Thanks for this helpful tutorial, I am learning much! In regard to the raster calculator: for calculating absolute values, you can simply use ABS(flowdir@1)
It depends what you need. There are tools that allow you to give multiple user-defined outlets or tools that can automatically find the pits and use them for delineation. Check out this playlist: ua-cam.com/play/PLeuKJkIxCDj2Gk0CkcJ-QeviE41aMZd-5.html&si=qWUtk1MEzQ4QKYLy With the PCRaster Tools plugin you can use pit tool and use the result as an input for the catchment tool. That's the most simple way.
Thanks for another great video! I see that we always have to specify the outlet. Is there a way to delineate sub-catchment for tributaries within a larger catchment, without specifying all its outlet individually?
Thanks. Have a look at Tutorial 6 on this page: courses.gisopencourseware.org/course/view.php?id=10 If you follow the SAGA approach (ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html) you can calculate the junctions in the Channel network and drainage basins tool and use that as an input to iteratively calculate the subcatchments of those points.
Hello, Sir. I can't produce a good watershed @ 20:15 because one of my streams wrongly flows to the opposite direction ( i also noticed it form the flow accumulation step). Is there any option to reverse its course? Thanks.
Hi, I have 2 points where I got different outputs. When I tried to change the values for flowdirabs it would not just change the label but the output as a whole, which was not in your case. The second was for the river, I got it on value 0 but not on 1. I apologise if it's hard to read but let me know if you would like me to rephrase, thanks.
These are negative flow accumulation values indicating that those cells have surface runoff from outside of the current geographic region. Meaning they don't belong to the catchment that we're going to derive.
@@jiongwang7645 FYI a step by step tutorial with data is available at GIS OpenCourseWare: www.gisopencourseware.org You can find it under the Advanced QGIS tutorials.
Maybe the resolution of your DEM is so high that it is a DSM instead of a DTM. You could resample to a courser resolution. Or get a layer with bridges (e.g. through QuickOSM), rasterize them and apply some mapalgebra to reduce the elevation at the bridge.
Thank you very much for the video Sir. Anyway, what is the unit of flow accumulation? Because on the channels creation step, you set the flow accumulation should be bigger than 50,000. Is it in m3/s?
Flow accumulation is the number of upstream cells that contribute to the cell. It's not m3/s, because there's no rainfall as input. So basically it is a unit of rain on each cell and then adds it up. A threshold >50k means that only cells that have more than 50k contributing cells are considered as a river. You need to calibrate that number by looking at existing maps or satellite images.
Hello Dr Hans. I have faced great difficulty of delineating watershed of a terminal lake, a lake which has inlets but no outlets (Lake Hawassa in Ethiopia). I tried repeatedly using all your suggestions, be it changing the DEM or alternating between PCRaster, SAGA, Whitebox, GRASS, SWAT and stream burning. I think the problem is with the filling process. But no idea how to go about it. Unfortunately there is no any UA-cam video on how to deal with such terminal lakes. Can you do one please. Thank you in advance.
@@HansvanderKwast Thanks but the filling process step is done before outlet designation. At the filling step, the lake is already filled and the streams are pushed away from the reality. I wish you could take a couple of your precious minutes to delineate Lake Hawassa, in Ethiopia (which is a typical terminal lake) and check for yourself using any one of your delineation procedures, which I am strictly following.
Hi thanks for the tutorial. I was able to make it to almost the very end but when I run r.waterout it kept showing SetColorTable() only supported for Byte or UInt16 bands in TIFF format. How to solve it?
Hey thank you very much for this video. For me tho it’s still not working. I can’t do a vector of streams and r.stream.extract is too much processing for my pc. The water outlet with my absolut flow direction (which is 0 to 128) is always 1-3 pixel big, what do I need to change?
I'm not sure what could go wrong. I prefer using the workflow with the PCRaster Tools plugin. You can find lots of videos on my channel or check this workshop: courses.gisopencourseware.org/course/view.php?id=60. I think it's more robust than other tools.
