Generating a D4 routing slope vector

[mvhulten]

We are testing out pysheds to use in our ParFlow static-file generator, but the (apparent) limitations of the function object cell_slopes() are:

• It returns a scalar the slope between each cell and its downstream neighbour, instead of a vector
• It does d8, dinf and mfd only and not d4, at least it does not recognise routing=d4

Is it possible to return a slope vector and use D4 routing?

[mvhulten]

I'd say that for D4 routing the vector slope should simply be any of $\nabla z = (±s, 0)$ or $\nabla z = (0,±s)$, depending on the downstream neighbour grid.flowdir(), where $s$ is the result of cell_slopes().

[mvhulten]

Flow direction

For D4 routing, I'm trying to fill in zeros at the diagonal: dirmap = (1, 2, 4, 8, 0, 0, 0, 0).

A believable spatial pattern comes out of this, ranging from -2 to 8 instead of -2 to 128 (seas and lakes are negative), but I don't know if it is correct.

edit: No, it is incorrect. I get zeros (reserved for missing values) where I'd otherwise get values larger than 8.