State Water Resources Research Institute Program

Project ID: 2009CA256B
Title: Development and Application of the Coupled Vadose Zone-Ground Water Flow Modeling Environment
Project Type: Research
Start Date: 3/01/2009
End Date: 2/28/2010
Congressional District: 43
Focus Categories: Groundwater, Water Use, Models
Keywords: Groundwater, Modeling
Principal Investigator: Simunek, Jirka (University of California)
Federal Funds: $ 15,514
Non-Federal Matching Funds: $ 40,828
Abstract: Problem Statement: Water flow through the variably-saturated (vadose) zone is an important part of the hydrologic cycle because it influences partitioning of water among various flow components. Depending upon hydrological, geological and soil characteristics, rain and snowmelt is partitioned at the land surface into runoff, infiltration, evapo-transpiration (ET), groundwater recharge, and vadose zone storage. Water flow in the vadose zone especially affects the transfer rates between the land surface and the groundwater table, which are two key hydrological boundaries. Evaluation of almost any hydrological process therefore requires that water flow through the vadose zone is appropriately taken into account. However, modeling of vadose zone flow processes is a complex and computationally demanding task that is often handicapped by the lack of data necessary to characterize the hydraulic properties of the subsurface environment. Consequently, vadose zone flow processes have rarely been properly represented in hydrological models. For example, models that simulate surface and near surface hydrology usually oversimplify the impact of vadose zone flow processes and rarely consider three-dimensional regional groundwater flow. Similarly, regional-scale groundwater models often simplify vadose zone flow processes by calculating groundwater recharge externally without proper consideration of changes in groundwater levels. To overcome this frequent simplification, there is an urgent need for methods that can effectively simulate water flow through the vadose zone in large scale hydrological models. This issue is especially important for groundwater models.

To overcome this problem, we have developed a new one-dimensional unsaturated flow package [Seo et al., 2007] for the three- dimensional modular finite-difference ground water model MODFLOW-2000 [Harbaugh et al., 2000]. MODFLOW was developed by the U.S. Geological Survey and is one of the most widely used groundwater flow models. The HYDRUS Package uses the computer program HYDRUS [Simùnek et al., 2005] to simulate water movement in variably-saturated porous media by numerically solving (using a finite-element method) the Richards equation. The HYDRUS package considers the effects of infiltration, soil moisture storage, evaporation, plant water uptake, precipitation, runoff, and water accumulation at the ground surface. Being fully incorporated into the MODFLOW program, the HYDRUS package provides MODFLOW with recharge fluxes at the water table, while MODFLOW provides HYDRUS with the position of the groundwater table that is used as the bottom boundary condition in the package. The HYDRUS package provides an optimal trade-off between computational effort and accuracy of model simulations for coupled vadose zone - groundwater problems. Being based on two most widely used models for simulating vadose zone flow (HYDRUS) and ground water flow (MODFLOW), the coupled software package has a tremendous potential to become widely used in both research and management, and to redefine entirely how the complex subsurface flow problems are evaluated.

Proposed Research: The vadose zone (HYDRUS-1D) and ground water (MODFLOW) models, and especially their coupled version (the HYDRUS package for MODFLOW), represent major advancements in their 3 respective fields of science. However, to fully realize the potential of this coupled package, several additional steps need to be carried out:

  1. The package needs to be extended so that it can simulate also solute (nutrients, pollutants) transport through the vadose zone and groundwater.
  2. While individual models (HYDRUS-1D and MODFLOW) are both supported by sophisticated graphical user interfaces (GUI), no such GUI exists for the coupled package. Such GUI needs to be developed so that the coupled package is adopted not only by the research community, but also by the practicing hydrologists and environmental consultants. This GUI should stay in the public domain to facilitate quick adoption of the coupled package.
  3. Test examples and tutorials should be developed demonstrating applicability and use of the coupled package.
  4. A short course should be developed and then regularly provided to the research community, as well as to practicing hydrologists and environmental consultants, to provide training in a proper use of the software package and about its possible applications.

Expected Results: There is a wide range of potential applications to which the coupled model (the HYDRUS package for MODFLOW) can be applied, these may include:

It is impossible to predict at present all potential applications to which the coupled model can be used and all benefits that can follow from its use. When we first released HYDRUS models about a decade ago, we could hardly imagine the wide spread of these models and the diversity of applications. It can be expected, partly because both original models are widely used and represent a state-of-the-art in their respective fields, that the coupled model will be similarly quickly adopted by the public if the four tasks listed above are carried out.

Progress/Completion Report, 2009, PDF

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