Diverting streams to go into different places increases the dwell time of the water by 45 days. when you have a rigid dry season, keeping the streams running for an extra 45 days is absolutely fucking huge. The clever part is that over the years the locals have managed to find the best place to divert water so it goes into springs that feed streams.
its a similar idea to a Johad, https://en.wikipedia.org/wiki/Johad which traps water for a number of days, increasing the dwell time and making the place wetter. The issue with Johads is that they can be a magnet for mosquitos. Johads differ in that they force the water into the soil rather than aquifers.
In LA, everything is done to make sure the water yeets into the sea as quick as possible. That water that would have hung around is replaced with the water from the LA aqueduct and the like. THe place gets drier, and hotter. I would wager, but can't assert that this water mismanagement is causing a large part of the drought in that area.