Effective rainfall (or precipitation) is equal to the difference between total rainfall and actual evapotranspiration. Effective rainfall can be calculated directly from the climatic parameters and the useable ground reserves (RU). At ground level, water from effective rainfall is split into two fractions: surface run-off and infiltration.
As with total rainfall, effective rainfall is expressed in height (in millimetres) related to a unit of time or in volume (for example billions of m3 per year).
Source : SOeS (site : http://www.statistiques.developpement-durable.gouv.fr)