User manual


This tutorial shows how to run a basic Itzï simulation using freely available dataset.

It assumes that GRASS 7 and Itzï are properly installed on your machine and that you possess a basic knowledge of GRASS.

Get ready

Here we will use the GRASS North Carolina data set. Please download the GRASS 7 version and extract it in your grassdata directory.

Then start GRASS in the PERMANENT mapset.

Adjust the region

Fit the lidar elevation raster map and set a resolution of 5m:

$ g.region -p [email protected] res=5 save=lidar_5m

Resample the DEM

Please note that this step is not strictly necessary. The Itzï simulation will be carried out in any case on the defined computational region extent and resolution. However the bilinear interpolation smooth the surface, which prevent high slope values that could occur if using the GRASS default nearest-neighbor sampling.

$ r.resamp.interp [email protected] output=elev_lid792_5m

Create a raster mask

Generate a drainage direction map and then create a watershed raster using the outlet point coordinates:

$ r.watershed elevation=elev_lid792_5m drainage=elev_lid792_5m_drainage
$ r.water.outlet input=elev_lid792_5m_drainage output=watershed coordinates=638888,220011

Create a raster mask to prevent calculation outside of the watershed:

$ r.mask rast=watershed

Create boundary condition maps

Create a vector map with the watershed outlet point:

$ echo '638888|220011' > watershed_out.txt
$ input=watershed_out.txt output=watershed_out

Using this vector map, create two raster maps for the boundary conditions. The first with a value corresponding to the type of condition, here 4 corresponds to a fixed water depth inside the domain. The second being the value of the depth wanted, here 0.

$ input=watershed_out type=point output=bctype use=val value=4
$ input=watershed_out type=point output=bcvalue use=val value=0

Create rainfall and friction maps

Create maps of uniform rainfall and friction coefficient:

$ r.mapcalc exp='rain=100'
$ r.mapcalc exp='n=0.05'

Run the simulation

Create a new parameter file and fill it with the created map. It should look like the following:

duration = 02:00:00
record_step = 00:05:00

dem = [email protected]
friction = [email protected]
rain = [email protected]
bctype = [email protected]
bcval = [email protected]

prefix = nc_itzi_tutorial
values = h, wse, v, vdir, boundaries

stats_file = nc_itzi_tutorial.csv

Run the simulation:

$ itzi run <parameter_file_name>

At the end of the simulation, Itzï should have generated five Space-Time Raster Dataset (STRDS) in the form:


The maps contained in those STDRS are following this naming convention:


Here is the example of the map nc_itzi_tutorial_h_0020:

NC depth

All the results can be processed using the GRASS tools for raster maps and / or space-time dataset. For instance, it is easy to generate an animation of the results using g.gui.animation.

Command line usage

Itzï is run from the command line.

Show the current version:

$ itzi version

Display the command line options:

$ itzi run -h

Configuration file

The parameters of a simulation are given through a configuration file in a format similar to Microsoft Windows INI files. An example is given in the tutorial above. The file is separated in sections described below.


Simulation duration could be given by a combination of start time, end time and duration. If only the duration is given, the results will be written as relative time STRDS. In case start time is given, the simulation will use a absolute temporal type.

Keyword Description Format
start_time Starting time yyyy-mm-dd HH:MM
end_time Ending time yyyy-mm-dd HH:MM
duration Simulation duration HH:MM:SS
record_step Time-step at which results are written to the disk HH:MM:SS

Valid combinations:

  • start_time and end_time
  • start_time and duration
  • duration only


Itzï does not support Lat-Long coordinates. A projected location should be used. The inputs maps could be given either as STRDS or single maps. First, the module try to load a STRDS of the given name. If unsuccessful, it will load the given map, and stop with an error if the name does not correspond to either a map or a STRDS.

