How to implement chunk_day in new syntax

General
1

How do I implement the old switch chunk_day using the new type of syntax sim.run()?

We use it since we sometime have memory issues in Windows env., or maybe this is not needed anymore?

2

Ok, found it.

“run_kwargs (dict) –

Note: Additional keyword arguments are currently not supported due to underlying function signature constraints. This parameter is reserved for future use.

In a future version, the following options may be supported: - save_state: bool - Save state at each timestep (planned) - chunk_day: int - Days per chunk for memory efficiency (planned)

For now, simulations use default settings: - save_state=False (states not saved at each step) - chunk_day=3660 (approximately 10 years per chunk)”

When do you plan to implement this again?

3

This topic has been tracked as a GitHub issue for the development team.

GitHub issue: How to implement chunk_day in new syntax · Issue #1195 · UMEP-dev/SUEWS · GitHub

The team will follow up there. Updates will be posted in the GitHub issue.

4

Hasn’t it been implemented yet? I’ve been using sim.run(chunk_day=31) with no error messages in 2025.11.20rc1.

5

Good question @Janka. In 2025.11.20rc1, sim.run() did accept chunk_day without raising an error, but it was actually silently ignored — the parameter got caught by the **run_kwargs catch-all and was never passed through to the simulation engine. So your runs were processing the full forcing period in a single pass regardless of the value you set.

This has been fixed (PR #1172, merged 30 Jan 2026) — chunk_day is now an explicit parameter on sim.run() and properly wired through to the backend. It’ll be available in the next public release.