GRASS GIS manual: r.traveltime

NAME

r.traveltime - Estimation of travel times/isochrones

KEYWORDS

SYNOPSIS

r.traveltime
r.traveltime help
r.traveltime dir=name accu=name dtm=name manningsn=name out_x=string out_y=string threshold=string b=string nchannel=string ep=string fdis=string out=name [--overwrite] [--verbose] [--quiet]

Flags:

--overwrite: Allow output files to overwrite existing files
--verbose: Verbose module output
--quiet: Quiet module output

Parameters:

dir=name: Flow direction map (e.g. derived by r.watershed)
accu=name: Flow accumulation map (e.g. derived by r.watershed)
dtm=name: Depressionless, filled terrain model (e.g. derived by r.fill.dir)
manningsn=name: Map with Manning's n value for surface roughness
out_x=string: x coordinate of basin outlet
out_y=string: y coordinate of basin outlet
threshold=string: Minimum number of cells (threshold) that classify cell as channel
b=string: Channel width
nchannel=string: Channel roughness (Manning's n)
ep=string: Excess precipitation [mm/h]
fdis=string: Reduction factor for equilibrium discharge, 0.0 < f <= 1.0 (default=1)
out=name: Output travel time map [minutes]

DESCRIPTION

r.traveltime computes the travel time of surface runoff to an outlet. The program starts at the basin outlet and calculates the travel time at each raster cell recursively. A drainage area related threhold considers even surface and also channel runoff. Travel times are derived by assuming kinematic wave approximation.
To derive channel flow velocities an equilibrium discharge for each cell is calculated (Q=Area*Excess_Prcipitation, Assumption: storm duration >= time of concentration). This assumption may result in overestimated velocities. Therefor a factor is implemented to reduce velocities biased towards too large values.
The results can be used to derive a time-area function. This might be usefull for precipitation-runoff calculations (estimation of flood predictions) with a lumped hydrologic model (user-specified unit hydrograph).

REMARKS

The program ist restricted to SI units (meters). The algorithm is recursive. Maybe it will not work with extensive datasets. It is assumed that the minimum slope is 0.001. For smaller gradients the program uses this value.

KNOWN ISSUES

The program does not work correctly if Manning's roughness grid is defined as double (float expected). To define a simple uniform roughness distribution try: r.mapcalc 'roughness=0.1f'
The region has to be set one row and column larger than the elevation map. See the example below to see how to do that with g.region.

EXAMPLE

This example uses the North Carolina sample dataset.

  g.region rast=elev_lid792_1m n=n+1 s=s-1 w=w-1 e=e+1 -p
  r.watershed elev_lid792_1m thresh=5000 accum=accum_5K   drain=draindir_5K
  r.fill.dir elev_lid792_1m elev=elev_filled dir=elev_dir
  r.mapcalc rough=0.1f
  r.traveltime --overwrite dir=draindir_5K@user1 accu=accum_5K@user1 \
        dtm=elev_filled@user1 manningsn=rough out_x=638741.43125 \
        out_y=220269.7 threshold=1 b=1 nchannel=0.1 ep=40 fdis=1 \
        out=travel_time