| Describe key physical parameters and equations
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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
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)
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.
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
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.
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
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