Q:

Why TauDEM specialized tools (e.g. D-Infinity Distance Down) are only available with D-Infinity flow model?

I would like to know why most TauDEM specialized tools have been developed just for the D-Infinity flow model.
Is it because the D8 would not make sense for these algorithms, or they just have not been implemented?
In the first case, what is the reason?

A:

Mostly this is a matter of implementation.  They have not been implemented.  Typically, each tool was historically developed in for a specific project or purpose and this development process has not resulted in complete consistency in the functionality available.  I think all the algorithms would make sense with D8 too.  Two of the tools (DinfAvalanche, DinfDistUp) have a parameter that sets a threshold for flow proportions to use in the accumulation.  Setting this threshold to 0.5 (or 0.499 to numerically be >= 0.5) gives a result equivalent to D8. This could be added to each tool (given time and funding), and it would be good if there was a systematic process of upgrading each tool to include this threshold parameter so that equivalent D8 options are available to users.  
If there is a developer who wants or needs this, these tools with an existing threshold could be used as a guide for implementation of others.  A fork and pull request in the github repository (https://github.com/dtarb/taudem) could be used to request inclusion of code changes in the official releases.  

