On the edge of the Mojave Desert, beneath 1,800 acres of scrubland and tumbleweeds, California’s giant public pension fund is trying to make a killing in the water business. CalPERS is the primary owner of the Willow Springs Water Bank, an underground reservoir that could hold as much water as Folsom Lake when fully developed. Its customers, mainly a collection of Los Angeles-area water agencies, pay fees to store water beneath the Kern County soil to bolster their supplies during dry periods.