Issue with r.watershed run by QGIS-Python-processing.runalg

Question

I would like to extract flow accumulation and flow direction layers from a DEM calling the grass r.watershed module from a QGIS-python script using the processing class. I obtain just an empty map, while, using the processing GUI I get the map I was expecting. Here is my code:

from PyQt4.QtCore import *
import processing

# "mydem" is the loaded dem layer
dem=QgsMapLayerRegistry.instance().mapLayersByName("mydem")[0]
processing.runalg("grass:r.watershed", dem, None, None, None, None, 10, None, None, 300, None, 1, None, None, None, 40, "C:\\Users\\flc\\Desktop\\QGIS_Plugins\\layers\\acc.tif", "C:\\Users\\flc\\Desktop\\QGIS_Plugins\\layers\\dir.tif", None, None, None, None, None, None)

The 6th argument is the threshold. It is not needed for the computation of flow accumulation and flow direction, but in my trials with the GUI (the processing GUI, not the GRASS GUI) I found that the algorithm works when it is specified. However, I tried a lot of combinations of parameters. Do you have any idea about how could it work?

Answer 1

Using the following code worked for me:

from PyQt4.QtCore import *
import processing

# "mydem" is the loaded dem layer
dem=QgsMapLayerRegistry.instance().mapLayersByName("mydem")[0]

fileName = dem.source()
fileInfo = QFileInfo(fileName)
baseName = fileInfo.baseName()
rlayer = QgsRasterLayer(fileName, baseName)

# Need to define the extent of the region
extent = rlayer.extent()
xmin = extent.xMinimum()
xmax = extent.xMaximum()
ymin = extent.yMinimum()
ymax = extent.yMaximum()

processing.runalg("grass:r.watershed", dem, None, None, None, None, 10, 0, 1, 300, False, True, False, False, "%f,%f,%f,%f"% (xmin, xmax, ymin, ymax), 40, "C:\\Users\\flc\\Desktop\\QGIS_Plugins\\layers\\acc.tif", "C:\\Users\\flc\\Desktop\\QGIS_Plugins\\layers\\dir.tif", None, None, None, None, None, None)

---

Running this algorithm from the Python Console contains boolean parameters which must be defined by True or False. You can check this by typing:

import processing
processing.alghelp("grass:r.watershed")

The output will tell you the parameters used:

ALGORITHM: r.watershed - Watershed basin analysis program.
    elevation <ParameterRaster>
    depression <ParameterRaster>
    flow <ParameterRaster>
    disturbed.land <ParameterRaster>
    blocking <ParameterRaster>
    threshold <ParameterNumber>
    max.slope.length <ParameterNumber>
    convergence <ParameterNumber>
    memory <ParameterNumber>
    -f <ParameterBoolean>
    -4 <ParameterBoolean>
    -m <ParameterBoolean>
    -a <ParameterBoolean>
    GRASS_REGION_PARAMETER <ParameterExtent>
    GRASS_REGION_CELLSIZE_PARAMETER <ParameterNumber>
    accumulation <OutputRaster>
    drainage <OutputRaster>
    basin <OutputRaster>
    stream <OutputRaster>
    half.basin <OutputRaster>
    visual <OutputRaster>
    length.slope <OutputRaster>
    slope.steepness <OutputRaster>

There's a couple of other things such as the the max.slope.length parameter which requires a minimum value of 1 and the GRASS_REGION_PARAMETER which needs to be defined.