Derive all subcatchments from a DEM using PCRaster in QGIS

This video shows how to use the PCRaster QGIS tools to derive all subcatchments from a DEM.
We'll convert a DEM in GeoTIFF format to PCRaster format. Then we'll derive the local drain direction map. Next, the streamorder tool is used to derive the Strahler orders. After calibration we determine the orders of the channels from which we derive the outlets of the subcatchments.
*NOTE THAT THE WORKFLOW CAN NOW BE DONE WITH THE PCRASTER TOOLS PLUGIN, WHICH HAS SOME BUGS FIXED*
0:00 Introduction and workflow
1:53 Convert to PCRaster format
2:20 Create local drain direction map (lddcreate)
3:20 Calculate Strahler orders (streamorder)
4:07 Derive channels from Strahler orders
5:32 Create lookup table with RAT
6:37 Reclassify Strahler orders (lookup)
7:24 Find downstream Strahler order (downstream)
7:52 Use raster calculator to find junctions
9:07 Give outlets unique ID (uniqueid)
9:56 Convert layer data type
10:30 Create nominal zero map (spatial)
10:49 Fill nodata (cover)
11:24 Derive subcatchments (catchment)
12:00 Derive subcatchments (subcatchment)
12:44 Give subcatchments their Strahler order (areamaximum)
Link to repository: github.com/jvdkwast/qgisrepos...
Linkt to PCRaster: www.pcraster.eu
#PCRaster #QGIS

[@MrLikeidontcare]

Great work as usual!

[@davidedimauro4804]

Good work Hans & Nyall!

[@davidhartmann4174]

Bedankt voor al je video’s! Ik studeer zelf Earth Surface and Water en deze video’s zijn erg leerzaam!

[@user-lb1wu8xg3p]

Hi, Hans. For the last 2 years i have always failed to installed PCRaster plugin in the QGIS. But, this is not the case with the latest version of 3.34 anymore. The instalation proccess were running without any issue and i have successfuly tested the PCRaster in subcatchment delineation as in this tutorial video. Thanks

[@manokito04]

Hello Sir Hans, your videos have really helped me a lot in exploring qgis, thank you so much! I'm quite curious in deriving the subcatchments, is there a way to calibrate or input somewhat a threshold that could result in lesser number of subcatchments?

[@HansvanderKwast]

The lower the Strahler threshold you use, the more subcatchments you get. So you can play with the threshold value and see what works best.

[@anwarmaqsood9764]

Hi. Is there a way to locate the last downstream cells of multiple streams/rivers which can be used as outlets for catchment delineation?

[@jzz0625]

Hi Hans, Great video, I don't understand the last step why a strahlerorder cathcment value '1' can be next to a strahler order '4' catchment. Should it not outflow through a catchment with one strahler order lower, as it does show in the StrahlerChannels.map layer? Keep up the great video's

[@HansvanderKwast]

Thanks. Smaller order tributaries can end up in higher order channels, see this video on the Strahler order method: kzbin.info/www/bejne/iaapnWt7jKmknLc. The order only increases if channels of the same order join. Two notes: (1) Catchments are nested. So downstream outlets have a catchment that cover those from the upstream. (2) Often users don't want to have this effect of downstream catchments covering the upstream ones. This is the difference between the catchment and subcatchment tools of PCRaster.

[@Somone_final_final_v2]

I've started trying to get these steps into a model/process worflow. Before I finished making it thought would ask if you've already made one as some steps seem to cause issues if use on the fly outputs.

[@HansvanderKwast]

Yes, you can find models and scripts in the Resource sharing repository. See kzbin.info/www/bejne/gZPNeqZ8gcSDma8

[@abdulelahalzerwi3576]

Hello, Thank you for the very informative videos you provided. I have a question that how to get subcatchments from a single one? Thank you

[@HansvanderKwast]

Can you please explain what you mean with "from a single one"? For deriving subcatchments also check this playlist: kzbin.info/aero/PLeuKJkIxCDj2Gk0CkcJ-QeviE41aMZd-5

[@kuducamper9934]

QGIS 3.14.15 doesn't have the 'scalar' option in Output Data Type when converting geotiff to .map format. When I used Raster > conversion > translate, the geotiff was converted to .map but was shifted north slightly!

[@valentinabau1874]

This is great! Thanks. But why when I find the strahler order of the catchment I get a raster whose min is -1.. ? Did I make a mistake? I am following exactly the same steps.

[@HansvanderKwast]

That is indeed strange. As far as I know the streamorder algorithm can't produce that. Did you find the minimum value in the Layer properties?

[@Alice-tp9ts]

Hello, thank you for all your videos they have been a great help. My first hurdle is my Raster attribute table has the edit toggle greyed out. I also am getting an error when trying to us the Lookup tool, resulting in no excel sheet produced. Do you have any solutions? thank you

[@HansvanderKwast]

Hi. The problem with the RAT is know and fixed. It will be working in the next release. See github.com/qgis/QGIS/issues/57795. Lookup tool reads a lookuptable in text format, not Excel and it is an input. The result is a reclassified raster. Lookup table from RAT exports the RAT to a text file in the lookup table format.

[@natureadventure4492]

Very informative and useful video. I have one problem. Not able to get option of add attribute table in the stream order output raster .map file. How can I get that?

[@HansvanderKwast]

You need to use the new Raster Attribute Table plugin. It's covered in this video: kzbin.info/www/bejne/gKuph6ijqK-bfs0

[@natureadventure4492]

Thank you

[@abigaila.jubilee6329]

Dear Prof Hans, I have downloaded the hydro plugin for points for catchment but couldn't find Ldd ( Local drain drainage) and the rest of the plugins that you're currently using in this video. How can I find the pulgins? Thanks very much

[@HansvanderKwast]

You need to install QGIS and PCRaster in conda. Check this playlist: kzbin.info/aero/PLeuKJkIxCDj2xbV45C45wz3N89FvmTuSu The first video gets you started. The last one is a more elaborate one also giving more context. The other videos highlight some themes.

