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)

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

Snow Model Meteorology (Inputs, Distributed Precip, and Snow/Elevation Bands) Frozen Soil (including Permafrost) Dynamic Lake/Wetland Model (new to 4.1.1) Flow Routing

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

Liang, X., 1994: A Two-Layer Variable Infiltration Capacity Land Surface Representation for General Circulation Models, Water Resour. Series, TR140, 208 pp., Univ. of Washington, Seattle.

Liang, X., D. P. Lettenmaier, E. F. Wood, 1996: One-dimensional Statistical Dynamic Representation of Subgrid Spatial Variability of Precipitation in the Two-Layer Variable Infiltration Capacity Model, J. Geophys. Res., 101(D16) 21,403-21,422.

Liang, X., E. F. Wood, and D. P. Lettenmaier, 1996: Surface soil moisture parameterization of the VIC-2L model: Evaluation and modifications, Global and Planetary Change, 13, 195-206.

Liang, X., E. F. Wood, and D. P. Lettenmaier, 1999: Modeling ground heat flux in land surface parameterization schemes, J. Geophys. Res., 104(D8), 9581-9600.

Cherkauer, K. A. and D. P. Lettenmaier, 1999: Hydrologic effects of frozen soils in the upper Mississippi River basin, J. Geophys. Res., 104(D16), 19,599-19,610.

Liang, X., and Z. Xie, 2001: A new surface runoff parameterization with subgrid-scale soil heterogeneity for land surface models, Advances in Water Resources, 24(9-10), 1173-1193.

Cherkauer, K. A., L. C. Bowling, and D. P. Lettenmaier, 2003: Variable infiltration capacity cold land process model updates, Global and Planetary Change, 38, 151-159, 2003.

Bowling, L. C., J. W. Pomeroy and D. P. Lettenmaier, 2004: Parameterization of blowing snow sublimation in a macroscale hydrology model, J. Hydromet., 5(5), 745-762, doi: 10.1175/1525-7541.

Bowling, L.C. and D.P. Lettenmaier, 2008: Modeling the effects of lakes and wetlands on the water balance of Arctic environments, J. Hydromet. , (accepted).

Lohmann, D., R. Nolte-Holube, and E. Raschke, 1996: A large-scale horizontal routing model to be coupled to land surface parametrization schemes, Tellus, 48(A), 708-721.

Nijssen, B., Lettenmaier, D.P., Liang, X., Wetzel, W. and Wood, E.F., 1997: Stream simulation for continental-scale river basins, Water Resources Research, 33(4), pp711-724.

Lohmann, D., E. Raschke, B. Nijssen and D. P. Lettenmaier, 1998: Regional scale hydrology: I. Formulation of the VIC-2L model coupled to a routing model, Hydrol. Sci. J., 43(1), 131-141.

Lohmann, D., E. Raschke, B. Nijssen, and D. P. Lettenmaier, 1998: Regional Scale Hydrology: II. Application of the VIC-2L Model to the Weser River, Germany, Hydrological Sciences Journal, 43(1), 143-157.

Abdulla, F.A., D.P. Lettenmaier, E.F. Wood and J.A. Smith, 1996: Application of a macroscale hydrologic model to estimate the water balance of the Arkansas-Red river basin, J. Geophys. Res., 101(D3), 7449-7459.

Wood, E.F., D. Lettenmaier, X. Liang, B. Nijssen and S.W. Wetzel, 1997: Hydrological modeling of continental-scale basins Annu. Rev. Earth Planet. Sci., 25, 279-300.

Nijssen, B.N., D.P. Lettenmaier, X. Liang, S.W. Wetzel, and E.F. Wood, 1997: Streamflow simulation for continental-scale river basins, Water Resour. Res., 33(4), 711-724.

Liang, X., E. F. Wood, D. Lohmann, D.P. Lettenmaier, and others, 1998: The Project for Intercomparison of Land-surface Parameterization Schemes (PILPS) Phase-2c Red-Arkansas River Basin Experiment: 2. Spatial and Temporal Analysis of Energy Fluxes, J. Global and Planetary Change, 19, 137-159.

Lohmann, D. E. Raschke, B. Nijssen and D.P. Lettenmaier, 1998: Regional scale hydrology: II. Application of the VIC-2L model to the Weser river, Germany, Hydrol. Sci. J., 43(1), 143-158.

Hamlet, A.F. and D.P. Lettenmaier, 1999: Effects of Climate Change on Hydrology and Water Resources in the Columbia River Basin, Am. Water Res. Assoc., 35(6), 1597-1623.

Hamlet, A.F. and D.P. Lettenmaier, 1999: Columbia River Streamflow Forecasting Based on ENSO and PDO Climate Signals, ASCE J. of Water Res. Planning and Mgmt., 125(6), 333-341.

Leung, L.R., A.F. Hamlet, D.P. Lettenmaier and A. Kumar, 1999: Simulations of the ENSO Hydroclimate Signals in the Pacific Northwest Columbia River Basin, Bull. Am. Met. Soc., 80(11), 2313-2329.

Mattheussen, B., R.L. Kirschbaum, I.A. Goodman, G.M. O'Donnell and D.P. Lettenmaier, 2000: Effects of land cover change on streamflow in the interior Columbia river basin (USA and Canada), Hydrol. Process., 14, 867-885.

Hamlet, A.F. and D.P. Lettenmaier, 2000: Long-Range Climate Forecasting and its Use for Water Management in the Pacific Northwest Region of North America,J. Hydroinformatics, 02.3, 163-182.

Maurer, E.P., G.M. O'Donnell, D.P. Lettenmaier, and J.O. Roads, 2001: Evaluation of the Land Surface Water Budget in NCEP/NCAR and NCEP/DOE Reanalyses using an Off-line Hydrologic Model. J. Geophys. Res., 106(D16), 17,841-17,862.

Maurer, E.P., G.M. O'Donnell, D.P. Lettenmaier, and J.O. Roads, 2001: Evaluation of NCEP/NCAR Reanalysis Water and Energy Budgets using Macroscale Hydrologic Simulations, In: Land Surface Hydrology, Meteorology, and Climate: Observations and Modeling, AGU series in Water Science and Applications, V. Lakshmi, J. Albertson, and J. Schaake eds.,pp. 137-158.

Nijssen, B.N., G.M. O'Donnell, D.P. Lettenmaier and E.F. Wood, 2001: Predicting the discharge of global rivers, J. Clim., 14, 3307-3323.

Nijssen, B.N., R. Schnur and D.P. Lettenmaier, 2001: Global retrospective estimation of soil moisture using the VIC land surface model, 1980-1993, J. Clim., 14, 1790-1808.