3. Model Parameters and Forcings

A tRIBS model application consists of a series of Voronoi polygons that discretely represent a watershed. At each TIN node associated with a Voronoi polygon, a set of governing equations describing the hydrologic and energy processes are computed. To solve these equations for each location, spatially distributed data describing surface topography, soils, vegetation and hydrometeorology are required, which can be obtained from field measurements, remote sensing data interpretation or model output such as numerical weather models.

3.1. Model Parameters

In the case of soil and vegetation cover, different data sources can be used to assign uniform or spatially-explicit parameter values for the governing equations. The soils parameters listed in Table 3.1 are necessary for carrying out the vertical infiltration and lateral flow redistribution, as well computing the soil heat budget within the sloped, heterogeneous, anisotropic soil columns assumed at each Voronoi polygon. Note that the parameter requirements vary with the specified hydrologic processes selected during a model run. For example, the surface heat parameters are only required if the radiation balance is computed. The order in which the soil parameters are listed in the tabular input should correspond to the list below and the units should match. Actual parameter names are not important for tabular input, whereas raster-based inputs assume slightly different naming conventions. Similar units need to be used when specifying soil parameters as raster inputs.

Table 3.1 tRIBS Soil Parameter Description

Parameter	Description	Unit
Ks	Saturated Hydraulic Conductivity	[mm/hr]
thetaS	Saturated Soil Moisture	[-]
thetaR	Residual Soil Moisture	[-]
m	Pore Distribution Index	[-]
PsiB	Air Entry Bubbling Pressure	[mm] (negative)
f	Hydraulic Decay Parameter	[1/mm]
As	Saturated Anisotropy Ratio	[-]
Au	Unsaturated Anisotropy Ratio	[-]
n	Porosity	[-]
ks	Volumetric Heat Conductivity	[J/msK]
Cs	Soil Heat Capacity	[J/m3K]

The vegetation or land-use parameters in Table 3.2 are necessary for carrying out the rainfall interception, bare soil evaporation and evapotranspiration within each Voronoi polygon, as well as determining the radiation and energy balance within the land surface. Note that the parameter requirements vary with the specified processes selected during a model run. For example, rainfall interception can be computed using a simple canopy storage method or the more complex Rutter model. The order in which the vegetation parameters are listed in the tabular input should correspond to the list below and the units should match. Actual parameter names are not important for tabular input, whereas raster-based inputs assume slightly different naming conventions. Similar units need to be used when specifying vegetation parameters as raster inputs.

Table 3.2 tRIBS Vegetation or Land Use Description

Parameter	Description	Unit
a	Canopy Storage - Storage Method	[mm]
b1	Interception Coefficient - Storage Method	[-]
P	Free Throughfall Coefficient - Rutter Method	[-]
S	Canopy Field Capacity - Rutter Method	[mm]
K	Drainage Coefficient - Rutter Method	[mm/hr]
b2	Drainage Exponent - Rutter Method	[1/mm]
Al	Albedo	[-]
h	Vegetation Height	[m]
Kt	Optical Transmission Coefficient	[-]
Rs	Stomata Resistance	[s/m]
V	Vegetation Fraction	[-]
LAI	Leaf Area Index	[-]
thetas	Stress Threshold for Evaporation [0 to 1]	[-]
thetat	Stress Threshold for Transpiration [0 to 1]	[-]

3.1.1. Input Formats

One form of data input for soil textural and vegetation data is through the use of ASCII grids consisting of soil or land use codes or indices. The soil (*.soi) and land use (*.lan) grids are specified in the Input File by using the keywords SOILMAPNAME and LANDMAPNAME. Table 3.3 presents an example of a soil or land use grid. As with other grid input, care should be taken to specify the grids in the same coordinate system as the topographic TIN data.

Table 3.3 Example of Soil or Land Use Class ASCII grid (*.soi and *.lan)

nrows	6
ncols	6
xllcorner	346035
yllcorner	3979905
cellsize	2000
NODATA_value	-9999
-9999	-9999	-9999	1	0	-9999
-9999	-9999	1	0	0	-9999
-9999	-9999	1	0	1	1
-9999	0	1	1	-9999	-9999
0	0	-9999	-9999	-9999	-9999

The parameter values for the soil and land use grids are read from a reclassification table inputted separately using the keywords SOILTABLENAME and LANDTABLENAME. The format of these soil reclassification (*.sdt) and land use reclassification (*.ldt) tables include a small header which specifies the number of cover types (#Types) and the number of variables for each type (#Params), as shown in Table 3.4 and Table 3.5. The header is followed by a matrix of parameter values where each row represents one cover type and each column represents one parameter. The order and units of these parameters are fixed. Since parameter values outside the appropriate range may results in inaccurate calculations, the user should be careful to select realistic values from literature sources prior to model use.

