinzhong, Wu, JingWei, Mao, DeQiang. Coupling methodology and application of a fully integrated model for contaminant transport in the subsurface system [J]. Journal of Hydrology.2013,501:56-72.)And then, an integrated nitrogen transformation and transport model has been established based on the fully coupled unsaturated-saturated water flow and solute transport model in this paper. Considering the different nitrogen transformation characteristics in the vadose zone and in the saturated zone, the model specifies the nitrogen transformation processes in these two zones separately. Nitrogen transformation rates are combined to the unsaturated and saturated nitrogen transport equation as source items. Both the transport equations of ammonium nitrogen (N) and nitrate N in the subsurface system are calculated in the model. The simulation results of the ammonium N distribution are utilized to the simulation of nitrate N. The model is tested by comparing the simulation results with a two-dimensional (2-D) nitrogen model NITROGEN2D. Since there are many parameters used in the nitrogen modeling, the sensitivity analysis of17major parameters is performed.Furthermore, the fully coupled unsaturated-saturated water flow model is applied to the prediction of the annual groundwater flow and the fully coupled water flow and solute transport model is adopted to the simulation of salt concentration in Yonglian Irrigation District. In the real-world applications, the numerical results show that the models are efficient to simulate the water flow and solute transport in regional-scale areas with small computational costs even with complex boundary conditions and variably topographies. The fully coupled nitrogen transformation and transport model is applied to two experimental field sites irrigated by treated sewage water to predict the water flow and the nitrate and ammonium nitrogen (N) distribution in the subsurface system. The model performs well in predicting the water flow and is effective to capture the temporal change of nitrate N and ammonium N in the soil or in the drainage water. It is concluded that the model has great flexibility to be applied to water flow and nitrogen transport simulation with complex boundary conditions, and can be used for further evaluation of treated sewage water irrigation impacts on subsurface water quality in the field sites.Lastly, the major research work and contributions are summarized, and the issues that need further investigation are presented. We also provide the views of the possible extensions of this study. |
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