interesting thing is that some of the driest Western states have laws against capturing rooftop runoff because their historical approach has been to manage the use of surface water such as streams and rivers. People's water rights are determined by who has been historically using a particular source of water the longest, and the most recent users are the first to be cutoff from such a source during a drought. Capturing rainwater theoretical reduces the amount of water that reaches surface water, and is thus a short-circuit of the process, and therefore illegal.

I didn't know this detail.

it's a crazy system.

Furthermore, certain kinds of gray water systems can be installed without permit.

I've occasionally experimented with water from our washing machine in our garden, but not recently. I don't feel guilty about the water that goes down the drain. Sewage is recycled here into near-potable and potable water. The near-potable water is used on certain high value crops and for groundwater recharge. Some of it is further treated by reverse osmosis, etc., to make high quality potable water. This sort of recycling costs less and uses less energy than desalinating ocean water.

Most new construction here, which is generally built on farmland or old industrial sites, now includes catchments that hold runoff from roofs, roads, and parking lots, allowing rainwater time to percolate into the ground.

Catching rainwater is something that can be accomplished at any scale, from a small single family home to a huge apartment complex.

One of the problems in California is ridiculously high property values. Developers don't like "wasting" valuable land for catchment basins. There's a large new shopping center in a city near us that has the catchment basins underneath the parking lots. That wasn't cheap!

The problem of runoff from roads and parking lots will be much less severe in the future when we finally get our shit together and abandon car culture, rebuilding our cities in such a way that most people can't be bothered to own a car.

use rainwater, but businesses and public entities cannot. Our office spent several years negotiating worth the state Public Health Department to set standards that would make rainwater harvesting available to everybody in the state. After reaching agreement with the officials in our work group, some higher-up vetoed the plan.Even the federal government declared that rainwater could not be used for livestock, in spite of the fact that water in ponds and other surface water are much more likely to be contaminated by runoff. Using grant money, we installed one model whole-house rainwater harvesting system that could be used for drinking, cooking, etc. in each of the four counties we service. But it's an uphill battle. It should be a no-brainer that public schools should harvest rain from their enormous roofs for toilets at least, but the resistance to it is very strong. It's an uphill battle.