Subcatchments#

Note

Engine: OpenSWMM 6 — refactored. This page documents the openswmm.engine.Subcatchments class. Legacy SWMM 5 users access subcatchments through the enum-driven getValue / setValue API on openswmm.legacy.engine.Solver — see Legacy SWMM 5 Solver.

Sub-areas of urban land that produce runoff in response to rainfall. The Subcatchments class is the entry point for reading and writing subcatchment geometry, infiltration parameters, landuse coverage, and per-step hydrology state.

Reference: openswmm_subcatchments.h.

Class signature#

class Subcatchments:
    def __init__(self, solver: Solver) -> None: ...

A single Subcatchments instance covers every subcatchment in the model. Address by string id or integer index.

Key methods#

Identity#

Method	Returns
`count()`	Number of subcatchments.
`get_index(id)()`	Integer index for a string id.
`get_id(idx)()`	String id for an integer index.
`add(id)()`	Append a new subcatchment.

Geometry & connectivity#

Method	Action / returns
`get_area()` / `set_area()`	Total subcatchment area.
`get_width()` / `set_width()`	Characteristic flow width.
`get_slope()` / `set_slope()`	Average surface slope.
`get_imperv_pct()` / `set_imperv_pct()`	Impervious-area fraction (0–100%).
`get_outlet()` / `set_outlet(node_idx)()`	Receiving node index.
`get_outlet_subcatch()` / `set_outlet_subcatch(sc_idx)()`	Receiving subcatchment (chained routing).
`get_gage()` / `set_gage(gage_idx)()`	Rain gage index that drives this subcatchment.

Surface roughness & depression storage#

Method	Action / returns
`get_n_imperv()` / `set_n_imperv()`	Manning’s n over impervious area.
`get_n_perv()` / `set_n_perv()`	Manning’s n over pervious area.
`get_ds_imperv()` / `set_ds_imperv()`	Depression storage on impervious area.
`get_ds_perv()` / `set_ds_perv()`	Depression storage on pervious area.

Infiltration#

The infiltration model is selected globally in [OPTIONS] INFILTRATION. Setters write the parameters appropriate for the active model.

Method	Action / returns
`get_infil_model()`	Active `InfilModel`.
`set_infil_horton(...)()` / `get_infil_horton()`	Max + min infiltration rate, decay constant, regen.
`set_infil_green_ampt(...)()` / `get_infil_green_ampt()`	Suction head, conductivity, initial deficit.
`set_infil_curve_number(cn)()` / `get_infil_curve_number()`	SCS Curve Number.

Per-step hydrology state#

Method	Returns
`get_rainfall()`	Current rainfall intensity over the subcatchment.
`get_runoff()`	Current runoff flow leaving the subcatchment.
`get_groundwater()`	Groundwater contribution to the receiving node.
`get_evap()`	Surface evaporation flux.
`get_infil()`	Infiltration flux.
`get_snow_depth()`	Snowpack water-equivalent depth.

Cumulative statistics#

Method	Returns
`get_stat_precip()`	Total precipitation depth so far.
`get_stat_runoff_vol()`	Total runoff volume so far.
`get_stat_max_runoff()`	Peak runoff seen so far.

For a richer cumulative-statistics surface (peak rates, durations, quality components) see Statistics.

Forcing#

Method	Action
`set_rainfall(idx, r)()`	Override rainfall for this subcatchment this step (one-shot).

For sticky cross-step rainfall overrides, use Forcing.subcatch_rainfall() (see Advanced forcing).

Landuse coverage & water quality#

Method	Action / returns
`get_coverage(idx, lu)()` / `set_coverage(idx, lu, frac)()`	Fraction of subcatchment covered by landuse `lu`.
`get_quality(idx, p)()`	Pollutant `p` concentration in surface runoff.
`get_ponded_quality(idx, p)()`	Pollutant `p` concentration in ponded surface water.

End-to-end example#

from openswmm.engine import Solver, Subcatchments

with Solver("site_drainage.inp", "site_drainage.rpt", "site_drainage.out") as s:
    sc = Subcatchments(s)
    print(f"{sc.count()} subcatchments")

    # describe each subcatchment
    for i in range(sc.count()):
        print(
            f"  {sc.get_id(i):<12}  "
            f"area={sc.get_area(i):7.3f}  "
            f"imperv={sc.get_imperv_pct(i):4.1f}%  "
            f"outlet={sc.get_outlet(i)}"
        )

    # accumulate runoff per subcatchment
    total = [0.0] * sc.count()
    dt = s.get_routing_step()
    while s.step():
        for i in range(sc.count()):
            total[i] += sc.get_runoff(i) * dt
    for i, vol in enumerate(total):
        print(f"  {sc.get_id(i):<12}  runoff vol = {vol:.3f}")

Common recipes#

Override rainfall on one subcatchment for the run#

One-shot per step:

s1 = sc.get_index("S1")
while s.step():
    sc.set_rainfall(s1, 0.5)        # in/hr or mm/hr per RAINFALL units

Sticky:

from openswmm.engine import Forcing, ForcingMode

forcing = Forcing(s)
forcing.subcatch_rainfall("S1", 0.5, ForcingMode.REPLACE, persist=True)
while s.step():
    pass
forcing.clear_all()

Switch infiltration model parameters at runtime#

# Change Horton parameters before initialize()
s.open()
sc.set_infil_horton(
    sc.get_index("S1"),
    f0=3.0,         # initial rate
    fmin=0.05,      # final rate
    decay=4.0,      # /hr
    regen=7.0,      # /day
)
s.initialize()

Sweep multiple coverage fractions for sensitivity analysis#

# Snippet only — wrap in a loop over scenarios.
sc.set_coverage("S1", lu_idx=0, fraction=0.4)   # 40% landuse 0
sc.set_coverage("S1", lu_idx=1, fraction=0.6)   # 60% landuse 1
# remaining must sum to ≤ 1.0; the engine fills the residual with default

Per-subcatchment runoff coefficient (post-run summary)#

for i in range(sc.count()):
    precip = sc.get_stat_precip(i)
    runoff = sc.get_stat_runoff_vol(i) / max(sc.get_area(i), 1e-12)
    rc = (runoff / precip) if precip > 0 else 0.0
    print(f"  {sc.get_id(i):<12}  precip={precip:.3f}  runoff={runoff:.3f}  RC={rc:.3f}")

Bulk arrays#

The Subcatchments class does not (yet) expose bulk-array accessors. Vectorise across the population manually:

import numpy as np

runoff = np.zeros(sc.count())
while s.step():
    for i in range(sc.count()):
        runoff[i] += sc.get_runoff(i)

If you need a strictly faster path, fall back to the OutputReader after the run and use the bulk subcatchment methods (Output reader (binary .out file)) — that’s typically fastest for post-processing.

EngineState requirements & exceptions#

Method group	Required state	Notes
identity / count	`OPENED` or later	Identity is fixed at parse time.
geometry / infiltration setters	`OPENED`	Modify before `initialize()` for predictable results.
hydrology accessors (`get_runoff` etc.)	`RUNNING` or `ENDED`	Need at least one runoff step.
cumulative statistics	`RUNNING` or `ENDED`	Accumulated since simulation start.
`set_rainfall`	`RUNNING`	One-shot.
quality accessors	`RUNNING` or `ENDED`	Require at least one pollutant in the model.

Common EngineError codes:

INVALID_INDEX — index out of range.
NOT_FOUND — string id not in the model.
INVALID_TYPE — landuse / infiltration mismatch.