the math doesn't really work in my head either for the environmental argument out west on the salmon streams.
[lots of math figures, none of which i dispute]
One also has to understand that roofs accelerate run off so diverting to a rain barrel is generally a good thing. Much of that roof water would have been absorbed and released in subsurface water flows typically prior to a roof being put up.
There may be some weird western thing going on in these salmon streams but I'm doubting that roof water retention is a negative. My guess is that it is a net positive and should be encouraged.
Of course I could be totally wrong.
I would also be interested to know where these homes on salmon streams acquire their water to begin with?
Ok, I'll respond, since this is an area I spent a lot of time working on.
I'll start by saying my personal opinion is that people should be able to store water that falls on their roof, but not necessarily divert water that has fallen on their entire property (which is what some people do).
First, a few things about the actual salmon streams in central California. Unlike many of the picturesque mega-rivers like the Colombia, Klamath etc. much of the salmon habitat are in small watersheds (often called creeks) that are just a few miles long. And there are dozens of them along every 100mi stretch of coastline. But just because they are small doesn't mean they aren't important, since historically these creeks had salmon runs numbering in the thousands, and downstream smolt (baby salmon) out-migrations numbering in the hundreds-of-thousands. Two other 'odd' things about these small creeks/rivers. They are often surrounded by completely lined with suburban development (people loved to build houses next to salmon creeks/rivers) and they have river mouths that are blocked by sand bars during the dry season, and are typically only open during the rainy season (typically November - April). For example, Santa Cruz gets about 31 inches of rain per year, of which 22 falls in just those four months. Typically less than an inch
total falls between June 1st and October 1st.
So - what's crucial for salmon is that they get these large-water pulses during heavy rainfalls to break open the sand bars and allow the adult salmon to come upstream and breed, and the young-of-year smolts to head out to sea. If the flow isn't high enough, it doesn't happen and that year becomes a bad salmon spawning year. There's a definite threshold at work.
So - that sets the scene. I'd say you are completely right that just the rain that falls on homes isn't a huge about relative to everything else around, but the core problem is that we've already monkeyed around with the system so much that it's not just about the rainfall on the roofs. Roads and driveways have their own system (storm-drains and culverts) which were built to take water quickly to the ocean, often bypassing the streams entirely. And then there's agriculture, which take the lion's share of water in California (estimates as high as 80%). Many of them either pump water from wells (which lowers the water table and reduces rain runoff into streams) or they take water out of the streams themselves. Then they use their own drainage ditches to divert and recycle as much water as they can back towards their fields, because they aren't stupid and a major cost is irrigation.
Getting back to the houses and rainfall on roofs.... most of these homes are on 1/4 acre (10,890 ft2) lots with large homes with big roofs (1000 ft2). That means ~10% of all water that falls can be falling on a person's roof, and another ~5-8% can be on the person's driveway which diverts most of its water into the storm-drains. So it is indeed a small fraction of the total, but not an insignificant fraction.
The laws are so complex and "Big-Ag" have so much power (indeed they get a written allotment every year based on the snowpack) that it's hard to restrict their usage - they have a definite economical need. Residential homes, however, are easier to regulate through muni-codes, and since a home isn't a for-profit business it's easy to say "no rain storage for you!".
A far more effective solution is to allow the rainwater to reach watersheds more naturally instead of diverting it back into the watersheds and by allowing streams to naturally migrate as they once did instead of channeling them through culverts. CalTrans (the department responsible for all road-building) is doing exactly this, but it's an infrasturcture problem - drains and culverts are being replaced once they have passed their natural service life (often waaaay past), and many are 50+ years old. There's also always the danger that runoff will kill all life in a stream if some a**hole decides to pour a gallon of RoundUp down the storm drain instead of disposing of it properly.
To finish up, Salmon in most places are screwed, with current runs in most places only ~1-5% of historical runs. Problem is, many are listed under the federal ESA act of 1973 as either endangered (e.g. Coho salmon) or threatened (central coast Steelhead) - which means we are
mandated to take steps to ensure protection of their habitat (and in many places augment the stock with hatchery-reared smolts). Hence - restrictions on collecting the water that falls on your roof. It's one thing that can be done, even though it will have only a small effect overall.
hope that at least gives you some insight.