Switching precipitation to Gridded may need a Mode line for every met variable

[jacobrbeatty]

When HEC-RAS 7.0 computes a plan whose precipitation is Gridded, it appears to require an explicit Met BC=<variable>|Mode= line for every meteorological variable, not just precipitation. A source unsteady file converted from a model whose precipitation was point or constant may carry no Mode line for evapotranspiration, and the run then fails as:

Writing Event Conditions ...
Error processing event conditions

RAS writes a 12,872-byte plan HDF containing only Results/Summary — no Results/Unsteady — and exits 0.

What I proved, and what I did not

Proved, by single-variable bisect on HEC-RAS 7.0 April 2026:

Converting a Montevallo 2D plan to gridded imported-raster precipitation, adding or removing one line flipped the outcome. The two flow files differ only by that line and a cosmetic Flow Title:

$ diff u10 u12
1c1
< Flow Title=u09_plus_u11_tail
---
> Flow Title=u10_minus_ET_mode
17d16
< Met BC=Evapotranspiration|Mode=None

Flow file	Result HDF
u10, with the line	110,564,922 bytes — 63,927 cells, 241 timesteps
u12, without it	12,872 bytes — no Results/Unsteady, exit 0

A working RAS-authored reference (CityBirmingham.u08/u10/u11, all gridded) carries Met BC=Evapotranspiration|Mode=None, Met BC=Air Density|Mode=Constant and Met BC=Air Pressure|Mode=Constant. The one gridded file there that lacks the ET line, u09, is not referenced by any plan.

Not proved: I hit this on the imported-raster path (Gridded Source = GDAL Raster File(s)). configure_gridded_dss_precipitation is the DSS variant. The code gap is the same in both — it sets five precipitation entries and nothing for the other variables:

desired_entries = [
    ("Precipitation Mode", "Enable"),
    ("Met BC=Precipitation|Mode", "Gridded"),
    ("Met BC=Precipitation|Gridded Source", "DSS"),
    # + Gridded Interpolation, Gridded DSS Filename, Gridded DSS Pathname
]
RasUnsteady._replace_met_precipitation_keys(unsteady_path, desired_entries)

But I have not run the DSS path and am not claiming the same failure there. It writes the same Met BC=Precipitation|Mode=Gridded block, so the same parser requirement plausibly applies.

Note on the existing test

tests/test_rasunsteady_gridded_dss_precipitation.py asserts the precipitation block is written correctly, and its fixture starts from Met BC=Precipitation|Mode=Constant with no ET line. Since it never runs HEC-RAS, a file RAS would reject still passes. Not a criticism of the test — it tests what it says it tests — just worth knowing the gap isn't covered.

Suggested fix

After writing the precipitation entries, ensure a Mode exists for the other met variables without overwriting one the source already set:

for variable, default in (
    ("Evapotranspiration", "None"),
    ("Air Density", "Constant"),
    ("Air Pressure", "Constant"),
):
    if RasUnsteady._get_met_bc_value(lines, variable, "Mode") is None:
        RasUnsteady._set_met_bc_line(lines, variable, "Mode", default)

_set_met_bc_line already places these correctly — it replaces in place, else inserts after the last Met BC=<variable>| line — on every realistic flow file I checked (31 of them).

Ordering caution, learned the hard way: the Met BC block must stay contiguous and sit after Wave Mode=. I split it during development and HEC-RAS abandoned the project, created a blank one with a steady flow file, and appended a second project block to the .prj — silently, exit 0. Worth a comment wherever these lines get written.

One observation rather than a defect: _set_met_bc_line's fallback is insert_idx = len(lines), so a file with no Met BC=<variable>| lines at all would append at end of file, after the Non-Newtonian block. I could not find a genuine flow file that hits this — the only candidate on my machine was a 113-byte "Geometry Only" stub — so it's latent. Might be worth hardening if flow files are ever generated from scratch.

Environment

HEC-RAS 7.0 April 2026 · ras-commander 0.98.2 · Windows 11 · Python 3.14
Model: Montevallo 2D, 63,927 cells, State Plane Alabama West (ft)

[gpt-cmdr]

Submit a pull request, and update any example notebooks that utilize these API calls.