Average annual runoff
Based on gauged data from hydrometric networks. Run-off in ungauged areas estimated from an empirical relationship relating run-off to precipitation and potential evaporation (Budyko, M.I. and Zubenok, L.I. : The determination of evaporation from the land surface. Izv. Akad, Nauk SSSR. In Ser. Geogr., No 6, p. 3-17).