[GRASS-dev] [GRASS GIS] #2719: r.watershed + D8 (unexpected results)

[GRASS GIS]

#2719: r.watershed + D8 (unexpected results)
--------------------+-------------------------
 Reporter:  180875  |      Owner:  grass-dev@…
     Type:  defect  |     Status:  new
 Priority:  normal  |  Milestone:
Component:  Raster  |    Version:  unspecified
 Keywords:          |        CPU:  x86-64
 Platform:  Linux   |
--------------------+-------------------------
 Dear GRASS Gurus,

 During the last days I've discovered a strange behaviour of r.watershed
 and D8. The used DEM is the result from a landscape evolution model and
 there are no depressions. I cannot use MFD as the D8-FlowDirections input
 data of a variety of tools I’ve written during the last years.

 I got obviously wrong values for the flow accumulation of r.watershed and
 D8. The number of cells in the Flow Accumulation grid at the outlet point
 was significantly smaller than the area of the analyzed catchment (spatial
 resolution in north south and east west direction is 1) also computed with
 GRASS using r.water.outlet and a conversion to Vector (Area).
 I checked Flow Accumulation at the the outlet with an alternative code of
 a colleague and the contributing drainage area was exactly the value of
 the catchment size computed with r.water.outlet.

 I further found some deviations in the flow direction grid. In some cases
 the flow routing leads to the lowest neighbouring cell which however might
 not be the steepest decent. r.watershed chooses sometimes a diagonal flow
 direction while the local channel gradient would be greater in horizontal
 direction.
 I started some test runs with very simple geometries. I've defined a ramp
 with r.mapcalc that dips in north direction and I expected that this will
 also be shown by the flow direction of r.watershed. However, this is not
 the case. The synthetic DEM features a watershed!!! See attachment.  It
 seems that r.watershed fills the region (without need). This strange
 behavior disappears by applying the "multiple flow direction" algorithm.

 I have tested with the latest versions grass70 , grass71 and grass645.
 Here is a test simple test case (grass71) that documents the misbehavior
 of r.watershed and D8.
 g.region w=0 e=1000 s=0 n=1000 res=1 -p
 r.mapcalc "DEM = 1000 - y()"
 r.watershed -s elevation=DEM at PERMANENT accumulation=ACC drainage=DIR

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2719>
GRASS GIS <https://grass.osgeo.org>