Proposing HAND index?

[allixender]

Hi,

We calculate hydrology and soil moisture-related metrics, and I was wondering if implementing HAND in addition to TWI and TRI would be possible for this package? Height Above Nearest Drainage (HAND) is basically a drainage normalizing terrain index.

• Nobre, A.D., Cuartas, L.A., Hodnett, M., Rennó, C.D., Rodrigues, G., Silveira, A., Waterloo, M., Saleska, S., 2011. Height Above the Nearest Drainage – a hydrologically relevant new terrain model. Journal of Hydrology 404, 13–29. https://doi.org/10.1016/j.jhydrol.2011.03.051

[evetion]

I would like to see HAND, it's on my (internal) wishlist. That said, unless you just set a accumulation threshold, it would require a few intermediate functions that derive a stream network first. Or do you have your own river network/mask that you want to use?

[allixender]

I could do both: For composability, I wouldn't necessarily require a single function, as long as the workflow is clear. Do you think I could import my river network/mask? I wouldn't be sure if the conventions for direction and accumulation would be compatible with the conventions used in Geomorphometry.jl. So, we can also just go through the steps, several of which are already available, right?

[evetion]

With the existing algorithms mostly using the priority flood algorithm, I can imagine that HAND is one more loop over the data:

• From lowest to highest determining drainage direction (and accounting for depressions)
• From highest to lowest accumulating flow
• From lowest to highest setting relative height (accumulate the height differences)

In terms of conventions, that can be problematic (although no flow direction can be used/imported here yet), let me know what you expect in terms of units etc. Happy to make further changes to this package to align it with others.