The first winter storm of 2017 to drop welcome rain over the rivers, pumps, pipes and canals that move California’s water north to south likely will open a new era of tension over how much water goes to fish or farms under a new U.S law. Legislation signed Friday by President Barack Obama dictates that the federal portion of California’s heavily engineered water systems gives agricultural districts and other human users the biggest possible share of the most fought-over resource in a state with a six-year drought.