Number of Stream Order

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03-12-2021 11:56 PM
ss_ayeni
New Contributor II

Hi!

I had delineated my watershed and calculated my stream order using Strahler method. I am aware that number of stream orders should decrease from the lowest order to the highest order in the watershed. In my result, I discovered that the number of second order streams is smaller than the number of third order streams. I am worried what the problem is. Or what exactly I did wrong! I look forward to your suggestions to overcome this challenge. Thank you.

Shamusideen

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JayantaPoddar
MVP Esteemed Contributor

I have run the whole workflow. And your answer is correct. There are more number of Third order streams than second order streams.

I can see so many first and second order streams connecting with the third-order stream, resulting in higher number of third-order streams.

You may classify (symbology) and label the stream feature class based on the grid-code to verify the order visually.

Jayanta_Poddar_1-1615658509091.png

 

Changing the threshold value for filtering the cells to create the stream network (Field Calculator), may result in a different number of segments of stream order.



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JayantaPoddar
MVP Esteemed Contributor

This may happen if many first-order (or second-order) streams directly combine with third-order streams, resulting in more number of third-order streams. There is nothing wrong in it.

Jayanta_Poddar_0-1615624268368.png

 



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ss_ayeni
New Contributor II

Thank you for your comments and time. I got all you said. Are you now saying it is scientifically acceptable to have something like this in terms of stream number: Order I = 40, Order II = 16, Order III = 20; Order IV = 2?

I am still asking because 1) all the papers I have read on stream number reported a downward trend 2) I do not have any theoretical explanation for this result indicated below. I really want to be sure I can defend this result. Thank you.

Shamusideen.

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JayantaPoddar
MVP Esteemed Contributor

Could you share the DEM of your region of interest? And what threshold value have you used for filtering the flow accumulation cells? 

You may put the data in a shared drive (Google Drive/Dropbox) if the file size is larger than 5 MB (attachment limit here).

Let me test the results before I confirm.



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ss_ayeni
New Contributor II

Yes, I can provide my DEM. I need your email address.

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JayantaPoddar
MVP Esteemed Contributor

I have run the whole workflow. And your answer is correct. There are more number of Third order streams than second order streams.

I can see so many first and second order streams connecting with the third-order stream, resulting in higher number of third-order streams.

You may classify (symbology) and label the stream feature class based on the grid-code to verify the order visually.

Jayanta_Poddar_1-1615658509091.png

 

Changing the threshold value for filtering the cells to create the stream network (Field Calculator), may result in a different number of segments of stream order.



Think Location
ss_ayeni
New Contributor II

Thank you very much for your time. We truly have similar stream network. I had already labelled the stream to feature to reveal the order of the streams. I also agree with you that if the threshold value is reduced, there may be different number of segments of stream order. As a matter of fact, I once tried this twice or thrice using different lower threshold values but got denser stream networks. 

Now, based on your experience with my DEM, is this not a departure from the rule that says stream number decreases downward as the stream order increases? I am just wondering how this result can be defended.

Once again, thank you. Note with appreciation that you have boost my confidence level in this current task with your input thus far because at a point I was already doubting myself.  

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JayantaPoddar
MVP Esteemed Contributor

Is this a flood-prone area? A lower bifurcation ratio (n/n+1) where n denotes the stream order, may result in higher chance of flooding.



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ss_ayeni
New Contributor II

My bifurcation ratio ranges from 0.8 to 10 with a mean bifurcation ratio of 4.4. This may suggest a certain degree of flooding in the area. 

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