California farmland lacks the technology to collect rainwater. If you catch the water high in the watershed you don’t have to deal with the deluge at the bottom. Also ground water can end up being aquifer water.
There’s a book out of copyright for a few decades called Farmers of Forty Centuries which tries to answer the question of “how is it the Chinese haven’t destroyed their fields in 400 years like we have?”
A lot of it is the slowing of the water, but also harvesting silt from the rivers and canals to build up material and keep the waterways from silting up.
A question I’ve had since reading that book is what has happened since the industrial revolution? Did manufacturing effluent break that process, and if so how extensively? Only in the deltas or how far upriver?