Your lot, your neighbourhood
Type your own lot on the left and watch both runoff methods move. On the right, the neighbourhood it sits in: canopy, tree removals, what it costs to buy here, and where the runoff goes. One set of controls drives both.
These controls set the neighbourhood and years for the panel on the right. The lot calculator on the left takes your own measurements.
Your lot
Change any figure and both methods recompute. Where they disagree, that gap is the point — not a number to split the difference on.
- Coefficient
- 0.410
- Runoff a year
- 55,671 gal
- One 1 in storm
- 1,277 gal
- Coefficient
- 0.420
- Runoff a year
- 57,029 gal
- One 1 in storm
- 1,308 gal
Storage is applied to the single storm only. Removing a tank's capacity from a whole year's runoff would assume one storm a year; this model has no storm count, so it does not claim an annual storage saving.
Where does it go?
Unknown
The figures above are water leaving a lot. Whether any of it reaches a sewer is a different question, and Cambridge does not publish what would answer it. Three things are missing:
- which sewer this parcel connects to
- which catchment the parcel sits in
- how this storm routes through that catchment
Cambridge is part combined sewer and part separated storm drain, and the split is not published. Runoff from a lot is not sewer inflow, and this page will not print a number that says it is.
Three published scenarios, for comparison
135,783
43.59 inches, the 1991–2020 normal at Boston Logan, over 5,000 square feet. This is the water the three scenarios divide up — none of them adds any.
Open ground
0% of the lot sealed
Two-family, as built
40% of the lot sealed
Full envelope
86% of the lot sealed
| The lot | Sealed | Runs off a year | Rational C | Share of the rain | × open ground |
|---|---|---|---|---|---|
| Open ground | 0% | 6,789 gal | 13,578 gal | 5% | 1.0× |
| Two-family, as built | 40% | 55,671 gal | 57,029 gal | 41% | 8.2× |
| Full envelope | 86% | 111,885 gal | 106,997 gal | 82% | 16× |
Built to the full envelope, the same lot sends about 16 times the water off-site that open ground does — roughly 105,096 more gallons a year into a pipe or a neighbour's yard. Which pipe is not published.
Shares and multiples use the Simple Method, which was derived for annual volumes. The Rational figure is shown beside it because the two methods disagree, and the page says by how much rather than averaging them.
Two methods, and where they disagree
This tracker refuses to reuse the Rational method's runoff coefficient as a volumetric ratio: it is a peak-flow coefficient from a rate equation, and using it to split a year's volume is a category error rather than a cautious approximation. The specification for this screen is built on exactly that coefficient.
So both are computed. The Rational coefficient the specification adopts, and the Simple Method coefficient (Schueler 1987), which was derived for annual volumes and is the right shape for this question. Neither is presented as the answer.
On the two lots anyone would actually build, the methods agree closely — 2% apart on the two-family house and 5% on the full envelope. On bare open ground they differ by 50%: one says a tenth of the rain runs off an unbuilt lot, the other a twentieth. That matters because the comparison this screen exists to make is between bare ground and full build, so the figure most sensitive to the choice of method is the one it would lead with.
Precipitation: NOAA 1991–2020 normals, Boston Logan. Coefficients: Rational method as adopted by the specification; Simple Method, Schueler 1987.
