LA is not a desert you are repeating a myth. it get very old after a while.
I know right...LA is semi arid at worst with many years above semi arid levels.
A nice chart showing rain totals from the 1820s till 2022
https://www.laalmanac.com/weather/we13.php
I’ll take a 3% cut of the profits for my offshore runoff polder solution to runoff capture. It would work the physics is solid and having an open end makes sediment disposal a trivial issue. You would need an outer breakwater ring to keep wave action to a minimum thus extending your mixing time into multiple weeks in the center of the freshwater bubble which is where you tap from away. Something like a 200 billion gallons just ran offshore out of the LA basin a single dike 5 miles in diameter and 100 feet deep would hold 7.84 square miles of water at a depth of 100 feet that’s 502,400 acrefeet of water or 163,707,040,000 gallons. The Dutch routinely build dikes that are hundreds of miles in length ask them for the know how. The LA basin drains into a single outlet at long Beach it’s a major port so ship locks would be needed or a bypass flow structure to an off channel dike polder either world work. Expanding to a Dutch know-how sized ten mile wide ring size makes for truly massive capture amounts. Over a million acrefeet California gets 7 million acrefeet from the Colorado per year for comparison. Ten miles is too large for a 827 square mile capture basin 5mi is the right size it will full capture a 6 inch rainfall at a statistical 500 year event like this. Which means every smaller event is fully captured and the larger 1000 year event just drains harmlessly out the open bottom end.