WET Hydrology & Hydrogeology

Earth's Water Resources consist of renewable and non-renewable water.

Non-renewable water resources are not replenished as part of the Earth's natural hydrological cycle or take an exceedingly long time to replenish. They are often referred to as fossil water. An example of such a water resource is the Ogallala Aquifer. Most fossil water reservoirs share one common characteristic: they are confined or exist in regions where insufficient perception occurs.  It is estimated that between 42 to 85 percent of stored water is fossil in origin.  

Renewable water resources, which comprise groundwater aquifers and surface water like rivers and lakes, are rechargeable due to the hydrological cycle. Unconfined aquifers are naturally recharged by rainfall or other surface water infiltrating the ground.

That is unless they are over used. Internal renewable water resources comprise the average annual flow of rivers and groundwater generated from endogenous precipitation (Oxford English Dictionary).