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Problème de délimitation de la grille du bassin versant

FaidessMwale

<\/HEAD>Je réalise la délimitation du bassin versant avec arcmap 9.3.1 et archydro 1.3.<\/SPAN>

Je suis correctement les étapes depuis le reconditionnement du MNT et tout fonctionne assez bien. Le problème survient lorsque j'arrive à la délimitation de la grille du bassin versant. Bien que le message indique que la délimitation de la grille du bassin versant est réussie, ce que j'obtiens ce sont des bassins délimités à certains endroits et des zones vides à d'autres endroits. Je pensais que peut-être j'avais fixé le seuil pour la définition du stream trop haut (4 % de l'accumulation maximale du flux) donc j'ai adopté la valeur par défaut de 1 %, mais cela ne fonctionne pas.<\/SPAN>

Quelqu'un peut-il m'aider s'il vous plaît ; je suis sur ce problème depuis plus d'une semaine maintenant.<\/SPAN>

Merci<\/SPAN>

Fai<\/SPAN><\/BODY><\/HTML>

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IxHour

I am carrying out watershed delineation with arcmap 9.3.1 and archydro 1.3.

I follow the steps properly from DEM reconditioning and things work just quite fine. The problem arises when i reach catchment grid delineation. although the message says catchment grid delineation is successful, what I get are catchments delineated in some places and empty patches in other places. I thought may be i set the threshold for stream definition too high(4% of maximum flow accumulation) so adopted the default 1% but it does not work.

Can somebody help please; I have been on this problem over a week now.

Thanks

Fai

I had the same problem when running catchment grid using BowlFdr and StrLnk. But it worked for me when i used the FdrFilled instead of the BowlFdr which is the result of the flow direction in lakes. Perhaps the Flow direction in Lakes function does not work well or the data was corrupted.

It is a good lesson learnt!
Hour Ix

MarkBoucher

but there are discontinuities along the main course within the lake area.

The discontinuities will appear to be there when zoomed out, but you could possibly see them when zoomed in closer. Its a screen resolution thing.

FaidessMwale

Thank you Jane.

The lowering of the stream definition threshold improved things tremendously but when I zoom in, in the small lake area (attached), I note that the defined streams follow the natural drainage quite perfectly but there are discontinuities along the main course within the lake area. I repeated this this to the lowest threshold (1 sq km) but with no lack.

ToC3.jpg

LornaMurison

What you really need to show on that image is the strlnk layer generated by ArcHydro. ArcHydro will generate a catchment for every segment of that layer. If you do not have a stlnk segment for the stream flowing into the lake(s) then try decreasing the stream generation threshold. Have a look at some of the highest flow accumulation values around where the river drains into the lake to get an idea of what value you should be using.
It seems as though the river layer you displayed in the second attachment doesn't agree with the flow direction grid. It makes it look as though the big lake at the top is connected to the river in catchment A, but if you look at the rivers shown in the first attachment, catchment A clearly does not drain anything from the lake.

FaidessMwale

Hie,

Thanks for replies.

yes, the black section in the northern part surrounds the lake, so it is a flat area. But other black areas are not along the shore line. I checked for stream segments. they are there but as you suggest, flow accumulation may not be enough to generate a stream. I also used a flow direction tool; the flow is not towsrds the catchment but away.

The thing is, the main river originate from a lake and flows for some distance into another small lake from which it resurfaces again and flows downstream (see attachment). I want to calculate the time of concentration for catchment around the main river below the big lake. From Hydrology(if I disregard the big lake), the catchment for the small lake should also be part of the river catchment. Now if i just use catchment A as defined by archydro, am I not disregarding a section of the catchment (which is the catchment of the small lake)? By the way, so archydro does not delineate catchments in low lying areas? what is the way to go around this problem?

Thank you

ToC2.jpg

MarkBoucher

I agree with Lorna.

It could be that the black areas drain away from the catchment outlet. Try the flow direction tool to see if the black areas actually drain via the watershed.

LornaMurison

Catchments are going to be delineated to where your stream segments are, it doesn't look like you have any in the black areas, is this the case? Are these areas close to the coastline? If they are, it's possible that flow accumulation in that area doesn't get high enough for streams to be defined because the streams begin close to the shoreline.

FaidessMwale

Hie

I repeated the process, I get better results (I dont know why) but still no catchments are delineated in certain areas.

I have attached a map. The black coloured parts represent undefined areas.

Any suggestion please?

Fai

ToC.jpg

FaidessMwale

Hie Mark,

I did; every step was followed diligently.

Fai

MarkBoucher

what I get are catchments delineated in some places and empty patches in other places.

Fai,
The empty patches could mean you didn't do the fill sinks step. Did you?
Mark

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