@13:30 you went WAY to fast for the color editing! I slowed it down to .25 speed and I'm still having trouble following. What exactly were you doing with the color picker and the stops???
Same for the layer styling... I labeled everything (North, North East...), but when I finish, North is not on top. The Value is 2, but your North is on top. I really wish you would have explained this in better detail instead of blow through it.
It's covered in other videos. For example this one: ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html and this one ua-cam.com/video/q54xTyV7gAE/v-deo.html. Also in the book QGIS for Hydrological Applications: locatepress.com/book/hyd. Info about encoding of flow direction by different tools: gis.stackexchange.com/questions/254939/legend-of-flow-direction-calculated-with-processing-toolbox-qgis-saga.
@@HansvanderKwast Thank you for the reply. Apologies for sounding snippy. I am trying to complete a catchment basin for my boss and it's taking days because all I've ever used is ArcMap + extensions, so I am having to teach myself QGIS with no one to help except UA-cam. I was able to complete up to the accumulation layer, but my values for the main channel are coming out as -1000 to -100000+, but I know for a fact this is the main channel the tributaries are feeding into. Some of the tributaries are giving a range in the 2000s, but others are -3000s. Where did I go wrong?
@@joshuah.9687 There can be many reasons. I prefer to use methods with the Strahler order, because they're more intuitive. For teaching I use my own PCRaster Tools plugin. It's very robust. Check this tutorial: courses.gisopencourseware.org/course/view.php?id=48§ion=1 You can also use the SAGA workflow: ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html. If you're in a hurry use the models for SAGA here: plugins.qgis.org/models/ or for PCRaster via the Resource Sharing plugin: ua-cam.com/video/KbjIsFL_PcM/v-deo.html If you're in a super hurry, just use Lekan: www.reos.site/en/reos-project/lekan/
Hi Dr Hans. I followed your steps on this video, but my projection might be off? when I ran the Fill on the cliped DEM it displays it at a nother location. Why would this be? Kind Regards Hanjo Stark
In the newer versions there's no separate way to start QGIS if you want to use GRASS. But you need to make sure you have GRASS installed. See ua-cam.com/video/pja_EX0tVZA/v-deo.html
Dear Dr.Hans. Many thanks for the video and for all your continuous free support in this field. I have followed the steps in this video to extract the streams network of my study area ( big area). Please, is it possible to convert the extracted raster (the layer channels of values 0 and 1 only as described at time16.36 of your video) to a vector layer of streams network? How to do that and convert only the streams (the 1 value) to a vector without including the 0 value and no data value extent? with many thanks in advance
Thank you very much for the video! I've found all your video's extremely helpful! I've been trying it out for my own project in Uganda but the result (both through GRASS tools and SAGA tools) channels are not always accurate to rivers displayed on basemaps. Even though a lot of rivers are the same, others are split in two different big rivers while on the basemaps it's just one river. I was wondering how I can alter the results so the channels are displayed more realistically. I've tried different SRTM layers, different tools, etc. but they all display 2 different rivers instead of 1.
You can try to improve the results by burning in the river network. There are 2 videos about that on my channel. Burning in the river network will force that the DEM is lower where the rivers are. Your problem can be caused by (1) human manipulation of the stream flow, (2) errors in the DEM, (3) resolution of the DEM in relation to local depressions, such as wetlands and lakes, ...
While specifying thd outlet .....You are searching for the capturing thd outlet point....However if we are already having the steam guage lat ang long data.....How to assign that as outlet data point..... please answer
how to calculate the area of the respective stream order? we can calculate the length of each order stream but how to calculate the area? 1st order , 2nd order etc?.
These methods are pixel based. So you don't get an area of the stream, it's limited to 1 pixel and it's spatial resolution. If you want real areas, you need to find a way to derive the channel width from the DEM or through remote sensing classifcation. Once you have the polygons, you can calculate the area. If the river is available as polygons in OpenStreetMap you can also use that.
