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

SEE ALSO

r.watershed, r.fill.dir

http://jesbergwetter.twoday.net/stories/4845555/

REFERENCES

  • Kilgore, J. L. (1997): Development and evaluation of a GIS-based spatially distributed unit hydrograph model, master thesis, Virginia Polytechnic Institute and State University.
  • Melesse, A. M., Graham, W. D. (2004): Storm runoff predicition based on a spatially distributed travel time method utilizing remote sensing and GIS, Journal of the American Water Resources Association, 8, 863-879.
  • Muzik, I. (1996): Flood modelling with GIS-derived distributed unit hydrographs, Hydrological Processes, 10, 1401-1409.

    • AUTHOR

    Kristian Foerster

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