On 17/04/2019 19:34, Mehrdad Varedi wrote:
Hi Everyone,
I would like to know how the flow path, flow accumulation, or topographic wetness is calculated in cities versus farms.
In other words how to take into account the effect of non-penetrable areas in forming the above-mentioned layers based on a DEM and penetrability score for the study area?

No answers yet, so I will give a try:

Each of the variables in your question are geomorphology characteristics. They depend only on elevation, slope, aspect. Flow accumulation is  an area, TWI is area divided by tan(slope), etc. These characteristics have nothing to do with infiltration (i.e. penetrable areas).


Having said that, the GRASS module r.watershed can take two input rasters: flow and retention, which change the output flow accumulation. If you have spatial data on soil retention properties, or disturbances to surface flow, you can apply them to r.watershed to get a more "realistic" accumulation grid. The defaults are 100 for retention (no infiltration, all water flowing into a cell also goes out) and 1.0 for flow (no disturbances, and no additional contribution for each cell)



Thanks for your help,

Mehrdad

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