Last Updated July 17, 2017

Formulation

Computing the Surface Flux Boundary

The key to modeling the vadose zone is predicting an accurate surface boundary condition. VADOSE/W computes this surface flux boundary by coupling ground heat, mass and vapor flow with actual climate data.

VADOSE/W extends the concepts found in the popular SoilCover program into two dimensions. Critical to the formulation of VADOSE/W is its ability to predict actual evaporation as a function of the soil water stress state, rather than simply using soil water content, drying time, or empirical user-defined relationships. Instead, VADOSE/W uses the rigorous Penman-Wilson method to compute actual evaporation as a function of soil water pressure, a stress state variable. It is the only 2D product using this state-of-the-art approach.

Actual and Potential Evaporation

Actual Evaporation (AE) is only equal to Potential Evaporation (PE) when the soil is saturated. If the soil at the ground surface is not saturated, the AE rate can be much less than the PE rate. Wilson (1990, 1994) showed that the only way AE can be predicted correctly for all soil types and climatic conditions is to base the calculation on both the negative pore-water pressures and temperatures in the ground. Wilson modified the Penman (1948) method to make the actual evaporation rate dependent on the relative humidity of the soil and the air. The relative humidity in the soil can only be known if the soil temperature and water pressure are known and solved for simultaneously. To solve this complex set of equations, it is necessary to include vapor flow in the soil. VADOSE/W meets all these requirements, and is fully coupled in two dimensions.

Wilson, G. W., 1990. Soil Evaporative Fluxes for Geotechnical Engineering Problems. Ph.D. Thesis, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.

Wilson, G. W., Fredlund, D. G. & Barbour, S. L. 1994. Coupled soil-atmosphere modelling for soil evaporation. Canadian Geotechnical Journal, 31(2): 151-161.

Penman, H. L., 1948. Natural evapotranspiration from open water, bare soil and grass. Proc. R. Soc. London Ser. A. 193: 120-245.

Gas Transport

VADOSE/W is formulated to analyze transient 2-dimensional oxygen or radon gas diffusion, dissolution and decay in response to changing heat and moisture conditions in the ground. The gas transport analysis is carried out simultaneously with the coupled heat and mass transfer solution. This feature can be used to determine gas concentrations and mass flows into or out of the ground in response to pre-set or user input concentration boundary conditions.

Features

  • Generate soil cover meshes based on cover thickness and soil type data.
  • Model complex soil cover stratigraphy, including pinchout layers.
  • Use adaptive time stepping during the solve process to help with convergence and the diurnal nature of climate boundary data.
  • Estimate soil properties based on grain size data or other input soil functions.
  • Use a scalable global climate database or enter site specific climate data.
  • Specify net solar radiation or potential evaporation as your climate data, or let VADOSE/W estimate the energy component.
  • Import and export DXF™, WMF, EMF, or bitmap graphics.
  • Density-dependent analysis with contaminant density different than groundwater density.
  • And many more!

Comprehensive Results

Computed Parameters

When VADOSE/W analyzes an evaporative flux problem, it computes data regarding:

  • Precipitation and infiltration
  • Snow accumulation and melt
  • Plant transpiration
  • Ground freezing and thawing
  • Potential and actual evaporation
  • Surface seepage, runoff and ponding
  • Groundwater recharge

Infiltration and ponding resulting from high rainfall event

Specific computed parameters include:

  • Temperature
  • Total Head
  • Pressure
  • Pressure Head
  • Boundary Flux
  • Liquid Velocity
  • Vapor Velocity
  • Ice Content
  • Water Content
  • Vapor Pressure
  • Conductivity
  • Gas Concentration / Flux

Comparison of upslope and downslope cover base flow on a shallow sloped cover

Soil surface results data includes (for each time interval, or cumulative since Day 1):

  • Precipitation
  • Net Radiation
  • Potential Evaporation
  • Actual Evaporation
  • Runoff
  • Infiltration
  • Snow Depth
  • Actual Transpiration

Upslope saturation profiles during a 365 day simulation in a shallow sloped cover.

Water Balance data includes cumulative:

  • Precipitation
  • Runoff
  • Boundary Fluxes
  • Evaporation
  • Storage
  • Water Balance
  • Plant Transpiration

Cover layer interface results data includes:

  • Volume of liquid flow
  • Volume of vapor flow
  • Total volume across layer
  • Total gas mass across layer

Integration with Other Applications

Use VADOSE/W pore-water pressures in SLOPE/W

Using finite element computed pore-water pressures in SLOPE/W makes it possible to model the effects of evaporative flux on stability. For example, you can analyze changes in stability as the pore-water pressure changes over time due to the evaporative flux process. Also use VADOSE/W data in CTRAN/W contaminant transport analysis.

Engineering Methodology Book

The included VADOSE/W engineering methodology book discusses the whys and hows of modeling, as well as the theory and formulations behind the VADOSE/W product.Vadose Zone Modeling with VADOSE/W is a full-length book about proper modeling techniques: how to think before, during and after setting up and solving a model. The book includes chapters devoted to:
  • Numerical Modeling: What, Why and How
  • Geometry and Meshing
  • Material Properties
  • Boundary Conditions
  • Analysis Types
  • Functions in GeoStudio
  • Numerical Issues
  • Visualization of Results
  • Modeling Tips and Tricks
  • Illustrative Examples
  • Theory