Watershed Delineation Problem

[tanvir ahamed]

Hello Everyone,

I hope everyone is doing well. I am having some trouble with ArcSWAT while delineating my watershed. The ArcSWAT model delineates my watershed with a smaller area compared to the actual study area. I'm unsure how to resolve this issue. Every time, I noticed that the model does not delineate some part of the watershed. Any idea how to resolve this issue?

Thank you

Tanvir

[Chris George]

It is not clear what you mean by your actual study area.

If it is an island, so the DEM is inevitably clipped, you can reduce the
threshold or try using a grid model, both without marking any outlets.

If it is inland you cannot dictate the result to the delineation
software. If it calculates that some parts flow outward then they will
be excluded unless they are large enough to meet your threshold.
Reducing the threshold will include more, as will using a grid model.
Or if you have suitable subbasin and stream maps you can model as an
existing watershed.

Chris

On 7/6/2025 4:06 PM, tanvir ahamed wrote:
> Hello Everyone, I hope everyone is doing well. I am having some trouble
> with ArcSWAT while delineating my watershed. The ArcSWAT model
> delineates my watershed with a smaller area compared to the actual study
> area. I'm unsure how to resolve this

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[tanvir ahamed]

Dear Chris,

Thank you for your kind response. My study area is not an island. I have experimented with various threshold values; however, the model consistently delineates a watershed with a smaller area than expected. In the attached image, I have marked the specific region that the model fails to include, regardless of the threshold value applied. I have also verified that my DEM is in TIFF format, has a resolution of 10 m × 10 m, and uses the projected coordinate system WGS 1984 UTM Zone 18N. 

I was wondering if you have any suggestions to resolve the issue?

Thank you

Tanvir