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The regional WEI+ is calculated according to the following formula:

WEI+ = (abstractions - returns)/renewable freshwater resources.

Renewable freshwater resources are calculated as ‘ExIn + P - Eta ± ΔS’ for natural and semi-natural areas, and as ‘outflow + (abstraction - return) ± ΔS’ for densely populated areas.

Where:

ExIn = external inflow

P = precipitation

Eta = actual evapotranspiration

ΔS = change in storage (lakes and reservoirs)

Outflow = outflow to downstream/sea.

It is assumed that there are no pristine or semi-natural river basin districts or sub-basins in Europe. Therefore, the formula ‘outflow + (abstraction - return) ± ΔS’ is used to estimate renewable water resources.