Updating the Coupling Algorithm between HYDRUS and MODFLOW in the HYDRUS Package for MODFLOW

Abstract

The HYDRUS-based flow package for MODFLOW (the HPM or the HYDRUS package) is an existing unsaturated zone flow package for MODFLOW. In MODFLOW with the HPM, the groundwater modeling domain is discretized into regular grids that can be combined into multiple zones based on similarities in soil hydrology, topographical characteristics, and the depth to the groundwater. Each of these zones is assigned one unsaturated soil profile (the HPM profile). In this model, after every MODFLOW time step, the flux at the bottom of the HPM profile is given as an input recharge flux to MODFLOW. MODFLOW simulates groundwater flow, and the water table depth at the end of the MODFLOW time step is assigned as the bottom boundary condition in the HPM profile. The current coupling algorithm assumes that the groundwater table in the HPM profile remains constant throughout the entire MODFLOW time step. This results in unrealistic sudden inflow and/or outflow fluxes at the bottom of the HPM profile after every time step. The objective of this study was to develop a methodology to eliminate the error in the determination of the recharge flux at the bottom of the HPM profile. This was achieved by updating or modifying the pressure head profile in the HPM profile after every MODFLOW time step. The effectiveness and the applicability of the new coupling algorithm were evaluated using different case studies. The new coupling algorithm is effective in eliminating unrealistic sudden variations in the bottom flux in the HPM profiles.Abbreviations: 1D, one-dimensional; 3D, three-dimensional; HPM, HYDRUS Package for MODFLOW.In the past few decades, the coupling of one-dimensional (1D) unsaturated zone models with three-dimensional (3D) groundwater flow models to simulate various processes in and interactions between unsaturated and saturated soil zones has received a lot of attention. This is mainly because of the computational complexity and requirements of fully 3D variably saturated flow models when modeling larger domains with unsaturated soil zones (e.g., SHE model [Abbott et al., 1986], MODFLOW-SURFACT [HydroGeoLogic, 1996], FEFLOW [Diersch and Kolditz, 1998], TOUGH2 [Pruess et al., 1999] MODFLOW (Harbaugh et al., 2000) is a widely accepted 3D groundwater flow model. There have been many attempts to incorporate unsaturated zone flow models into MODFLOW (e.g., Havard et al., 1995;Facchi et al., 2004;Niswonger et al., 2006;Twarakavi et al., 2008;Lin et al., 2010;Zhu et al., 2011;Xu et al., 2012). The basic principle behind linking independent models for unsaturated and saturated soil zones is the exchange of information regarding the recharge flux from the unsaturated zone to the saturated zone and the elevation of the water table at appropriate time steps.The HYDRUS-based flow package for MODFLOW (further referred to as the HPM or the HYDRUS package) was developed by Seo et al. (2007) and Twarakavi et al. (2008) to simultaneously evaluate transient water flow in unsaturated and saturated zones. In this package, the subroutines from the comput...

Estimation of Conservative Contaminant Travel Time through Vadose Zone Based on Transient and Steady Flow Approaches

“…One advantage of numerical models of flow and transport in unsaturated zones is that both water and contaminant fluxes reaching the groundwater table can be estimated. Additionally, one dimensional model for unsaturated zone processes can be integrated with 3D models for flow and transport in the saturated zone, such as MODFLOW or MT3D, which is currently undertaken by the authors [18,38,39].…”

Investigating Atrazine Concentrations in the Zwischenscholle Aquifer Using MODFLOW with the HYDRUS-1D Package and MT3DMS

“…In MODFLOW with the HYDRUS-1D package [49,57] and MT3DMS [43], the one-dimensional HYDRUS-1D [46] model for water flow and solute transport modeling in the unsaturated zone is linked to MODFLOW [58], a three-dimensional model for water flow modeling of the saturated zone, and loosely coupled to MT3DMS (for solute transport modeling of the saturated zone) [53,54].…”

“…The time-averaged bottom flux from the HYDRUS-1D profile is given as the recharge flux to MODFLOW, which then simulates groundwater flow. The average water table depth at the end of the MODFLOW time step is assigned as the bottom pressure head boundary condition in the HYDRUS-1D profile for the next MODFLOW time step [49,53,54,57]. The solute flux reaching the water table is further used for solute transport modeling in the saturated soil zone using MT3DMS.…”

“…Simulation models that can analyze the movement of water and solute through both unsaturated and saturated soil zones are important for a contaminant transport analysis. There exist several numerical models that simulate these two zones independently [42][43][44][45][46][47] or simultaneously [48][49][50][51][52][53][54].…”

“…The MODFLOW model with the HYDRUS-1D package and MT3DMS [53,54] was chosen for this study mainly due to its capability of combined modeling of water flow and solute transport in the unsaturated and saturated soil zones and its well-adapted computational complexity. The intention of using this simulation model in this study was to incorporate the effect of the highly heterogeneous characteristics of the study domain, varying land use, and several other controlling factors (e.g., locations and rates of atrazine applications, precipitation, potential evapotranspiration, and changing water table elevations) on the transport of atrazine in the study area, followed by simulating the long-term behavior of atrazine in the aquifer.…”