HI Hans, Can you please explain, what does this (-) Flow accumulation mean? do we have to convert this "-" accumulation also into "+" values? Problem: 1) I have a catchment which shows higher negative flow accumulation than "+" values, this "-" flow accumulation defines my river network exactly than positive flow accumulation 2) Why D8 is better than MFD Method?
Negative flow accumulation means that it's outside your catchment. More info here: grass.osgeo.org/grass76/manuals/r.watershed.html D8 is not better than MFD. What you want to use depends on your area. In science you start with the most simple model. In this case D8. MFD might give better results in flatter areas. There are also other algorithms. See for example this document: rivix.com/Topics/D8_vs_Dinf.php
The algorithms don't change. But to avoid errors I recommend to use the LTR versions. The problems have normally more to do with the steps: - do not use a DEM without reprojecting from GCS to a projection with x, y in meters. Interpolarate voids, otherwise no continuity of the flow. Fill the sinks, otherwise a lot of disrupted streams. Don't use a very high resolution, especially if you have a large catchment. Don't use spaces, dots, special characters in folder and file names. The area should also not be too flat or too human modified. Outlets should be chosen in the delineated stream, otherwise it will not follow the model.
@@HansvanderKwast thanks a lot for your tips. I have two questions, is it the same reprojecting the dem than simply using the right click feature of exporting and choosing the desired SRC? I‘ve not seen in your videos the voids interpolation, does the fill tool make also that?
@@SuperDanisse Right click is available from version 3.16. The videos are made with older versions. This video shows how to interpolate voids: ua-cam.com/video/FwJSBoTn1y4/v-deo.html
I am not getting osm standard map eventhough I had installed the latest version of plugins and the status of QGIS official plugin repository is unavailable....plzz suggest any solution for this problem..
Probably something wrong with your installation. Normally this is solved when you make a new profile. Go to Settings | Use Profiles | New Profile... and choose a simple name without spaces or strange characters. Note that you need to start QGIS with GRASS for the procedure in the video.
Thanks for this helpful tutorial, I am learning much! In regard to the raster calculator: for calculating absolute values, you can simply use ABS(flowdir@1)
Greetings and thank you for your video ! Any idea about issues with GRASS saying the layers were not correctly generated ?
Dear Hans
Thank for your tutorials, please how can delineate multiple catchments at once
It depends what you need. There are tools that allow you to give multiple user-defined outlets or tools that can automatically find the pits and use them for delineation. Check out this playlist: ua-cam.com/play/PLeuKJkIxCDj2Gk0CkcJ-QeviE41aMZd-5.html&si=qWUtk1MEzQ4QKYLy
With the PCRaster Tools plugin you can use pit tool and use the result as an input for the catchment tool. That's the most simple way.
Thanks for another great video! I see that we always have to specify the outlet. Is there a way to delineate sub-catchment for tributaries within a larger catchment, without specifying all its outlet individually?
Thanks. Have a look at Tutorial 6 on this page: courses.gisopencourseware.org/course/view.php?id=10
If you follow the SAGA approach (ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html) you can calculate the junctions in the Channel network and drainage basins tool and use that as an input to iteratively calculate the subcatchments of those points.
Hello, Sir.
I can't produce a good watershed @ 20:15 because one of my streams wrongly flows to the opposite direction ( i also noticed it form the flow accumulation step). Is there any option to reverse its course? Thanks.
That's due to the DEM and maybe the filling process. Try another DEM or another workflow (PCRaster, SAGA, Whitebox).
Hello Hans, would you help us to find command area from a given canal/river in QGIS??
Hi, I have 2 points where I got different outputs. When I tried to change the values for flowdirabs it would not just change the label but the output as a whole, which was not in your case. The second was for the river, I got it on value 0 but not on 1.
