SWMM Water Quality Skill

Purpose

Complete SWMM engine coverage for pollutant buildup/washoff simulation and load reporting. This skill provides:

1. validate_wq_config.py — validate a WQ config JSON before passing it to the builder.
2. extract_wq_loads.py — extract WQ load summaries from a SWMM RPT.

The water-quality sections ([POLLUTANTS], [LANDUSES], [COVERAGES], [BUILDUP], [WASHOFF], [LOADINGS]) are emitted by skills/swmm-builder/scripts/build_swmm_inp.py via the --water-quality-json flag (see also build_inp tool's water_quality_json argument).

Agent tool: read_wq_loads

Read pollutant load summaries from a completed run's .rpt file. Returns wq_present=false for non-WQ runs.

read_wq_loads(rpt_path="runs/my_run/model.rpt")

Returns a structured JSON with:

• wq_present (bool)
• pollutants — sorted list of pollutant names
• runoff_quality_continuity — mass-balance rows (metric + per-pollutant kg)
• quality_routing_continuity — routing mass-balance rows
• subcatchment_washoff — per-subcatchment loads (kg per pollutant)
• link_loads — per-link transport loads (kg per pollutant)
• outfall_loads — per-outfall flow stats + pollutant loads

WQ config JSON schema

Top-level keys (all required when the key is present; empty arrays are valid):

{
  "pollutants": [...],
  "landuses": [...],
  "coverages": [...],
  "buildup": [...],
  "washoff": [...],
  "loadings": []
}

pollutants entries

Field	Type	Default	Notes
name	string	required	No spaces
units	string	required	MG/L, UG/L, or #/L
c_rain	float	0	Concentration in precipitation
c_gw	float	0	Concentration in groundwater
c_ii	float	0	Concentration in RDII
k_decay_per_day	float	0	First-order decay (1/days)
snow_only	bool	false	Buildup during snow only
co_pollutant	string	"*"	Co-pollutant name or "*"
co_fraction	float	0	Co-pollutant fraction (0–1)
init_conc	float	0	Initial dry-weather concentration

landuses entries

Field	Type	Default	Notes
name	string	required
sweep_interval	float	0	Days between sweeping (0 = no sweeping)
availability	float	0	Fraction of buildup removed by sweeping (0–1)
last_sweep	float	0	Days since last sweep at start

coverages entries

Field	Type	Notes
subcatchment	string	Must reference an existing subcatchment
landuse	string	Must reference a defined land use
percent	float (0–100)	Percent coverage; per-subcatchment sum must be ≤ 100

buildup entries

Field	Type	Notes
landuse	string	Must reference a defined land use
pollutant	string	Must reference a defined pollutant
func_type	string	POW, EXP, or SAT (EXT not supported in v1)
c1	float	Max buildup (kg/ha or count/ha when normalizer=AREA)
c2	float	Rate constant
c3	float	Third coefficient (unused for EXP/SAT)
normalizer	string	AREA or CURBLENGTH

washoff entries

Field	Type	Notes
landuse	string	Must reference a defined land use
pollutant	string	Must reference a defined pollutant
func_type	string	EXP, RC, or EMC
c1	float	Coefficient 1
c2	float	Coefficient 2 (0 for EMC)
sweep_removal	float (0–1)	Fraction removed by sweeping
bmp_removal	float (0–1)	Fraction removed

loadings entries (optional)

Field	Type	Notes
subcatchment	string	Must reference an existing subcatchment
pollutant	string	Must reference a defined pollutant
init_buildup	float	Initial buildup mass

Scripts

• scripts/validate_wq_config.py — standalone CLI validator
• scripts/extract_wq_loads.py — RPT load extractor

Executed examples

Validate a WQ config JSON

# Write a minimal WQ config JSON:
cat > /tmp/wq_example.json << 'EOJSON'
{
  "pollutants": [{"name": "TSS", "units": "MG/L", "c_rain": 0, "c_gw": 0,
                  "c_ii": 0, "k_decay_per_day": 0, "snow_only": false,
                  "co_pollutant": "*", "co_fraction": 0, "init_conc": 0}],
  "landuses": [{"name": "Residential", "sweep_interval": 0, "availability": 0, "last_sweep": 0}],
  "coverages": [{"subcatchment": "S1", "landuse": "Residential", "percent": 100}],
  "buildup": [{"landuse": "Residential", "pollutant": "TSS", "func_type": "EXP",
               "c1": 15, "c2": 0.5, "c3": 0, "normalizer": "AREA"}],
  "washoff": [{"landuse": "Residential", "pollutant": "TSS", "func_type": "EMC",
               "c1": 50, "c2": 0, "sweep_removal": 0, "bmp_removal": 0}],
  "loadings": []
}
EOJSON

python3 skills/swmm-water-quality/scripts/validate_wq_config.py \
    --wq-json /tmp/wq_example.json
# Output: {"ok": true, "pollutant_count": 1, "landuse_count": 1, ...}

Extract WQ load summaries from a completed run RPT

python3 skills/swmm-water-quality/scripts/extract_wq_loads.py \
    --rpt tests/fixtures/wq/wq_smoke.rpt
# Output: {"ok": true, "wq_present": true, "pollutants": ["TSS"],
#          "runoff_quality_continuity": [...], ...}

Build an INP with water quality sections

python3 skills/swmm-builder/scripts/build_swmm_inp.py \
    --subcatchments-csv <subcatchments.csv> \
    --params-json <params.json> \
    --network-json <network.json> \
    --water-quality-json /tmp/wq_example.json \
    --out-inp /tmp/wq_model.inp \
    --out-manifest /tmp/wq_model_manifest.json

Validation constraints

Enforced by both validate_wq_config.py and build_swmm_inp.py:

• Referential: every [BUILDUP]/[WASHOFF] landuse/pollutant must exist
• Referential: every [COVERAGES]/[LOADINGS] subcatchment must exist
• Enum: units ∈ {MG/L, UG/L, #/L}
• Enum: buildup func_type ∈ {POW, EXP, SAT} (EXT rejected with message)
• Enum: washoff func_type ∈ {EXP, RC, EMC}
• Range: coverage percent ∈ [0, 100]; per-subcatchment sum ≤ 100
• Range: sweep_removal, bmp_removal, co_fraction ∈ [0, 1]