QGIS and watershed delineation in Queensland

[Ted Webb]

I am a GIS rookie and thus far unable to install QGIS on my old Windows 7 computer, but that is a work in progress!!  

However I do have a few rural watershed related questions for now!

Assuming I have loaded the Queensland DEM file and I have entered the coordinates for the drain point of the watershed of interest, what other piece of information regarding rainfall rates do I need to enter?  And where can such information be found?  I live near (25KM) Gatton, QLD.

I'm also assuming the geographical boundary (area) of the watershed will be a dependent on drain point coordinates and independent of rainfall rate.  Flow (Q) of the watershed will be determined by area and rainfall rate.    In other words if QGIS returns a watershed area of 3 square KM, then it will always be  3 square KM for that drain point and will not change as a function of rainfall rates.  However, changing rainfall rate for the watershed will change the output flow.

Am I on the right track?  I welcome your responses!

Thank you

[Emma Hain]

Hi Ted

I'd like to point you in the direction of Hans van der Kwast  https://www.youtube.com/c/HansvanderKwast as he has a playlist on Hydrological Applications that may assist answer your question.

Kind regards

Em

Emma Hain — Product Manager/Senior GIS Analyst
em...@north-road.com

North Road Cartography • Development • Spatial Analysis north-road.com QGIS User Conference 2025 2-3 June, Norrköping, Sweden FOSS4G 2025 17-23 November, Auckland, New Zealand

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[Ted Webb]

Emma,

Thank you so much for your helpful reply!  I will certainly follow the link that you provided.

Have a good day!

Ted

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[Ted Webb]

Emma,

Again, thank you for your help.  I'm going to ask one more favor!  I am still trying to determine the area of the watershed in question and resorting to a manual topo map approach.  I'm wondering if you (or another member of the group) might have the DEM for southern Queensland and can give me the catchment area for a pourpoint of -27.372066,  152.319128  This is about 25 KM from Gatton QLD?  I will use this computed area to see how it coincides with my manual topo determination for the area.

Thank you,

Ted

[Adam Steer]

G'day Ted,all

The area of the catchment will vary based on what DTM /resolution and method is used. I wouldn't expect a computed area and a manually generated area to precisely match.

A lot of analysts use SRTM data (30m) which is available for everywhere for free. Otherwise take a look in Qld govt map services (QLD globe) and GA's ELVIS datasets (https://elevation.fsdf.org.au) - you might find more detailed elevation there as public, open data.

And applying what you learn in the videos Emma pointed to should get the rest of the way.

Catchment analysis is something many people in this forum generally do as consulting work.

As Even Roault (GDAL developer) puts it: software is free, time is generally not. So there's plenty of (paid) expertise and mentoring around if needed also.

Having said that, help on a self education journey quite often falls into the "sometimes free" part of time ;). And we're also always learning / coming across new things.

Hope that helps!

Adam

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[Ted Webb]

Adam,

Thank you so much for your input, very much appreciated.  I will certainly take your thoughts on board and visit the link you provided.

Ted

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