The land surface is modelled as a grid of large (>1km), flat, uniform cells Sub-grid heterogeneity (e.g. elevation, land cover) is handled via statistical distributions. Inputs are time series of daily or sub-daily meteorological drivers (e.g. precipitation, air temperature, wind speed). Land-atmosphere fluxes, and the water and energy balances at the land surface, are simulated at a daily or sub-daily time step Water can only enter a grid cell via the atmosphere Non-channel flow between grid cells is ignored The portions of surface and subsurface runoff that reach the local channel network within a grid cell are assumed to be >> the portions that cross grid cell boundaries into neighboring cells Once water reaches the channel network, it is assumed to stay in the channel (it cannot flow back into the soil) This last point has several consequences for VIC model implementation:
Grid cells are simulated independently of each other Entire simulation is run for each grid cell separately, 1 grid cell at a time, rather than, for each time step, looping over all grid cells Meteorological input data for each grid cell (for the entire simulation period) are read from a file specific to that grid cell Time series of output variables for each grid cell (for the entire simulation period) are stored in files specific to that grid cell 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 and 1998)
global,
user_def.h File Meteorological Forcing Files Soil Parameter File Vegetation Library File Vegetation Parameter File (Optional) Initial State File (Optional) Elevation Band File (Optional) Lake/Wetland Parameter File
Meteorology (Inputs, Distributed Precip, and Snow/Elevation Bands) Frozen Soil (including Permafrost) Dynamic Lake/Wetland Model (new to 4.1.1) Flow Routing
geographic locations or configurations of land cover types are not considered; VIC lumps all patches of same cover type into 1 tile 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 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
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).
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