The Simulations: Setup, Parameters, Initial Conditions and Calibration
The Variable Infiltration Capacity (VIC) land surface model (Liang et al., 1994, 1996; Cherkauer et al., 2002) was used to generate spatially and temporally consistent fields of soil moisture and other water budget flux and state variables. The VIC model simulates the terrestrial water and energy balances and distinguishes itself from other land surface schemes through the representation of sub-grid variability in soil storage capacity as a spatial probability distribution, to which surface runoff is related (Zhao et al., 1980), and by modeling base flow from a lower soil moisture zone as a nonlinear recession (Dumenil and Todini, 1992). The VIC model has been applied extensively at regional (Abdulla et al., 1996; Maurer et al., 2002) and global scales (Nijssen et al., 2001; Sheffield et al., 2004b), including snow and ice dominated regions (Bowling et al., 2003; Su et al., 2006).
Horizontally, VIC represents the land surface by a number of tiled land cover classes. The land cover (vegetation) classes are specified by the fraction of the grid cell which they occupy, with their leaf area index, canopy resistance, and relative fraction of roots in each of the soil layers. Evapotranspiration is calculated using a Penman-Monteith formulation with adjustments to canopy conductance to account for environmental factors. The subsurface is discretized into multiple soil layers. Movement of moisture between the soil layers is modeled as gravity drainage, with the unsaturated hydraulic conductivity a function of the degree of saturation of the soil. Cold land processes in the form of canopy and ground snow pack storage, seasonally and permanently frozen soils and sub-grid distribution of snow based on elevation banding are represented in the model. Seasonally and permanently frozen soils are represented in the VIC model according to the algorithm of Cherkauer and Lettenmaier (1999). Soil temperatures are calculated using a finite difference solution of the heat diffusion equation for a user-specified number of nodes that are independent of the soil moisture layers. Soil ice content is estimated based on node temperatures and infiltration and baseflow are restricted based on the reduced liquid soil moisture capacity.
For this study, the VIC model was run globally at 1.0 degree spatial resolution and 3-hourly time step, for the period 1950-2000.
The values of soil and vegetation parameters and their spatial distribution were specified following Nijssen et al. (2001). Soil textural information and bulk densities were derived by combining the 5-min Food and Agricultural OrganizationUnited Nations Educational, Scientific, and Cultural Organization (FAOUNESCO) digital soil map of the world (FAO 1995) with the World Inventory of Soil Emission Potentials (WISE) pedon database (Batjes 1995). The remaining soil characteristics, such as porosity, saturated hydraulic conductivity, and the exponent for the unsaturated hydraulic conductivity equation were based on Cosby et al. (1984).
Three soil layers were used in the simulation with the top layer being 30 cm thick. The second layer, the main storage layer, was between 0.5 to 1.5 m and the lower layer, which provides moisture for subsurface runoff, was between 0.1m and 0.25m. These two layers are adjusted during the calibration process to result in routed streamflow that satisfactorily match observations at the large basin scale.
Vegetation types were taken from the Advanced Very High Resolution Radiometer (AVHRR)-based, 1-km, global land classification of Hansen et al. (2000). Vegetation parameters such as height, and minimum stomatal resistance were assigned to each vegetation class based on a variety of sources described in Nijssen et al. (2001). Leaf area index values were based on Myneni et al. (1997) and were allowed to vary by month, but were kept constant from year to year.
Model Initial Conditions and Spinup
For each simulation, initial conditions for land surface states (soil moisture, snow, surface temperature) were created by forcing the model with a 47-year (1901-1947) spinup meteorological dataset. This forcing dataset was generated based on the CRU monthly precipitation and temperature record for 1901-47. These monthly values were used to bias correct years of sub-daily data chosen at random from the 1948-2000 period. This results in values that match the observed monthly precipitation and temperature record for 1901-47 but with daily and sub-daily variability that is statistically consistent with the period 1948-2000. This spinup period allows states, such as deep soil moisture, to wet up or dry down to reasonable values and then adjust to the climate at the end of the 1940s.
The model contains a number of parametrizations that require calibration. These are generally related to the variable infiltration capacity curve that VIC derives its name from and other parameters that control subsurface drainage and baseflow. A simple but efficient calibration scheme was implemented to calibrate the model against a climatology of gridded runoff estimates globally. Calibration was carried out over a sparse grid of pixels and the parameter values interpolated to the remaining pixels. The results were validated against streamflow for a number of large basins scattered across the globe.