A new plan to reroute how water moves from wetter Northern California to drier Southern California would ferry it through a single, 45-mile (72-kilometer) underground tunnel, wrapping around the state’s existing water delivery system and dumping the water straight into the main aqueduct that sends it south to vast swaths of farmland and millions of people. The scaled back proposal released Wednesday would build one tunnel to take water from the Sacramento River, the state’s largest, to the California Aqueduct for delivery to millions of people and farmland further south.