What is the difference between rainwater and stormwater?

Which form is correct: stormwater or storm water ?

Can I use words stormwater and rainwater as a synonym?

  • Mateusz, your question is on the brink of being closed. Your question is on topic in my estimation, but the rules here require that you show some prior research effort. What references have you consulted? How did they fall short? We also need to know who your audience is. The distinction is mostly a technical one of interest to planners and developers concerning the regulations imposed on them. – Phil Sweet May 4 '18 at 0:57
  • EPA definition of stormwater - epa.gov/greeningepa/epa-facility-stormwater-management – Phil Sweet May 4 '18 at 1:00
  • The overall term is 'surface water' when drainage is in view. I suspect that you do not need to specify either 'storm' or 'rain'. – Nigel J May 4 '18 at 13:49

I've not seen "storm water" used as such, but when talking of major drainage infrastructure that can handle sudden extreme rainfall in a city we use the term "storm drains". I wouldn't be too surprised to encounter "storm water" or "stormwater" in such a context. Perhaps it's a customary term for urban councils or water authorities. Rainwater is the more general phenomenon, and if you're dealing with texts for architects or urban planning it would refer to the general kind of drainage means used on buildings and landscaped spaces - drain pipes and gutters and so on. I've never thought about making one or two words out of this, but the two together seems instinctively to make more sense when talking about the substance itself, known for its distinct properties of purity (or not!).

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    I am also unfamiliar with 'stormwater' but would suggest that seaside communities and ones at the side of tidal rivers would also, potentially, suffer from flooding caused by large storm-driven waves which would not be rainwater and would even be salty. 'Stormwater' would be a good word for water driven ashore by storms which could be augmented by rainwater as well. – BoldBen May 3 '18 at 18:07

I'll briefly weigh in from the architectural / urban planning side of this discussion:

Stormwater is the water that drains off a land area from rainfall. This includes rain that falls on rooftops, directed through gutters and downpipes onto land or into drains, as well as rain falling on ground surface areas such as roads, driveways, footpaths, gardens and lawns.

Rainwater refers only to the rain that falls on the roof, which can be harvested into a storage tank prior to contact with the ground. Rainwater quality is much higher, since groundwater generally contains a many more contaminants including soil, organic matter, fertilisers from gardens, oil residues from driveways and the like.

As you can see, the primary difference is not just volume, but also contaminants and carried solids, floating detritus, branches and so on.

In the various inter-related AEC (Architecture Engineering Construction) trades and specializations, I've not seen "storm water" used as two words when referring to this phenomenon, but have always seen it tied as one "stormwater" and similarly "rainwater".

Hope that helps.

External reference to support specific language: http://www.nationalpolyindustries.com.au/knowledge-base/what-is-the-difference-between-stormwater-and-rainwater/index.html

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