Forcing Data Files Forcing data input to the VIC model has been completely rewritten for version 4.0. There are no longer any pre-defined types of forcing file, it is now up to the user to define the forcing file in the global control file. Forcing data files can be in short-int Binary or ASCII column formats, but the VIC model must be informed as to the contents of each of the columns in the file. This is done in the global control file using the FORCE_TYPE parameter.
Possible forcing file data types are:
| AIR_TEMP | sub-daily air temperature (C) |
| ALBEDO | surface albedo (fraction) |
| DENSITY | atmospheric density (kg/m3) |
| PREC | precipitation (mm) |
| PRESSURE | atmospheric pressure (kPa) |
| SHORTWAVE | shortwave radiation (W/m2) |
| TMAX | daily maximum temperature (C) |
| TMIN | daily minimum temperature (C) |
| VP | atmospheric vapor pressure (kPa) |
| WIND | wind speed (m/s) |
| SKIP | used to indicate a data column which is not read into the model |
Three examples of file type definitions are provided below using a standard daily input file containing precipitation, daily maximum and minimum air temperature and wind speed:
Below is an example of a 4 column daily forcing file:
0.000 -10.00 -2.00 2.00 0.000 -8.50 -1.10 2.00 5.000 -7.90 0.10 2.00 10.000 -8.20 -3.10 2.00 ...
To read this file the global control file must have the following lines:
FORCING1 FORCING_DATA/LDAS_ONE_DEGREE/data_ N_TYPES 4 FORCE_TYPE PREC FORCE_TYPE TMAX FORCE_TYPE TMIN FORCE_TYPE WIND FORCE_FORMAT ASCII FORCE_DT 24 FORCEYEAR 1950 FORCEMONTH 1 FORCEDAY 1 FORCEHOUR 0 FORCING2 FALSE
Short integer binary files are significantly smaller than an equivalent ASCII column file. However, floating point data values must be converted by a multiplication factor in order to be stored in a single short int value. If a forcing value is always positive (such as precipitation), then using UNSIGNED short int values means that a larger range of positive values can be stored. To properly extract the data stored in the forcing file, both the multiplication factor, and whether the data is SIGNED or UNSIGNED must be included in the global control file.
One further piece of information must be provided to make sure that VIC reads the binary file correctly even if the file was created on a diffrent machine than the one which runs the simulation. There are two different archetectures present on our systems. Most of the workstations store data in big-endian format (most significant byte first), while the PC based systems (including all of the FreeBSD systems) store data in little-endian format (least significant byte first). VIC can determine the endian of the simulation machine, but the global control file must tell it the format of the machine on which the binary files were created.
Below is an example of what would be in the global control file in order to read the previous example if it had been encoded in short int binary form:
FORCING1 FORCING_DATA/LDAS_ONE_DEGREE/data_ N_TYPES 4 FORCE_TYPE PREC UNSIGNED 40 FORCE_TYPE TMAX SIGNED 100 FORCE_TYPE TMIN SIGNED 100 FORCE_TYPE WIND SIGNED 100 FORCE_FORMAT BINARY FORCE_ENDIAN LITTLE FORCE_DT 24 FORCEYEAR 1950 FORCEMONTH 1 FORCEDAY 1 FORCEHOUR 0 FORCING2 FALSE
As the final example, assume that you have daily precipitation, and minimum and maximum air temperature, but you also have hourly wind speed. To save space, it makes the most sense to save the data sets separately (otherwise the daily data must be converted to hourly, or the hourly to daily). VIC is capable of reading forcing data from two sources with two different time steps. Below is an example of what the global control file would look like:
FORCING1 FORCING_DATA/LDAS_ONE_DEGREE/data_ N_TYPES 4 FORCE_TYPE PREC FORCE_TYPE TMAX FORCE_TYPE TMIN FORCE_TYPE SKIP FORCE_FORMAT ASCII FORCE_DT 24 FORCEYEAR 1950 FORCEMONTH 1 FORCEDAY 1 FORCEHOUR 0 FORCING2 FORCING_DATA/LDAS_ONE_DEGREE/data_ N_TYPES 2 FORCE_TYPE SKIP FORCE_TYPE WIND FORCE_FORMAT ASCII FORCE_DT 1 FORCEYEAR 1950 FORCEMONTH 1 FORCEDAY 1 FORCEHOUR 0
