Appendix 1: Hydrologic Modeling
The Variable Infiltration Capacity (VIC) Model was used to generate global energy and water
balance data. The VIC model is a macro scale hydrology model operated on a gridcell basis.
The model was run at a quarter degree spatial resolution for all drainage basins containing
potential glacier runoff contribution. Temporally the model was run from January 1, 1998 to
December 31, 2008. Daily output was produced but the model was run at a 3-hour computation
time step to aid in the determination of snow and energy balance variables.
Quarter-degree daily precipitation, minimum and maximum temperatures, and wind drive the
VIC model. Precipitation input data was sourced from the Tropical Rainfall Measurement
Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) in the 50º S – 50º N latitude
band (Huffman et al., 2007). Outside of this band, passive microwave-based daily precipitation
from the Global Precipitation Climatology Project (GPCP) one decimal degree product was
interpolated to quarter-degree using the inverse distance squared algorithm from Shepard (1984 –
SYMAP) (Huffman et al., 2001). Temperature and wind data were sourced from the NCEP
Reanalysis 2 dataset provided by the NOAA/OAR/ESRL/PSD, Boulder, Colorado, USA website
(http://esrl.noaa.gov/psd) (Kanamitsu et al., 2002). Wind was interpolated from the Gaussian
grid to the regular quarter degree grid using the SYMAP algorithm. The daily temperature
minima and maxima were interpolated and adjusted using a pseudo-adiabatic lapse rate of
0.065º/1000m using the SYMAP algorithm and the Global Land One-kilometer Base Elevation
Project (GLOBE) digital elevation model (DEM).
Global soil parameters were derived as described in Nijssen et al. (2001a) using the 2009
Harmonized World Soil Database and remaining structure-based parameters from Cosby et al.
(1984). Calibration parameters (soil depths, baseflow partitioning into linear and non-linear
flow, and baseflow velocities) were taken from Nijssen et al. (2001b) calibration over the global
domain as a starting point. Global vegetation parameters were derived as in Su et al. (2005). A
maximum of five elevation bands per grid cell were created with the GLOBE DEM.
The VIC model does not include parameters for glaciers. For a correct modeling of gridcell
water and energy balance, glaciers had to be mimicked within the existing VIC framework and
snow model. Gridcells containing glaciers were identified using the combined Global Land Ice
Measurements from Space (GLIMS) and Digital Chart of the World (DCW) glacier data
described in the main text. For each gridcell containing glaciers, the fractional glacier area was
used to create a modified snowpack area.
The area of the glacier coverage and modified snowpack coverage does not exactly match due to
VIC limitations. Although VIC is a flat-Earth model, each gridcell was divided into a maximum
of five elevation bands. Each elevation band has values for elevation and fractional gridcell
coverage; the average of the band’s elevations equal the gridcell average elevation. The assigned
coverage of the modified snowpack overestimates the actual glacier area because snowpack
modifications must be applied per whole elevation band. The modifications are applied to
successive elevation bands until the modified elevation band area is greater than or equal to the
true glacier area. The modified snowpack is located in the highest elevation bands (mountain
glaciers) unless the total difference between the lowest and highest elevation band is less than
200 meters (valley or tidewater glaciers). The differentiation between mountain and tidewater
glaciers using a 200-meter gridcell elevation difference is an assumption made based on the size
of the gridcells.
Snowpack modification to create glaciers was conducted by changing the snow water equivalent.
A snow water equivalent value for the modified snowpack was set artificially high to ensure the
snowpack never fully melted. Albedo of the “glacier” follows the default VIC calculation for
snow albedo, U.S. Army Corps of Engineers (1956) empirical snow albedo decay curves.
During the melt season, with little snowfall, the modified snowpack surface albedo decays to
known glacier ice albedo values. The sustained artificial snow surface ensures more accurate
modeling of energy fluxes, especially turbulent heat fluxes. The temperature of the modified
snowpack, the modified snow surface, and the lowest soil temperature node were initially set to
the average annual temperature to reduce iterative computations for normalizing gridcell
variables. The initial soil moisture condition of gridcells with glacier coverage was set to
saturated; non-glaciated gridcells were initially set to field capacity.
The VIC model was run for a total of 22 years in water balance mode. In water balance mode
the model attempts to close the total water balance for each gridcell. The meteorological
forcings for the 22-year “spin-up” are the same 1998-2008, 11-year input dataset repeated. The
spin-up is conducted in order to build water storage variables, e.g. soil moisture. Following the
22 year spin-up the model was run in energy balance mode with frozen soil calculations. In
energy balance mode the model attempts to close the total water and total energy balances.
Model output variables were checked for realistic values. Incoming shortwave radiation and net
longwave radiation were compared to the International Satellite Cloud Climatology Project
(ISCCP) dataset. Total runoff plus baseflow for each gridcell was compared to previously
created runoff datasets to check for order of magnitude accuracy. Routing of the total runoff
used the GLOBE DEM disaggregated to a quarter degree with all sinks filled.
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