In the hydrological jargon, the Budyko hypothesis refers to the hypothesis according to which the long- term water and energy balances of the Earth’s surface are largely dependent upon a single aridity ratio u = E x / P (where P represents long-term precipitation [mm/yr] and E x long-term maximum evaporation [mm/yr]). In this historical note, we analyze the contribution of Evald Oldekop (1885–1952), who not only inspired Budyko’s work, but also proposed the concept of maximum catchment evaporation more than three decades before Thornthwaite introduced the concept of potential evapotranspiration.