Multiple lake outlets ignored warning
Yuxiang Ren
SWAT+ Issue Report Template
1. Primary Issue Location
Component: QSWAT+
Area of impact: Model input / predefined watershed and channel network /
lake-reservoir definition
2. Software Versions
GIS Interface: QSWAT+ 3.03
3. Project Characteristics
Watershed Details: 2500km2
Modeling Aim: water balance,crop growth
4. Description of Issue
When I use a predefined watershed and channel network to describe a watershed containing four connected lakes, QSWAT+ gives three warnings about separate outlets for Polygon NULL.
The warning messages are:
2026-06-09T12:12:29 INFO WARNIING: Polygon NULL has separate outlets at (597184.6263250151, 2907165.0) and (595815.0, 2891032.489867424): ignoring second
2026-06-09T12:12:29 INFO WARNIING: Polygon NULL has separate outlets at (597184.6263250151, 2907165.0) and (623275.5686421097, 2832145.5686421096): ignoring second
2026-06-09T12:12:30 INFO WARNIING: Polygon NULL has separate outlets at (597184.6263250151, 2907165.0) and (605871.8089067113, 2877218.191093289): ignoring second
The point: (597184.6263250151, 2907165.0) is the outlet of Lake 3. It seems that QSWAT+ treats this point as the first outlet of Polygon NULL (I also cant find the NULL in watershed or subbasin shp), and then treats the other lake outlet points as additional outlets of the same polygon and ignores them as “second” outlets.
I then tried to continue the workflow despite these warnings. In the Create HRUs step, the Landuse and Soil part can be completed. However, when moving to the HRUs section, QSWAT+ reports routing/outlet errors such as:
CRITICAL Failed to find outlet for basin 3099 on path [3099, 5002, 5003, 1002, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099]
2026-06-09T12:14:00 CRITICAL Path has loop length 4
2026-06-09T12:14:01 CRITICAL Cannot find watershed outlet for subbasin 1
Similar Cannot find watershed outlet for subbasin ... errors occur for all subbasins.
To test whether this issue is related to the multiple connected lakes, I made a simplified test case. I clipped the watershed to an area containing only one lake, Lake 1. In this simplified model, I did not provide an outlets/inlets shapefile. In that case, QSWAT+ did not report these outlet warnings, and HRUs could be created successfully.
Therefore, I suspect that the problem may be related to how the outlets of multiple connected lakes are represented or interpreted in the predefined watershed/channel setup.
I have tried the following two options, but neither solved the problem:
- Using channels.shp without DSNODEID, and not providing an outlets/inlets shapefile.
- Using channels.shp with DSNODEID, and providing an outlets/inlets shapefile.
In both cases, the warning about separate outlets for Polygon NULL still occurs, and the subsequent HRU creation still fails.
My main question is:
Which input files, attribute fields, or routing relationships should I check or modify to prevent the lake outlets from being ignored and to avoid the following warning when multiple connected lakes exist in the watershed?
WARNIING: Polygon NULL has separate outlets at (597184.6263250151, 2907165.0) and (605871.8089067113, 2877218.191093289): ignoring second
5. Supporting Data (Optional)
Link to project folder:
https://drive.google.com/drive/folders/1xbs7q-JJeg95kLhUUu4f6EWiYMEYijjh?usp=sharing
The Google Drive folder contains the input files used for this test case, including:
- DEM: strm_erhai.tif
- Subbasins shapefile: sub_f4.shp
- Watershed shapefile: watershed_f4.shp
- Channels shapefile: channels_f4.shp
- Lakes shapefile: lakes.shp
Chris George
The NULL arises from the BasinNo field in your channels shapefile. It
looks like you are not complying with the requirements for predefined
watersheds.
QSWAT+ will not allow multiple outlets from a lake. It chooses the
channel with the largest upstream drainage area. If you want multiple
outlets you would add the extra ones in the SWAT+ editor, and set the
percentage flows for the outlets from each lake to sum to 100.
The failure to find an outlet is because of the loop in the downstream
path from subbasin 3099, as you can see.
I suggest you check the requirements for predefined watersheds, and make
sure your files meet them. If the loop error recurs you should check
your files to see if there is really a loop in the way the channels are
connected. You should also use the latest QSWAT+.
Chris
On 6/9/2026 12:40 PM, Yuxiang Ren wrote:
> SWAT+ Issue Report Template 1. Primary Issue Location Component: QSWAT+
> Area of impact: Model input / predefined watershed and channel network /
> lake-reservoir definition 2. Software Versions GIS Interface: QSWAT+ 3.
