A public agency and a powerful farmer are gearing up for a high-stakes court battle to determine who owns the largest share of Colorado River water in the West, complicating the river’s future as seven western states scramble to avoid severe water shortages. There’s a long history of fighting over water in California’s Imperial Valley, which has a legal right to more than 1 trillion gallons of Colorado River water each year — twice as much as the rest of California, and as much as Arizona and Nevada combined.