I apologise if it's hard to read but let me know if you would like me to rephrase, thanks.
Big fan. Very helpful thank you very much sir
What happened to the upper left corner that downstream values are very dark (low) at 14:25?
These are negative flow accumulation values indicating that those cells have surface runoff from outside of the current geographic region. Meaning they don't belong to the catchment that we're going to derive.
@@HansvanderKwast Thanks a lot for detailed explanation. Your video is very helpful.
@@jiongwang7645 FYI a step by step tutorial with data is available at GIS OpenCourseWare: www.gisopencourseware.org
You can find it under the Advanced QGIS tutorials.
After the Raster calculator step I am getting an error "Raster couldnot create destination file". How can I solve this problem?
Hi! Great video! How can I remove bridges from DEM? Problem is that when channel hits a bridge it redirects to a nearby catchment area.
Maybe the resolution of your DEM is so high that it is a DSM instead of a DTM. You could resample to a courser resolution. Or get a layer with bridges (e.g. through QuickOSM), rasterize them and apply some mapalgebra to reduce the elevation at the bridge.
@@HansvanderKwast it's 1x1m grid. But smaller bridges aren't removed. I Will try to modify it. Thank you.
Thanks a lot dear Dr.Hans
Do you think that GRASS tools plugin is suitable to do the same process.
This procedure was done with te GRASS tools in the Processing Toolbox.
@@HansvanderKwast Thanks for this really helpful video. Could you comment on why you chose this process over the tools in r.watershed?
@@jeremymorris1307 r.watershed creates all basins. If you want to delineate the catchment of a specific outlet you need to use this procedure.
Thank you very much for the video Sir. Anyway, what is the unit of flow accumulation? Because on the channels creation step, you set the flow accumulation should be bigger than 50,000. Is it in m3/s?
Flow accumulation is the number of upstream cells that contribute to the cell. It's not m3/s, because there's no rainfall as input. So basically it is a unit of rain on each cell and then adds it up. A threshold >50k means that only cells that have more than 50k contributing cells are considered as a river. You need to calibrate that number by looking at existing maps or satellite images.
Hello Dr Hans. I have faced great difficulty of delineating watershed of a terminal lake, a lake which has inlets but no outlets (Lake Hawassa in Ethiopia). I tried repeatedly using all your suggestions, be it changing the DEM or alternating between PCRaster, SAGA, Whitebox, GRASS, SWAT and stream burning. I think the problem is with the filling process. But no idea how to go about it.
Unfortunately there is no any UA-cam video on how to deal with such terminal lakes. Can you do one please. Thank you in advance.
You need to delineate the catchments with outlets at the lake shores. Don't include the lakes, because they will be filled.
@@HansvanderKwast Thanks but the filling process step is done before outlet designation. At the filling step, the lake is already filled and the streams are pushed away from the reality. I wish you could take a couple of your precious minutes to delineate Lake Hawassa, in Ethiopia (which is a typical terminal lake) and check for yourself using any one of your delineation procedures, which I am strictly following.
@@miraclebuoy Just mask the lake in the DEM so it has nodata values.
Hi thanks for the tutorial. I was able to make it to almost the very end but when I run r.waterout it kept showing SetColorTable() only supported for Byte or UInt16 bands in TIFF format. How to solve it?
That's no problem. You don't need a color table. Check if the output has been generated.
Hey thank you very much for this video. For me tho it’s still not working. I can’t do a vector of streams and r.stream.extract is too much processing for my pc.
The water outlet with my absolut flow direction (which is 0 to 128) is always 1-3 pixel big, what do I need to change?
I'm not sure what could go wrong. I prefer using the workflow with the PCRaster Tools plugin. You can find lots of videos on my channel or check this workshop: courses.gisopencourseware.org/course/view.php?id=60. I think it's more robust than other tools.
@13:30 you went WAY to fast for the color editing! I slowed it down to .25 speed and I'm still having trouble following. What exactly were you doing with the color picker and the stops???
