Tekabe Seifu The error message you're encountering, "Error Number: 91 - Error Message: Object reference not set to an instance of an object," is a common runtime error in programming and GIS software. It typically occurs when the software is trying to access an object or variable that hasn't been initialized or doesn't exist.

Here are some steps to troubleshoot and potentially resolve this issue:

1. Check Data Sources: Ensure that all the necessary data sources and layers required for watershed delineation in ARCSWAT are correctly specified and accessible. Verify that the DEM (Digital Elevation Model) data is correctly linked.

2. Data Integrity: Make sure the DEM data is in the correct format and is not corrupted. Errors can occur if there are issues with the DEM file itself.

3 Software Version Compatibility: Ensure that the version of ARCSWAT you are using (2012) is compatible with ARCGIS 10.7.1. Sometimes, using versions that are not fully compatible can lead to errors.

4. Environment Settings: Check if any required environment settings or variables are missing or misconfigured. This can include paths to data directories or temporary storage locations.

5. Updates and Patches: Make sure you have applied any necessary updates or patches for both ARCSWAT and ARCGIS. Sometimes, software updates address known issues.

6. Permissions: Ensure that you have the necessary permissions to access and modify data and files related to your project. Lack of permissions can lead to errors.

7. Documentation and Support Consult the official documentation for ARCSWAT and ARCGIS for troubleshooting guidance. You can also reach out to their support or user community for assistance. They may have specific solutions or workarounds for this error.

8. Debugging: If you have programming experience, you can try to debug the specific line of code mentioned in the error message (Procedure: error line). This may provide insights into what's causing the issue.

9. Backup and Restore: Consider restoring your project to a previous state if you have a backup. Sometimes, this can help if the error was introduced due to recent changes.

Remember to back up your project and data before making significant changes or modifications. Additionally, provide as much detail as possible when seeking assistance from forums, user communities, or technical support. Information about your project setup, data sources, and the steps leading to the error can be invaluable in diagnosing and resolving the problem.