UKC

River catchment areas using Q GIS

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 David Barratt 10 Jan 2014
Anyone ever used QGIS for defining hydrological catchments based on an underlying DTM? I've found this question asked a number of times on various forums, but none of the answers satisfy me. Currently I just do it manually by transforming the DTM in 1-10m contours and interpreting what I see... but surely there is an automated way of doing it...

Ideally, I want to define a point and click 'climb' or something... and then it just outputs a catchment polygon.

I have QGIS version 2 and have GRASS installed as a plugin.

Very obscure for UKC, I know.
OP David Barratt 10 Jan 2014
In reply to David Barratt:

Hmmm... I had a feeling nobody would know.
 Jonny2vests 10 Jan 2014
In reply to David Barratt:

Well, it's been a while and I didn't want to guess at an answer, I was hoping someone else might chip in. What I do remember is that the PhD student that sat next to me did flood modeling in ArcGIS and he always said that simple use of DTMs like that, especially in Urban Areas was farcically simplistic. His name is Jochen Schubert and he went to Nottingham if you want to look up his thesis, it'll probably be online.
OP David Barratt 10 Jan 2014
In reply to Jonny2vests:

Thanks, the actual flow modeling (while not something that I do using GIS) is reasonably simple as far as I can see. What I am trying to do SHOULD be even simpler. If I get really stuck, I might look your friend up, although Arc and Quantum are different so he may be unable to help. Maybe I'll continue doing it manually, I trust my judgment more than a computers anyway!
 daWalt 10 Jan 2014
In reply to David Barratt:

don't often see "hydrological" in these forum posts, I do that sort of stuff for a living.
what are you doing this for? (out of interest)

It’s straightforward (built in function) using Spatial Analyst in ArcGIS,
Can’t help on QGIS tho
Generally it’s not totally reliable, can be woefull where there’s urban stuff or it’s a really flat area.
Mostly it’s just a quick to do it by hand as you have to check everything anyway.
OP David Barratt 10 Jan 2014
In reply to daWalt:

Thanks, pity I'm stuck with Q GIS for now! I'm a hydrologist so also do it for a living. On this occasion it's for low flows analysis. CatchmentsUK is 'ok' but it only uses a 1:50,000 DTM. most of my catchments are small enough that I can just spend 15 mins doing it manually by transforming my DTM into 1m contours or something. I'll have a hunt for the arc equivalent tool, or try and convince work just to buy me Arc (that will never happen).
 Jim Fraser 10 Jan 2014
In reply to David Barratt:

Fascinating! To look at the news, you would think there was nobody doing this for a living in the UK.

So you guys are telling us that there is a genuine technical discipline where people know that water flows downhill? I would never have known.

Presumably, none of you are in Inverness? We have a real problem here with that type of knowledge. Town built on 250m of gravel and we still manage to flood it. That takes skill.
TOS 10 Jan 2014
In reply to Jim Fraser:

> Fascinating! To look at the news, you would think there was nobody doing this for a living in the UK.

I think you'll find the problem is more the councils who don't listen to these guys and bury their heads in the sand.
 jamie84 10 Jan 2014
In reply to David Barratt:

Hi, I do this for a living too! In QGIS v2, use the geoprocessing toolbox there are hydrology functions under the 'domain specific tab' which I've previously used for catchment definition. It involves more than one function from memory - my notes are in work at the moment. Similar to Arc.
 toad 10 Jan 2014
In reply to David Barratt: If you're still struggling next week, PM me and I'll pass you on to someone who might be able to help.

I'm afraid he's a boulderer, though...

OP David Barratt 11 Jan 2014
So there's a few of us on here! I'm based in Edinburgh so not to blame for Inverness! And I would agree, the most important piece of knowledge is that water flows down hill! (shhh... keep that one quiet)

Jamie84 - yeah, I've found that tool box, there're A LOT of tools in there! I can get lovely raster maps showing flow direction and channels etc. but as for a polygon climbing from a single point... it fails me.

Toad - I may well PM you!

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