In the eastern United States, groundwater generally is plentiful and is managed and used as a shared resource. In that sense, groundwater is not intended to be used at rates not sustainable. In the west and owing to lessened recharge rates and differing regulatory schemes (i.e., the law of prior appropriations), groundwater generally is used at rates not sustainable over longer periods of time. Large-scale irrigation in the central United States has contributed to groundwater withdrawals exceeding the aquifer recharge rates. This is termed groundwater mining, the immediate effect of which is declining water levels and/or potentiometric heads in the central and western aquifers.