Land Cover

• can subdivide each grid cell's land cover into arbitrary number of "tiles", each corresponding to the fraction of the cell covered by that particular land cover (e.g. coniferous evergreen forest, grassland, etc.)
• geographic locations or configurations of land cover types are not considered; VIC lumps all patches of same cover type into 1 tile
• versions 4.1.0 and later include a lake/wetland cover type
• fluxes and storages from the tiles are averaged together (weighted by area fraction) to give grid-cell average for writing to output files
• for a given tile, jarvis-style veg stomatal response used in computing transpiration
• 4.1.0 and later can consider canopy energy balance separately from ground surface

Soil

• arbitrary number of soil layers, but typically 3
• infiltration into the top-most layers controlled by variable infiltration capacity (VIC) parameterization
• top-most layers can lose moisture to evapotranspiration
• gravity-driven flow from upper layers to lower layers
• ARNO baseflow formulation for drainage from bottom layer
• 4.1.0 and later can simulate spatially-distributed (laterally) soil freezing
• 4.1.0 and later can simulate frozen soil and permafrost processes such as melting of excess ground ice

Snow Model

VIC considers snow in several forms: ground snow pack, snow in the vegetation canopy, and snow on top of lake ice. Main features:

• Ground snow pack is quasi 2-layer; the topmost portion of the pack is considered separately for solving energy balance at pack surface
• 4.1.0 and later can consider spatially-distributed (laterally) snow coverage
• 4.1.0 and later can consider blowing snow sublimation

For more information about the snow pack formulation, click here.

Meteorological Input Data

• Can use sub-daily met data (prcp, tair, wind) at intervals matching simulation time step
• Can use daily met data (prcp, tmax, tmin, wind) for daily or sub-daily simulations
• Disaggregates daily met data to sub-daily via Thornton & Running algorithm and others (computes incoming sw and lw rad, pressure, density, vp)

Distributed Precipitation

VIC can consider spatial heterogeneity in precipitation, arising from either storm fronts/local convection or topographic heterogeneity. Here we consider the influence of storm fronts and local convective activity. This functionality is controlled by the DIST_PRCP option in the global parameter file. Main features:

• Can subdivide the grid cell into a time-varying wet fraction (where precipitation falls) and dry fraction (where no precipitation falls).
• The wet fraction depends on the intensity of the precipitation; the user can control this function.
• Fluxes and storages from the wet and dry fractions are averaged together (weighted by area fraction) to give grid-cell average for writing to output files.

For more information about the distributed precipitation formulation, click here.

Elevation Bands

VIC can consider spatial heterogeneity in precipitation, arising from either storm fronts/local convection or topographic heterogeneity. Here we consider the influence of topography, via elevation bands. This is primarily used to produce more accurate estimates of mountain snow pack. This functionality is controlled by the SNOW_BAND option in the global parameter file. Main features:

• Can subdivide the grid cell into arbitrary number of elevation bands, to account for variation of topography within cell
• Within each band, meteorologic forcings are lapsed from grid cell average elevation to band's elevation
• Geographic locations or configurations of elevation bands are not considered; VIC lumps all areas of same elevation range into 1 band
• Fluxes and storages from the bands are averaged together (weighted by area fraction) to give grid-cell average for writing to output files
• However, the band-specific values of some variables can be written separately in the output files

For more information about the snow/elevation band formulation, click here.

Soil Thermal Solution

VIC can use either the approximate soil temperature profile of Liang et al. (1999) or a finite difference solution that takes soil ice content into account, vis a vis Cherkauer et al. (1999).

• Liang et al. (1999): set QUICK_FLUX to TRUE in global parameter file; this is the default for FULL_ENERGY = TRUE and FROZEN_SOIL = FALSE.
• Cherkauer et al. (1999): set QUICK_FLUX to FALSE in global parameter file; this is the default for FROZEN_SOIL = TRUE.
  • By default, the finite difference formulation is an explicit method.
  • By default, the nodes of the finite difference formulation are spaced linearly.

For more information about the frozen soil formulation, click here.

Permafrost Enhancements (new in 4.1.1)

These apply to the case QUICK_FLUX = FALSE and FROZEN_SOIL = TRUE, i.e. the formulation of Cherkauer et al. (1999).

• New global parameter file option: IMPLICIT: allows for implicit solution
• New global parameter file option: EXP_TRANS: allows for exponential node spacing

Excess Ice and Subsidence Model (new to 4.1.1)

• Excess ice is the concentration of ice in excess of what the soil can hold were it unfrozen
• Models the melting of excess ice in a soil layer that causes the ground to subside

Temperature Heterogeneity: "Spatial Frost"

• Linear (uniform) distribution of soil temperature around a mean
• Allows some moisture movement in soiil when the average temperature is below freezing

Dynamic Lake/Wetland Model (new to 4.1.1)

• Multi-layer lake model of Hostetler et al. 2000
• Energy balance model
• Mixing, radiation attenuation, variable ice cover
• Dynamic lake area (taken from topography) allows seasonal inundation of adjacent wetlands
• Currently not a part of channel network

River Routing Model

• Routing of stream flow is performed separately from the land surface simulation, using a separate model, typically the routing model of Lohmann, et al. (1996; 1998)
• Each grid cell is represented by a node in the channel network
• The total runoff and baseflow from each grid cell is first convolved with a unit hydrograph representing the distribution of travel times of water from its points of origin to the channel network
• Then, each grid cell's input into the channel network is routed through the channel using linearized St. Venant's equations