Overdrafting is the process of extracting groundwater beyond the safe yield or equilibrium yield of the aquifer. Since every groundwater basin recharges at a different rate depending upon precipitation, vegetative cover and soil conservation practices, the quantity of groundwater that can be safely pumped varies greatly among regions of the world and even within provinces. Some aquifers require a very long time to recharge and thus the process of overdrafting can have consequences of effectively drying up certain sub-surface water supplies. Subsidence occurs when excessive groundwater is extracted from rocks that support more weight when saturated. This can lead to a capacity reduction in the aquifer.