> 03 3. Project Characteristics
> This Message Is From an External Sender
> This message came from outside your organization.
> ZjQcmQRYFpfptBannerEnd
>
> *SWAT+ Issue Report Template*
>
> *1. Primary Issue Location*
>
> *Component:* QSWAT+
> *Area of impact:* Model input / predefined watershed and channel
> network / lake-reservoir definition
>
> *2. Software Versions*
>
> *GIS Interface:* QSWAT+ 3.03
>
> *3. Project Characteristics*
>
> *Watershed Details:* 2500km2
>
> *Modeling Aim:* water balance,crop growth
>
> *4. Description of Issue*
> When I use a predefined watershed and channel network to describe a
> watershed containing four connected lakes, QSWAT+ gives three warnings
> about separate outlets for Polygon NULL.
>
> The warning messages are:
>
> 2026-06-09T12:12:29 INFO WARNIING: Polygon NULL has separate outlets
> at (597184.6263250151, 2907165.0) and (595815.0, 2891032.489867424):
> ignoring second
>
> 2026-06-09T12:12:29 INFO WARNIING: Polygon NULL has separate outlets
> at (597184.6263250151, 2907165.0) and (623275.5686421097,
> 2832145.5686421096): ignoring second
>
> 2026-06-09T12:12:30 INFO WARNIING: Polygon NULL has separate outlets
> at (597184.6263250151, 2907165.0) and (605871.8089067113,
> 2877218.191093289): ignoring second
>
> The point: (597184.6263250151, 2907165.0) is the outlet of Lake 3. It
> seems that QSWAT+ treats this point as the first outlet of Polygon NULL
> (I also cant find the NULL in watershed or subbasin shp), and then
> treats the other lake outlet points as additional outlets of the same
> polygon and ignores them as “second” outlets.
>
> I then tried to continue the workflow despite these warnings. In the
> However, when moving to the *HRUs* section, QSWAT+ reports routing/
>
> CRITICAL Failed to find outlet for basin 3099 on path [3099, 5002,
> 5003, 1002, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL,
> 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL,
> 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL,
> 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL, 4099, 1004, NULL,
> 4099]
>
> 2026-06-09T12:14:00 CRITICAL Path has loop length 4
>
> 2026-06-09T12:14:01 CRITICAL Cannot find watershed outlet for subbasin 1
>
> Similar Cannot find watershed outlet for subbasin ... errors occur for
> all subbasins.
>
> To test whether this issue is related to the multiple connected lakes, I
> made a simplified test case. I clipped the watershed to an area
> containing only one lake, Lake 1. In this simplified model, I did not
> provide an outlets/inlets shapefile. In that case, QSWAT+ did not report
> these outlet warnings, and HRUs could be created successfully.
>
> Therefore, I suspect that the problem may be related to how the outlets
> of multiple connected lakes are represented or interpreted in the
> predefined watershed/channel setup.
>
> I have tried the following two options, but neither solved the problem:
>
> inlets shapefile.
> 2. Using channels.shp with DSNODEID, and providing an outlets/inlets
>
> In both cases, the warning about separate outlets for Polygon NULL still
> occurs, and the subsequent HRU creation still fails.
>
> My main question is:
>
> Which input files, attribute fields, or routing relationships should I
> check or modify to prevent the lake outlets from being ignored and to
> avoid the following warning when multiple connected lakes exist in the
> watershed?
>
> WARNIING: Polygon NULL has separate outlets at (597184.6263250151,
> 2907165.0) and (605871.8089067113, 2877218.191093289): ignoring second
>
>
> *Link to project folder:*
> https://drive.google.com/drive/folders/1xbs7q-
> JJeg95kLhUUu4f6EWiYMEYijjh?usp=sharing <https://urldefense.com/v3/
> __https://drive.google.com/drive/folders/1xbs7q-
> JJeg95kLhUUu4f6EWiYMEYijjh?usp=sharing__;!!KwNVnqRv!
> EPwo0A2A7sdYQtYi_SW8SabzpSVz9aJ-dyOYYChmEr2we98TUNjgm68Q7Tl3BgdgngK_aZv-
> TYrC7OMP4hjHn5g$>
> The Google Drive folder contains the input files used for this test
> case, including:
>
> * Subbasins shapefile: sub_f4.shp
> * Watershed shapefile: watershed_f4.shp
> * Channels shapefile: channels_f4.shp
> * Lakes shapefile: lakes.shp
>
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