Where California’s towering Sierra Nevada surrender to the sprawling San Joaquin Valley, a high-stakes detective story is unfolding. The culprit isn’t a person but a process: the mysterious journey of snowmelt as it travels underground to replenish depleted groundwater reserves.   The investigator is a NASA jet equipped with radar technology so sensitive it can detect ground movements thinner than a nickel. The work could unlock solutions to one of the American West’s most pressing water challenges — preventing groundwater supplies from running dry.