Dwayne, thank you for the detailed reply!
I've seen purple pipes used to carry recycled wastewater in California. Do you know if the stormwater needs to meet any water quality criteria before it's used for irrigation?
On No.4, why is it less of a problem if a stormwater irrigation system breaks vs. the treated water? Is it because something different is being irrigated, or because of the existing backup system?
Bart, thank you for replying.
The question came to my mind while driving by stormwater retention ponds in a new development, so urban and suburban environments were what I was thinking about. I was also thinking about stormwater that ends up in combined sewers. You are also right that the stormwater becomes a part of the hydrologic cycle, especially if it infiltrates into the groundwater or is released back to the receiving water.
I came across this article on the future Sites reservoir in California in May issue of the Civil Engineering Magazine. Is it something similar to what you had in mind when talking about collecting wet weather flows to supplement dry weather base flow?
Growing and maintaining plants more naturally can greatly increase the mass of live and dead roots in the soil. More live and dead root mass means the soil can absorb (percolate) rain faster and hold (capture, store) rain. This fact means the people of Ventura County can modify how they maintain the soil of their farms, orchards, golf courses, road medians, bioswales, residential yards, parks, and schools to capture stormwater. They can grow water storage with volumes that address drought and the likelihood that we will experience less rain, but at higher intensity.
Consider the percolation results claimed for carbon farming. During minute 8:40 – 9:40 of the carbon-cowboys web video, the farmer claims percolation improved from 1" of rain per hour to 8" of rain per hour. The second website has more how-to from a finalist in the Virgin Earth Challenge.
Table 1 shows the people of Ventura County can capture thousands of times more rain with percolation than with rain barrels. We can have useful seasonal rain capture with improved percolation, enough to make a difference in how we manage our groundwater aquifers.
Table 1 – Comparison of rain capture with barrels and improved soil percolation
Rain capture volume with improved percolation
Residential roof with two rain barrels
Yard which can perc 1" of rain
Yard which can perc 4" of rain
Golf Course, park, school which can perc 1" of rain
Golf Course, park, school which can perc 4" of rain
Orchard or farm which can perc 1" of rain
Orchard or farm which can perc 4" of rain
The percolation landscaping could become a best management practice (BMP) which improves other drainage BMPs (bioswales, traffic and parking lot islands which are lower so that all the water drains to them, instead of away from them, etc.). These bioswales and low islands would be improved two ways: 1) they perc more water before ponding and overflowing and 2) they store water to stay green without irrigation into July and August every year. Expect a few years of plant roots increasing soil carbon before the effect is noticeable.
If you have money and want quick results, use Silva Cells and mix in biochar when converting islands and tree wells from high points to low points.
 https://www.greenbiz.com/article/rise-soil-carbon-cowboys and http://www.virginearth.com/finalists/savory-institute/