Dowsing is as legit as tarot cards, astrology, and palm-reading. They hit often enough to fool the gullible.

You are partly correct, large amounts of water do move through the pore spaces in rock. However, in addition to that type of flow, almost all rock formations have a network of cracks and fractures, and water often moves preferentially along these pathways. How much porous flow vs fracture flow you get in an aquifer depends on the nature of the rock and the degree of fracturing in the aquifer. In wells, intersecting a few good fractures can be the difference between a productive well and one that does not produce sufficient flow to be useful.

This is also true in the Ogallala Aquifer which underlies large areas of Kansas, Oklahoma, and Texas (and other aquifers around the world as well). This isn't a problem unique to California.

In many places, the groundwater withdrawals are outstripping the rate at which these groundwater reservoirs can be replenished by precipitation. Dryer and warmer climates are exacerbating this problem as well. As the interstitial spaces in the aquifers are depleted of water, they lose the structural strength that the water provided and subside. The worst thing is that these losses of interstitial space are likely permanent, so even if the aquifer is re-watered, it will have a lower capacity than it used to have.

We are heavily reliant on groundwater in the US for agriculture and municipal water, and unsustainable groundwater use is a serious problem that is largely overlooked by the media and unfamiliar to most Americans.