Climate change and other environmental pressures are already putting the pinch on water resources in California, the Southwest and other arid parts of the world. Over-tapped groundwater, rivers and lakes are forcing water managers to find new supplies. Some of these can be costly, like treating wastewater for drinking water. Or they can come with a hefty price tag and outsized environmental footprint, like desalination or new dams. There’s another option on the table, though: stormwater. If we do the accounting right, runoff from precipitation is a cost-effective supplementary water resource, experts say.