The state’s water conservation districts don’t need the approval of property owners or voters to charge their customers fees to fund programs aimed at protecting groundwater, the California Supreme Court ruled on Monday. But the justices in a unanimous decision also said the districts cannot charge cities disproportionately more than farmers for conservation efforts. The decision ensures the water districts have a source of funding to undertake projects to replenish ground water — a key irrigation source for farmers that became even more vital during California’s historic drought.