Same for the layer styling... I labeled everything (North, North East...), but when I finish, North is not on top. The Value is 2, but your North is on top. I really wish you would have explained this in better detail instead of blow through it.
It's covered in other videos. For example this one: ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html and this one ua-cam.com/video/q54xTyV7gAE/v-deo.html. Also in the book QGIS for Hydrological Applications: locatepress.com/book/hyd. Info about encoding of flow direction by different tools: gis.stackexchange.com/questions/254939/legend-of-flow-direction-calculated-with-processing-toolbox-qgis-saga.
You can also download the ramp from the QGIS Style Hub: plugins.qgis.org/styles/116/
@@HansvanderKwast Thank you for the reply. Apologies for sounding snippy. I am trying to complete a catchment basin for my boss and it's taking days because all I've ever used is ArcMap + extensions, so I am having to teach myself QGIS with no one to help except UA-cam. I was able to complete up to the accumulation layer, but my values for the main channel are coming out as -1000 to -100000+, but I know for a fact this is the main channel the tributaries are feeding into. Some of the tributaries are giving a range in the 2000s, but others are -3000s. Where did I go wrong?
@@joshuah.9687 There can be many reasons. I prefer to use methods with the Strahler order, because they're more intuitive. For teaching I use my own PCRaster Tools plugin. It's very robust. Check this tutorial: courses.gisopencourseware.org/course/view.php?id=48§ion=1
You can also use the SAGA workflow: ua-cam.com/video/2Ub0c7Ss-T4/v-deo.html.
If you're in a hurry use the models for SAGA here: plugins.qgis.org/models/ or for PCRaster via the Resource Sharing plugin: ua-cam.com/video/KbjIsFL_PcM/v-deo.html
If you're in a super hurry, just use Lekan: www.reos.site/en/reos-project/lekan/
Hi Dr Hans. I followed your steps on this video, but my projection might be off? when I ran the Fill on the cliped DEM it displays it at a nother location. Why would this be?
Kind Regards
Hanjo Stark
Did you try to set the proper 'extent' of the map when using r.fill.dir tool?
hi, how to have QGIS with GRASS?, I've downloaded the latest one but I don't see the 'QGIS with GRASS', it only shows 'QGIS Desktop 3.22.2'
In the newer versions there's no separate way to start QGIS if you want to use GRASS. But you need to make sure you have GRASS installed. See ua-cam.com/video/pja_EX0tVZA/v-deo.html
Dear Dr.Hans. Many thanks for the video and for all your continuous free support in this field. I have followed the steps in this video to extract the streams network of my study area ( big area). Please, is it possible to convert the extracted raster (the layer channels of values 0 and 1 only as described at time16.36 of your video) to a vector layer of streams network? How to do that and convert only the streams (the 1 value) to a vector without including the 0 value and no data value extent? with many thanks in advance
You have to use the r.thin and r.tovect tools to do the conversion. See ua-cam.com/video/pq_Ug2M4bmY/v-deo.html
@@HansvanderKwast Many thanks dear Dr.Hans.
Thank you very much for the video! I've found all your video's extremely helpful! I've been trying it out for my own project in Uganda but the result (both through GRASS tools and SAGA tools) channels are not always accurate to rivers displayed on basemaps. Even though a lot of rivers are the same, others are split in two different big rivers while on the basemaps it's just one river. I was wondering how I can alter the results so the channels are displayed more realistically. I've tried different SRTM layers, different tools, etc. but they all display 2 different rivers instead of 1.
You can try to improve the results by burning in the river network. There are 2 videos about that on my channel. Burning in the river network will force that the DEM is lower where the rivers are. Your problem can be caused by (1) human manipulation of the stream flow, (2) errors in the DEM, (3) resolution of the DEM in relation to local depressions, such as wetlands and lakes, ...