The following inputs are mandatory:

  • Digital elevation model in meters
  • Friction, expressed as Manning’s n
Keyword Description Format
dem Elevation in meters map or strds
friction Manning’s n (friction coefficient) map or strds
start_h Starting water depth in meters map name
rain Rainfall in mm/h map or strds
inflow Point inflow in m/s (ex: for 20 m3/s on a 10x10 cell, velocity is 0.2 m/s) map or strds
bctype Boundary conditions type map or strds
bcval Boundary conditions values map or strds
infiltration Fixed infiltration rate in mm/h map or strds
effective_pororosity Effective porosity in mm/mm map or strds
capillary_pressure Wetting front capillary pressure head in mm map or strds
hydraulic_conductivity Soil hydraulic conductivity in mm/h map or strds
drainage_capacity Drainage capacity in mm/h (new in 16.9) map or strds

Boundary conditions type are defined by an integer as follow:

  • 0 or 1: Closed boundary (default if no map is given)
  • 2: Open boundary: velocity at the boundary is equal to the velocity inside the domain
  • 3: Not implemented yet
  • 4: User-defined water depth inside the domain


The “open” and “closed” boundary conditions are applied only at the border of the GRASS computational region.


infiltration and any of the Green-Ampt parameters are mutually exclusives. Likewise, if any of the Green-Ampt parameter is given, all the others should be given as well.


Keyword Description Format
prefix Prefix of output STRDS string
values Values to be saved. Each one will be a STRDS comma separated list

The possible values to be exported are the following:

Keyword Description Format
h Water depth meters
wse Water surface elevation (depth + elevation) meters
v Overland flow speed (velocity’s magnitude) m/s
vdir Velocity’s direction. CCW from East degrees
qx Volumetric flow, x direction. Positive if going East m³/s
qy Volumetric flow, y direction. Positive if going South m³/s
boundaries Flow coming in (positive) or going out (negative) the domain due to boundary conditions. Average since the last record m/s
infiltration Infiltration rate. Average since the last record mm/h
rainfall Rainfall rate. Average since the last record mm/h
inflow Average user flow since the last record m/s
drainage_cap Average flow due to drainage capacity since the last record (new in 17.1) m/s
verror Total created volume due to numerical error since the last record (new in 17.1)


Additionally to output a map at each record_step, h and v also produce a map of maximum values.

Water depth maps, apart from map of maximum values, do not display values under the hmin threshold (See options)


Keyword Description Format
stats_file Statistics file CSV table


Keyword Description Format Default value
hmin Water depth threshold in metres positive float 0.005
cfl Coefficient applied to calculate time-step positive float 0.7
theta Inertia weighting coefficient float between 0 and 1 0.9
vrouting Routing velocity in m/s positive float 0.1
dtmax Maximum superficial flow time-step in seconds positive float 5
dtinf Infiltration time-step in seconds positive float 60

When water depth is under hmin, the flow is routed at the fixed velocity defined by vrouting (see Technical details).


New in 16.9

Setting those parameters allows to run simulation outside the GRASS shell. This is especially useful for batch processing involving different locations and mapsets. If Itzï is run from within the GRASS shell, this section is not necessary.

Keyword Description Format
grass_bin Path to the grass binary string
grassdata Full path to the GIS DataBase string
location Name of the location string
mapset Name of the mapset string

With Linux, grass_bin could be simply “grass”. On Windows, the full path is necessary.

Statistics file

The statistic file is presented as a CSV file. As of version 17.1, the values that are output are shown in the table below.

Water entering the domain is represented by a positive value. Water that leaves the domain is negative. Volumes are in m³.

Keyword Description
sim_time Elapsed simulation time
avg_timestep Average time-step duration since last record
#timesteps Number of time-steps since the last record
boundary_vol Water volume that passed the domain boundaries since last record
rain_vol Rain volume that entered the domain since last record
inf_vol Water volume that left the domain due to infiltration since last record
inflow_vol Water volume that entered or left the domain due to user inflow since last record
drain_cap_vol Water volume that entered or left the domain due to drainage capacity since last record
domain_vol Total water volume in the domain at this time-step
created_vol Water volume created due to numerical errors since last record
%error Percentage of the domain volume variation due to numerical error. Corresponds to created_vol / (domain_vol - old_domain_vol) * 100