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?
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!).
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