[@abigaila.jubilee6329]

@@HansvanderKwast Thanks so much Prof. I'm currently using QGIS 3.16 using family computer. I have managed to understand command prompt about using previous environment in Anaconda. Right now, I have reached the Strahler order following the guidelines in this video. But I don't see any attribute table when I have right clicked the Strahler using QGIS 3.16. What should I do? Thanks very much

[@HansvanderKwast]

@@abigaila.jubilee6329 The Raster Attribute Table is a very new feature in QGIS. You need to install an experimental plugin in order to use it. Check this video: kzbin.info/www/bejne/gKuph6ijqK-bfs0. Also check the description of the video for more details.

[@danieldoucelin8485]

Hello prof Hans ! First of all, I'd like to thank you for sharing this precious videos. They really helped me !!! I'm facing an issue on this topic when converting to PCRaster Format. It only succeeds when I'm not saving the file (temporary file). Do you know why ? Regards, Daniel.

[@HansvanderKwast]

The temporary format is probably still tiff and can not be accepted by the gdal python code that needs to write the prcraster format properties.

[@paulasantandreu2891]

Really good video, as always! However, when I try to create the LDD layer, my computer always freezes, and I cannot get results. Is that common?

[@HansvanderKwast]

Lddcreate is quite calculation intensive. You probably have a large DEM. While QGIS freezes, it's probably still processing. Try to wait longer. It could take really long with large data. You can test it on a small DEM to see if it works.

[@paulasantandreu2891]

@@HansvanderKwast Thanks Hans, I left it the whole weekend and it worked. However, now when I'm trying to create the "new attribute table" for the Strahler layer, QGIS doesn't show me the option. Do you know if I could do this step in another way?

[@HansvanderKwast]

You need to first style the layer with the Layer Styling panel. Then the option should show up. I assume you've installed the Raster Attribute Table plugin already.

[@HansvanderKwast]

Also check my new playlist QGISHydro 2 Chapter 4.

[@paulasantandreu2891]

@@HansvanderKwast Solved! (it was the plugin)

[@markdominicgalerio1234]

Dear prof Hans, I am encountering a problem with regards to the attribute table. When I right click my created "Strahler" layer, "Open Attributes Table" does not come out. It is also disabled or can not be clicked under "Layers" tab. How can I resolve this? Thanks!

[@HansvanderKwast]

Did you install the Raster Attribute Table plugin? If so, try to remove and add the layer again. Also check this video: kzbin.info/www/bejne/hKCcemaGfdSZqdU

[@markdominicgalerio1234]

@@HansvanderKwast Apologies for the late response. Thank you very much for your help prof Hans!

[@raghukambaiah7031]

Hello Sir, kindly upload a video of Weighted Overlay Analysis using QGIS

[@aparnasharma8026]

Hi, I'm unable to convert junctionid to nominal, as it is raising an exception of crs_wkt. Can you please suggest how to overcome this? Thanks.

[@HansvanderKwast]

The error is for some reason related to your projection. What you can do is to delete the xml file of your input layer, so all projection info is removed. Then run the tool again and assign the correct projection back to the input and created output.

[@andreasalgado8244]

Can I know the size of the subcatchments? because I can´t see that in the attribute table

[@andreasalgado8244]

I mean the area of them

[@HansvanderKwast]

You can use the areaarea tool from the PCRaster Tools plugin or convert the raster layer to polygon vector and calculate $area in the attribute table.

[@mlmichel98]

Polygonize (raster to vector) - makes subcatchments into a shp file to use in civil 3d after final step

[@user-uk7gj5gf8c]

Please I have a problem with fill it not function en my computer how can resolve the problem

[@brolimrkz]

must clip the sea area?

[@HansvanderKwast]

Always good to mask water bodies.

[@zainabmansoor462]

it is not work there is an AttributeError: 'str' object has no attribute 'crs' in the lookup, please answer me, for my senior project

[@HansvanderKwast]

There's a bug with the lookup tool. We'll fix it in the next release. Meanwhile you can use other tools in QGIS that doe the same (for example reclassify by table). Or use the lookup tool from the Resource Sharing repository for PCRaster.

[@HansvanderKwast]

Also note that the output is created, but not loaded in QGIS. You can drag it from the browser panel to the Map canvas and continue the procedure with the reclassified layer.

[@zainabmansoor462]

@@HansvanderKwast Thank you veryy much. I am sorry for bothering you. But there is an AssertionError in the downstream. I have searched about it and found nothing. Would you please tell me if I can solve it? Thank you again.

[@HansvanderKwast]

@@zainabmansoor462 Please copy the error and paste it here so I can check.

[@zainabmansoor462]

@@HansvanderKwast Traceback (most recent call last): File "C:\Users/zainab/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\pcraster_tools\processing\algorithms\pcraster_downstream_algorithm.py", line 101, in processAlgorithm self.set_output_crs(output_file=outputFilePath, crs=input_ldd.crs(), feedback=feedback, context=context) File "C:\Users/zainab/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\pcraster_tools\processing\algorithm.py", line 69, in set_output_crs return PCRasterAlgorithm.set_output_crs_wkt(output_file, crs_wkt, context, feedback) File "C:\Users/zainab/AppData/Roaming/QGIS/QGIS3\profiles\default/python/plugins\pcraster_tools\processing\algorithm.py", line 80, in set_output_crs_wkt assert ds AssertionError Execution failed after 0.02 seconds