Table 3.4 Soil Reclassification Table Structure (*.sdt)

#Types	nParams
ID	Ks	thetaS	thetaR	m	PsiB	f	As	Au	n	ks	Cs

Table 3.5 Land Use Reclassification Table Structure (*.ldt)

#Types

nParams

ID

a

bI

P

S

K

b2

Al

h

Kt

Rs

V

LAI

Note that the soil parameters relate to the hydraulic and thermal properties in the upper portions of the soil profile. Most of these can be directly related to the surface soil texture. The first nine parameters are essential for running the Unsaturated Zone Model while the last two are required if the keyword GFLUXOPTION = 1. Note that these land use parameters relate to the interception and evaporation properties of the vegetative cover or land use type. The first two parameters are required if the keyword OPTINTERCEPT = 1, while the next four are required if OPTINTERCEPT = 2. The final five parameters are required for various options of the keyword OPTEVAPOTRANS. The last two parameters have been added to specify the soil moisture stress threshold for soil evaporation and plant transpiration in units of relative soil moisture (varying from 0 to 1).

Gridded soil data can be used as an alternative to the tabular soil parameter input. To activate the use of the gridded soil data the user must the keyword OPTSOILTYPE = 1 in the Input File (*.in). If OPTSOILTYPE = 0 then the use of the tabular data will be selected. The information is provided through the use of a text file for reading soil grid input (*.gdf) specified through the keyword SCGRID. The structure of the soil grid data file or GDF is shown in Table 3.6.

Table 3.6 Soil Parameter GDF File Structure

#Params
Latitude	Longitude	GMT
KS	Grid File Pathname	Grid Extension
TS	Grid File Pathname	Grid Extension
TR	Grid File Pathname	Grid Extension
PI	Grid File Pathname	Grid Extension
PB	Grid File Pathname	Grid Extension
FD	Grid File Pathname	Grid Extension
AR	Grid File Pathname	Grid Extension
UA	Grid File Pathname	Grid Extension
PO	Grid File Pathname	Grid Extension
VH	Grid File Pathname	Grid Extension
SH	Grid File Pathname	Grid Extension

An alternative input format type for dynamic land cover data is with the use of grid data. This option in the tRIBS model is used with the keyword OPTLANDUSE = 1, while the more static land cover is specified with OPTLANDUSE = 0. The use of dynamic land cover variables maybe convenient for inputting remotely sensed vegetation fields. Information is provided through a text file for reading in land cover grid input (*.gdf) as specified through the keyword LUGRID in the Input File. The structure of the Grid Data File or GDF is presented in Table 3.7.

Table 3.7 Land Cover GDF File Structure

#Params
Latitude	Longitude	GMT
AL	Grid File Pathname	Grid Extension
TF	Grid File Pathname	Grid Extension
VH	Grid File Pathname	Grid Extension
SR	Grid File Pathname	Grid Extension
VF	Grid File Pathname	Grid Extension
CS	Grid File Pathname	Grid Extension
IC	Grid File Pathname	Grid Extension
CC	Grid File Pathname	Grid Extension
DC	Grid File Pathname	Grid Extension
DE	Grid File Pathname	Grid Extension
OT	Grid File Pathname	Grid Extension
LA	Grid File Pathname	Grid Extension

In the above *.gdf files, note that the first line specifies the total number of parameters to be inputted, while the second line is used to input a representative absolute latitude, longitude and GMT values for all the input grids. The next #Params lines are used to specify the parameter code, the file pathname of the land cover parameter grid (including the basename of the file) and the extension given to the particular grid. The NO_DATA flag is used to specify the grids that are not available for a particular parameter.

3.2. Model Forcings

In the case of hydrometeorological forcings, model inputs can be achieved in a number of different ways: (1) point input of hydrometeorological observations; (2) grid input of meteorological observations or numerical model results, or (3) point input of stochastic climate simulations. The model can handle the meteorological forcing in the point or grid format and has internal routines to assign this information to Voronoi polygons or TIN nodes via Thiessen resampling or nearest neighbor approaches.

Table 3.8 lists the hydrometeorological model forcings. The primary hydrometeorological parameter is rainfall at a specified temporal resolution, typically hourly. Sub-hourly forcing can be specified despite having no minute column, by simply providing the data in order using the same hour in the hour column. The requirement of the other meteorological parameters depends on the processes selected for the model run. Some of the parameter information is redundant, for example dew point temperature and relative humidity are interchangeable. When incoming solar radiation is used, sky cover is not neeed. Other information can be input directly or computed within the model, for example net radiation, using the other meteorological measurements. The naming convention for each variable is used when specifying raster-based inputs. Units should be preserved.

