A drone whirred to life in a cloud of dust, then shot hundreds of feet skyward for a bird’s-eye view of a vast tomato field in California’s Central Valley, the nation’s most productive farming region. Equipped with a state-of-the-art thermal camera, the drone crisscrossed the field, scanning it for cool, soggy patches where a gopher may have chewed through the buried drip irrigation line and caused a leak. In the drought-prone West, where every drop of water counts, California farmers are in a constant search for ways to efficiently use the increasingly scarce resource.