Distinction is made between renewable and non-renewable water resources. Non-renewable water resources are not replenished at all or for a very long time by nature. This includes the so-called fossil waters. Renewable water resources are rechargeable due to the hydrological cycle unless they are overexploited, comprising groundwater aquifers and surface water like rivers and lakes. Internal renewable water resources comprise the average annual flow of rivers and groundwater generated from endogenous precipitation.