SWAT+ Toolbox Multi-Site Calibration: “Sequence contains no elements” Error

[Sangjoon Bak]

Dear SWAT+ Development Team,

I am currently trying to perform a multi-site calibration using SWAT+ Toolbox with monthly evapotranspiration observations from multiple spatial locations. I was able to successfully set up the observation data for Basin 1, but I encountered an issue when adding the observation data for Basin 3.

As shown in the attached screenshot, the observation data for Basin 1 (Monthly Evapotranspiration) is correctly recognized and displayed. However, the observation data for Basin 3 (Monthly Evapotranspiration) is displayed as NaN, even though the observation data are available for the calibration period. When I proceed with the calibration, SWAT+ Toolbox stops and returns the following error:

Sequence contains no elements

I have checked the observation data for Basin 3, but I have not been able to identify what is causing this issue.

Interestingly, I also tested the multi-site automatic calibration using different spatial levels. When I set the observation data at the HRU level, the multi-site automatic calibration works properly. However, when I use the LSU level or Basin level, the calibration does not work as expected.

This makes me wonder whether the issue may be related to how the different spatial units are handled in SWAT+ Toolbox. In particular, I would like to clarify the meaning of "Basin" in the multi-site calibration setup. Does "Basin" in the SWAT+ Toolbox correspond to a subbasin in the SWAT+ model structure, or does it represent a different spatial aggregation level?

I would greatly appreciate your guidance on the following questions:

1. Why does the observation data for Basin 3 appear as NaN while Basin 1 is read correctly?
2. Why does multi-site automatic calibration work at the HRU level but not at the LSU or Basin level?
3. Does Basin in the SWAT+ Toolbox correspond to a subbasin in the SWAT+ model structure, or are these different spatial concepts?
4. Is multi-site automatic calibration currently supported at the LSU and Basin levels?
5. If Basin-level multi-site calibration is supported, is there a specific configuration or observation-data format required?

1. Primary Issue Location

• Component: SWAT+ Toolbox – Calibration / Multi-Site Calibration
• Area of impact: Observation data reading and automatic calibration

2. Software Versions

• GIS Interface: 3.40
• SWAT+ Editor: 3.0.8
• SWAT+ Revision: 61.0.1 / 61.0.2
• SWAT+ Toolbox: v3.2.0

3. Project Characteristics

• Modeling Aim: Multi-site calibration using monthly evapotranspiration observations at different spatial levels (HRU, LSU, and Basin).

4. Description of Issue

Step to Reproduce:

1. Set up monthly evapotranspiration observation data for multiple sites (Basin 1 and Basin 3).
2. Select Multi-Site Calibration in the Calibration tab.
3. Select NSE as the objective function.
4. Select Calibration by Latin-hypercube Sampling Iterations (CALSI).
5. Run the automatic calibration.
6. Basin 1 observation data is displayed normally, but Basin 3 shows NaN.
7. The calibration then stops with the error: "Sequence contains no elements."
8. When the observation data are configured at the HRU level, multi-site automatic calibration works properly.
9. However, the same calibration does not work properly when the observation data are configured at the LSU or Basin level.

Could anyone please help me understand what might be causing this issue and whether this behavior is expected in the current version of SWAT+ Toolbox?

Thank you very much for your help and guidance.

Best regards,
Sangjoon Bak