### Chimaps in a few lines of code (1)

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Since Willett’s et al. (2014) paper on the growth and decay of drainage basins, chimaps have become a popular tool to illustrate the direction of divide migration. Whether a drainage basin is a victim or aggressor, i.e., will loose or gain catchment area in the future, is indicated by the variable chi in the vicinity of the divides. chi (Χ) is a transformed horizontal coordinate measured from the outlet of a drainage basin. The transformation uses upslope area to linearize the concave upward profile for a well-chosen value of the mn-ratio. If all drainage basin outlets have the same elevation, then chi values should be roughly the same on both sides of the divides in a steady-state situation. In this case, no drainage basin reorganization should be expected. Yet, if chi values differ, then one should expect that divides migrate from the basins with lower chi values (i.e. the aggressors) towards those with higher chi values (i.e. the victims) (see Figure below).

Now how is it possible to calculate chimaps using TopoToolbox? It is actually pretty easy, but there is no ready-made function. And there is a reason for this. The description above shows that plotting chimaps involves several modular steps, each of which should be carefully done. These steps are:

1. Load DEM and derive flow directions (FLOWobj).
2. Choose a suitable threshold for stream initiation and derive a stream network (STREAMobj).
3. Make sure that all outlets have the same elevation.
4. Make sure that all drainagebasins are complete, i.e. that the DEM covers their entire extent. Remove incomplete drainage basins.
5. Choose a suitable value for the mn-ratio.
6. Calculate the transformed variable chi (Hint: Don’t use chiplot, but check the low-level function STREAMobj/chitransform!).
7. Plot the results.

I am on field work in Nepal right now and I don’t have access to a suitable DEM. I will thus go through these steps in my next blog entries.

#### References

Willett, S. D., McCoy, S. W., Perron, J. T., Goren, L. and Chen, C.-Y. (2014): Dynamic Reorganization of River Basins, Science, 343(6175), 1248765. [DOI:10.1126/science.1248765].