It is difficult to determine the amount of renewable water produced internally from the total renewable water resources (external and internal). However the FAO gives a rather precise definition of this indicator. Internal Renewable Water Resources (IRWR) includes:
- Average precipitation: long-term double average over space and time of the precipitation falling on the country in a year.
- Surface water produced internally: long-term average annual volume of surface water generated by direct runoff from endogenous precipitation.
- Groundwater produced internally: long-term annual average groundwater recharge, generated from precipitation within the boundaries of the country (estimated by the annual infiltration rate (in arid countries) or river base flow (in humid countries).
- Overlap between surface water and groundwater: part of the renewable water resources common to both surface water and groundwater. It is equal to groundwater drainage into rivers (typically, base flow of rivers) minus seepage from rivers into aquifers.
- Total internal renewable water resources: long-term average annual flow of rivers and recharge of aquifers generated from endogenous precipitation. Double counting of surface water and groundwater resources is avoided by deducting the overlap from the sum of the surface water and groundwater resources.