Manual Method for Computing Upstream Radiation Weighted by Distance
D. Nagel
August 20, 2006
Important Note: This procedure is one used to develop the final procedure. This is not
the final procedure used for the project. A Python script and C program are used for the
final procedure.
This document describes a method for computing a distance weighted radiation value for
each pixel along a raster TauDEM stream network. The radiation value at each pixel
along the stream channel is a summarized upstream accumulation (of radiation) that
decays with upstream distance. The procedure requires the use of TauDEM software
and a raster grid that represents radiation.
1) DEM preparation. Start with a 30 m DEM clipped to a watershed boundary. If
watershed is clipped right at boundary, uncheck the edge contamination function in
TauDEM. If watershed is buffered, maintain the edge contamination function. The DEM
should be named “dem”. Your folder should be an empty ArcInfo workspace, except that
it will contain the DEM.
2) Run TauDEM. Run the following functions:
• Select base DEM grid …
• Fill pits
• D8 flow directions
• D8 contributing area
• Grid network order and flow path lengths
• Full river network raster
3) Create the decay multiplier grid.
Enter the following in Spatial Analyst > Map Algebra > Single output:
demdec = 0.979010 + (demp mod 2) * .006102
e folding Length
(m)
Cell Length (m)
Multiplier straight
Multiplier diagonal
Delta Mult
250
30
0.8869204
0.8439132
0.0430072
500
30
0.9417645
0.9186475
0.0231170
1000
30
0.9704455
0.9584610
0.0119845
2000
30
0.9851119
0.9790102
0.0061017
4000
30
0.9925281
0.9894494
0.0030786
6000
30
0.9950125
0.9929539
0.0020586
The coefficients are derived from “Multiplier diagonal” and “Delta Mult”. These are
dependent on e folding Length, which is the upstream distance at which weight is 1/e.
For this example we used an e folding Length of 2000 m.
4) Create the DEM angle file.
Enter the following in Spatial Analyst > Map Algebra > Single output:
demang = (demp – 1) * (3.1415926536 / 4)
3.1415926536 = pi
Note: This step creates a file with the “ang” ending. This is a replacement for the “ang”
file that TauDEM creates from the Dinf Flow Direction routine. This new version of the
“ang” file is created because it follows the D8 flow direction (“p” file).
5) Obtain radiation data that is co-registered with DEM data. Radiation file should
only have data along TauDEM streams with 0 everywhere else. Use demsrc as a mask
file for extracting the radiation pixels. The output should be called strm_rad.
6) Calculate the accumulated radiation using decaying accumulation function of
TauDEM.
TauDEM > Specialized grid analysis > Decaying accumulation
D-Inf Flow Direction Grid: demang (note: this is the “ang” created in step 4
above)
Decay Multiplier Grid: demdec
Use Weight Grid (check): strm_rad
Output: rad_acc (radiation accumulated)
7) Calculate the accumulated effective number of cells using decaying
accumulation function of TauDEM.
TauDEM > Specialized grid analysis > Decaying accumulation
D-Inf Flow Direction Grid: demang (note: this is the “ang” created in step 4
above)
Decay Multiplier Grid: demdec
Use Weight Grid (check): demsrc
Output: strm_acc (effective accumulated cells)
8) Compute average radiation
Use: Spatial Analyst > Math > Divide
Input: rad_acc / strm_acc
Output: rad_avg
This yields a distance weighted average radiation for each cell.
Procedures and Methods Disclaimer of Liability
Neither the United States Government nor any of its employees makes any warranty,
express or implied, for any purposes regarding these procedures or methods. This
includes warranties of merchantability and fitness for any particular purpose.
Furthermore, neither the United States Government nor any of its employees assumes
any legal liability or responsibility for the accuracy, completeness, or usefulness of any
information or products derived from these procedures.