Table 3.8 tRIBS Hydrometeorological Parameter Description

Parameter	Description	Unit
PA	Atmospheric Pressure	[mb]
TD	Dew Point Temperature	[C]
RH	Relative Humidity	[%]
VP	Vapor Pressure	[mb]
XC	Sky Cover	[tenths] (0 to 10)
US	Wind Speed	[m/s]
TA	Air Temperature	[C]
TS	Surface Temperature	[C]
NR	Net Radiation	[W/m2]
R	Rainfall	[mm/hr]
IS	Incoming Solar Radiation	[W/m2]

3.2.1. Input Formats

Meteorological input into tRIBS can from point data or grid data, depending on the data sources available. The two data inputs are treated differently in the model. Table 3.9 shows the two forms of meteorological data input and storage.

Table 3.9 Meteorological Data Input Methods

Characteristic	Point Data	Grid Data
Input	Station Descriptor File (*.sdf)	ASCII grids (*.txt, *.lan, *.soi)
	Meteorological Data File (*.mdf)
Storage	Assignment to storage objects	Direct assignment to tCNode
Manipulation	Thiessen point resampling	Grid resampling
Examples	tHydroMet, tRainGauge	tRainfall, tVariant, tInvariant

The format of the Station Descriptor Files (*.sdf) and the Meteorological Data Files (*.mdf) is modified slightly depending on whether these contain meteorological or rain gauge data.

Table 3.10. Weather Station SDF Structure

#Stations	#Params
StationID	FilePath	AbsLat	RefLat	AbsLong	RefLong	GMT	RecordLength	#WeatherParams	Other

Table 3.11. Rain Gauge SDF Structure

#Stations	#Params
StationID	FilePath	RefLat	RefLong	RecordLength	#RainParams	Elevation

Note the following: #Stations is the number of total stations to be read, #Params is the number of parameters for each of the subsequent lines, StationID must be unique values for each station (starting at 0), the FilePath refers to the MDF file for that particular station and must be relative to the location of the executable, the AbsLong and AbsLat must be in decimal degree (lat/long), the RefLong and RefLat must be in the same coordinate system as the input grids and watershed TIN, Greenwich Mean Time (GMT) is difference in hours between the location and the Greenwich Meridian (negative number in Western Hemisphere), the RecordLength is the length of the time series in the MDF file, the #WeatherParams and #RainParams are the number of parameters in the MDF file including the date and time, and Other is used for inputting additional station information, such as station elevation, if desired. These keywords are not included in the file, just the parameter value.

Table 3.12 Weather Station MDF Structure

Y	M	D	H	PA	TD/RH/VP	XC	US	TA	TS	NR
…	…	…	…	…	…	…	…	…	…	…

Table 3.13 Rain Gauge MDF Structure

Y	M	D	H	R
…	…	…	…	…

Note the following: the parameter names must be a placed in a header for each MDF file, the TD/RH/VP imply that either one of these parameters can be inputted into that particular field, there must be RecordLength number of lines following after the header in intervals, missing data must be inputted with the NO_DATA flag = 9999.99, and the units must be retained as indicated, including for IS, NR and TS. Rainfall (R) is typically specified in its own MDF file. Notice that the file does not contain a minute column. Nevertheless, sub-hourly data can be inputted into the model at intervals that are multiples of the TIMESTEP. For example, for 15-minute data, the user should specify four rows for each hour (same H) in order. A similar approach is taken for sub-hourly rain gauge data.

An alternative input format type for meteorological data is with the use of grid data. This option in the tRIBS model is used with the keyword METDATAOPTION = 2, while the more traditional weather station data is specified with METDATAOPTION = 1. The additional information is provided through a text file for reading in meteorological input (*.gdf) as specified through the keyword HYDROMETGRID in the Input File. The structure of the Grid Data File or GDF is presented in Table 3.14.

Table 3.14 Meteorological GDF File Structure

#Params
Latitude	Longitude	GMT
PA	Grid File Pathname	Grid Extension
TD	Grid File Pathname	Grid Extension
XC	Grid File Pathname	Grid Extension
US	Grid File Pathname	Grid Extension
TA	Grid File Pathname	Grid Extension
IS	NO_DATA	NO_DATA
TS	NO_DATA	NO_DATA
NR	NO_DATA	NO_DATA
RH	NO_DATA	NO_DATA

Note that the first line specifies the total number of parameters to be inputted, while the second line is used to input a representative absolute latitude, longitude and GMT values for all the input grids. The next #Params lines are used to specify the parameter code, the file pathname of the weather grid (including the basename of the file) and the extension given to the particular grid. The NO_DATA flag is used to specify that weather grids are not available for a particular parameter. All the keywords used to represent the parameters are fixed as well as the units.