For years, Californians have mismanaged the aquifers that supply the state with about 40 percent of its water supplies. Declining water levels from over-pumping have left less water for agriculture, urban, and other uses in many areas of the state. But the problems do not stop with groundwater users. Groundwater and surface water are closely connected, so declining groundwater can reduce streamflow at critical times of the year, and can devastate rivers, streams, and wetlands.