Research On Regional-scale Integrated Modeling Of Unsaturated-saturated Water Flow And Solute Transport And Its Application

The research on regional-scale saturated-unsaturated water flow and solute transport is a long-lasting challenge in subsurface groundwater resources and environment.One of the most efficient methods is the numerical modeling technique.The soil water is the major recharge source and consumption place for groundwater system,and it is also the major source and buffer zone for groundwater pollution.Therefore,the soil water and groundwater are often simulated together as a whole.The Richards' equation is the fundamental governing equation that describes unsaturated water flow.But the intensively input parameters and extensive calculation consumption leads to a challenge when solving the regional-scale problems with Richards' equation.In fact,the water budget models are the most widely implemented methods of regionalscale problems.However,some water budget models are too simple and fail to obtain the correct results.It is urgent to make a complete summary and improvement of the water budget models.Besiders,there are some difficulties for practical applications of present saturated-unsaturated models,such as difficulty of convergence,mass balance error,failure of coupled with GIS system.What's more,it is seldom in the study on long-term regional scale unsaturated-saturated water flow and salt transport in agricultural area.As a result,it is hard to recognize the salt accumulation process in a holistic perspective.This work reviewes the progress in modeling saturated-unsaturated water flow and solute transport theories,modeling methods and practical application.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 objective of our work is to establish a regional scale integrated model for saturated-unsaturated water flow and solute transport,and the purpose of our work is to help address the issue of soil water and salt transport problem in large agricultural irrigation area.The balance models which describe soil water flow and salt transport are developed,as well as the regional scale saturated-unsaturated water flow and solute transport model.An integrated scheme is proposed for the practicability and expansibility of the model.Furthermore,the model and loosely coupled SaltMod have been used to study the soil water flow and salt transport rules in Hetao Irrigation District,China.The detailed studies are presented as follows.Firstly,the soil water balance model UBMOD has been developed based on hybrid numerical and statistical methods.The model considers three terms that impact soil water movement,including the gravitational potential,the external forces and the matric potential.The model can deal with complicated boundary conditons in heterogeneous situations,and a simple crop model has been coupled with UBMOD.The efficiency and effective of the model have been demonstrated.Besides,the model only needs four physical parameters and can keep mass balance well,the upward flux neglected by other water budget models can also be simulated in UBMOD,which makes the model particularly suitable for large-scale simulation.Secondly,another model called UBMOD?S has been developed based on UBMOD.The mobile-immobile hypothesis has been adopted in the model for describing the physical nonequilibrium solute transport phenomenon.The model inherits the merits of UBMOD,which makes the model keeps mass balance well and can be used with coarse discretization in space and time with perfect results.The model is suitable for large-scale simulation as well.Thirdly,we develop a coupling scheme to couple the soil water balance model UBMOD with MODFLOW iteratively based on mass conservation law.The integrated model CoupMod coupled with GIS is proposed,too.CoupMod wraps numbers of data processing tools and numerical calculation models,which improves the practicability and expansibility.The strength and weakness of the regional scale saturated-unsaturated water flow coupling scheme are evaluated.The scheme has satisfactory computational accuracy and mass balance error,which indicate the model is a convenient and effective tool for regional scale water movement problems.Moreover,three Quasi-3D saturated-unsaturated solute transport coupled schemes based on mass conservation rules have been proposed.The second coupling scheme which has a trade off between computational accuracy and efficiency has been described in detail and has been encoded.The strength and weakness of the CoupMod are evaluatedFurthermore,we study the soil water flow and salt transport rules in Hetao Irrigation District,China with CoupMod.The data in year 2007 has been used to calibrate the model.Then we use the model to forecast the groundwater and salt dynamic under the situation with limited water diversion.The results indicate that with the weaken of salt leaching,the soil salt will accumulate in farm land.Lastly,the well-canal conjunctive irrigation can supply the irrigation water and control soil salinization,which can be used as one of the effective measures under the limitation of water diversion.In this study,the mass balance model,SaltMod,is loosely coupled for simulating groundwater and salt dynamics under well-canal conjunctive irrigation in Hetao Irrigation District.The 15 years observation data in Long Sheng wellcanal irrigated area has been used to calibrate and validate the model.Then the model is used to predict groundwater level and soil salinity for the next 100 years of Long Sheng and the whole district.The results indicate that,the soil salinization in root zone of farmland can be effective controlled under well-canal conjuctive irrigation with the mineralization of groundwater is less than 2 g/L,and the plentitude irrigation using the diversion water is been implemented twice a year in awtumn.