Technical NoteThe Budyko hypothesis before Budyko: The hydrological legacy of Evald Oldekop

- We review the legacy of Evald Oldekop, who first used the aridity index in hydrology.
- Oldekop was the first to propose the concept of maximum evaporation.
- Oldekop rewrote Schreiber's equation to make it a function of the aridity ratio.
- Oldekop proposed his own formula to compute catchment actual evaporation.

SummaryIn 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 φ = Ex/P (where P represents long-term precipitation [mm/yr] and Ex 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.