Thank you very much for your answer! I burned it and it displayed very accurate rivers now compared to my basemap! You're my hero
While specifying thd outlet .....You are searching for the capturing thd outlet point....However if we are already having the steam guage lat ang long data.....How to assign that as outlet data point..... please answer
I've made a new video to answer your question: ua-cam.com/video/5XmPnKCMZHg/v-deo.html. Hope it's useful.
how to calculate the area of the respective stream order? we can calculate the length of each order stream but how to calculate the area? 1st order , 2nd order etc?.
These methods are pixel based. So you don't get an area of the stream, it's limited to 1 pixel and it's spatial resolution. If you want real areas, you need to find a way to derive the channel width from the DEM or through remote sensing classifcation. Once you have the polygons, you can calculate the area. If the river is available as polygons in OpenStreetMap you can also use that.
@@HansvanderKwast thank you so much sir.
How can I delineate river and catchment area of a plain area using QGIS
Probably only manually by digitizing and expert knowledge.
HI Hans, Can you please explain, what does this (-) Flow accumulation mean? do we have to convert this "-" accumulation also into "+" values?
Problem:
1) I have a catchment which shows higher negative flow accumulation than "+" values, this "-" flow accumulation defines my river network exactly than positive flow accumulation
2) Why D8 is better than MFD Method?
Negative flow accumulation means that it's outside your catchment. More info here: grass.osgeo.org/grass76/manuals/r.watershed.html
D8 is not better than MFD. What you want to use depends on your area. In science you start with the most simple model. In this case D8. MFD might give better results in flatter areas. There are also other algorithms. See for example this document: rivix.com/Topics/D8_vs_Dinf.php
@@HansvanderKwast Thanks I will have a look on this documents
I am also getting negative accumulations - did you manage to fix it, if so how?
@@cameronciaglia7171 I just converted it into positive values.
@@maiyourathaankarunanandara3382 how you converted negative values to positive
How to find out the centroid of the catchment in QGIS?
In the main menu of QGIS go to Vector | Geometry Tools | Centroids
@@HansvanderKwast thank you so much sir.
I just discovered your channel and I love it. Does it matter the version of QGis?
The algorithms don't change. But to avoid errors I recommend to use the LTR versions. The problems have normally more to do with the steps:
- do not use a DEM without reprojecting from GCS to a projection with x, y in meters. Interpolarate voids, otherwise no continuity of the flow. Fill the sinks, otherwise a lot of disrupted streams. Don't use a very high resolution, especially if you have a large catchment. Don't use spaces, dots, special characters in folder and file names. The area should also not be too flat or too human modified. Outlets should be chosen in the delineated stream, otherwise it will not follow the model.
@@HansvanderKwast thanks a lot for your tips. I have two questions, is it the same reprojecting the dem than simply using the right click feature of exporting and choosing the desired SRC? I‘ve not seen in your videos the voids interpolation, does the fill tool make also that?
@@SuperDanisse Right click is available from version 3.16. The videos are made with older versions. This video shows how to interpolate voids: ua-cam.com/video/FwJSBoTn1y4/v-deo.html
I am not getting osm standard map eventhough I had installed the latest version of plugins and the status of QGIS official plugin repository is unavailable....plzz suggest any solution for this problem..
Sounds like QGIS can't connect to your internet.
Not getting the catchment boundary after applying r.outlet
Check if you have chosen an outlet in a delineated stream that corresponds with the filled DEM.
i had the same error as you. you have to make sure your project CRS is the same as the selected layer CRS (in this case the flow direction layer)
i am not getting build virtual raster option
Probably something wrong with your installation. Normally this is solved when you make a new profile. Go to Settings | Use Profiles | New Profile... and choose a simple name without spaces or strange characters. Note that you need to start QGIS with GRASS for the procedure in the video.
where can i get the bounding box shp sir??
akhirnya ada yg nanya ini wkwk
You have to create a bounding box polygon yourself, based on your knowledge on the approximate extent of the study area.
Txuuu