Hi Ken Many thanks for your reply. your time and effort are much appreciated.
I use -a option to ensure positive flow accumulations and using SFD, but still got different results. The reason why needs to change flow direction is that the DEM imperfect, especially when there is a lot of lakes in the watershed. Without change flow direction, the lake will be divided into different subbasins. And the reason why I want to start from flow direction instead of DEM is that there are several hydrological pre-conditioned datasets, the flow direction in this dataset already adjusted for most of the lakes. ---- Some personal suggestions---- I find a solution for my work, but based on this experience feels like divide functions like r.stream.extract into four functions might provide user more feasibility: such as a function that defines flow direction DEM, a function define flow accumulation from flow direction, a function define streams from flow accumulation and a function define subbasin based on flow direction and stream network. These are just personal suggestions. Cheers Ming Ken Mankoff <mank...@gmail.com> 于2020年11月25日周三 下午6:01写道： > > On 2020-11-25 at 04:17 -08, ming han <dustm...@gmail.com> wrote... > > And another problem I got is that the flow accumulation I got from > > r.accumulate and r.watershed is different when r.accumulate using flow > > direction from r.watershed. Again is there anyway r.watershed supports > > using flow direction, so we can get the consistent result. > > There may be reasons for these differences? For example, if SFD v. MFD, or > the "-a" flag to r.watershed? > > > We need this when we need to adjust the flow direction from > > r.watershed or r.stream.extract. and then we need to determine new > > flow accumulation with an adjusted flow direction dataset. If the > > result is inconsistent, not sure what is the solution is. > > It isn't clear why you are adjusting the flow direction. Is this required? > > >> But, If I only want to use flow direction to drive streams, which > >> function I should use? > > https://grass.osgeo.org/grass78/manuals/r.water.outlet.html but this only > works for 1 outlet. > > -k. >
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