# Oldekop, Evald

**Evald Oldekop** (1885-1952) was an Estonian hydrologist 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.

## Contents

## Biography[edit]

Evald Oldekop was born in the Estonian city of Tallin in 1885. He studied hydrology at the University of Tartu under the direction of Prof. B.I. Sreznewski and produced a remarkable thesis entitled “Evaporation at the surface of river basins.” From 1912 to 1921, he worked abroad in Tachkent (Uzbekistan), and then returned to Estonia where he died in 1952.

## Oldekop's formula[edit]

The name of Oldekop is linked to the following formula, which links *catchment-scale long-term values* of precipitation *P*, streamflow *Q*, potential evaporation *E0* (Oldekop used the expression *maximum* evaporation), all in mm/year:

*Q* = *P* - *E0*.tanh(*P*/*E0*)

By assuming a watertight catchments (no deep aquifer losses), on can write actual evaporation *Ea* = *P* - *Q*, and Oldekop's formula can be transformed into an actual evaporation formula:

*Ea* = *E0*.tanh(*P*/*E0*)

## Hydrological Achievements[edit]

Oldekop is known for his formula, but his actual contribution to hydrology consists in four innovations:

(i) he proposed the concept of maximum evaporation, based on his analysis of Schreiber’s original formulation, analyzing that the original empirical parameter corresponded to the “maximum possible evaporation, only dependent on climate.” Oldekop obtained this climatic value by analyzing the annual series of precipitation minus streamflow on a given catchment. Although Oldekop does not provide a meteorological formulation for the maximum evaporation, the concept is analogous to that of potential evaporation, which was presented almost four decades later by Thornthwaite (1948);

(ii) he rewrote Schreiber’s equation to make it a function of the aridity ratio (the Schreiber equation as used in de Groen and Savenije (2006), Oudin et al. (2008) or in Gerrits et al. (2009) is in fact Schreiber’s equation as rewritten by Oldekop);

(iii) he proposed a new formulation, better adapted to his but also to other catchment data to compute actual catchment evaporation, again as a function of the aridity ratio. Promoting the aridity ratio as the main explaining factor of evaporation and streamflow efficiency makes him the ‘ancestor’ of the Budyko hypothesis;

(iv) last, he was the first to gather a large catchment data from all over the world: where Rafter (1903) and Keller (1906) only used catchment data from their respective countries, Oldekop was the first to attempt collating a ‘worldwide’ (Europe, North and Central America) dataset from published reports. In L'vovitch (1979 /1974/) words, it was ‘a first attempt at a global summary’.

In the Russian literature, Oldekop’s thesis used to be widely cited until the 1970s, for example by Mikhaïl Budyko himself (see e.g. Budyko, 1963 /1948/; Budyko, 1974; Budyko, 1977), or by L'vovitch (1979 /1974/) who was another prominent Soviet scientist of the 1960s and 1970s.

## Anecdotes[edit]

Evald Oldekop is buried in the Rahumäe cemetery in Tallinn.

Note on the gravestone the title ‘hüdroloog’ and the famous formula.

## References[edit]

Budyko, M.I., 1963 /1948/. Evaporation under natural conditions. Israel Program for Scientific Translations, Jerusalem, 130 pp.

Budyko, M.I., 1974. Climate and Life. Academic Press, New York, 508 pp.

Budyko, M.I., 1977. Global ecology (Глобальная экология). Misl', Moscow, 327 pp.

de Groen, M.M., Savenije, H.H.G., 2006. A monthly interception equation based on the statistical characteristics of daily rainfall. Water Resour. Res., 42(12).

Gerrits, A.M.J., Savenije, H.H.G., Veling, E.J.M., Pfister, L., 2009. Analytical derivation of the Budyko curve based on rainfall characteristics and a simple evaporation model. Water Resour. Res., 45.

Keller, H., 1906. Precipitation, flow and evaporation in central Europe (Niederschlag, Abfluss und Verdunstung in Mitteleuropa). Jahrbuch für die Gewässerkunde Norddeutschlands. Besondere Mitteilungen, 1(4): 1-43.

L'vovitch, M.I., 1979 /1974/. World water resources and their future. American Geophysical Union, Washington.

Oudin, L., Andréassian, V., Lerat, J., Michel, C., 2008. Has land cover a significant impact on mean annual streamflow? An international assessment using 1508 catchments. J. Hydrol., 357: 303-316.

Penck, A., 1896. Research about evaporation and flow from large land areas (Untersuchungen über Verdunstung und Abfluss von grösseren Landflächen). Geographische Abhandlungen., hgg. v. Penck, Wien, 5(5): 461-508.

Rafter, G.W., 1903. The relation of rainfall to run-off. Water Supply and Irrigation Paper. USGS, Washington, pp. 104.

Schreiber, P., 1904. On the relationship between precipitation and river flow in central Europe (Über die Beziehungen zwischen dem Niederschlag und der Wasserführung der Flüsse in Mitteleuropa). Zeitschrift für Meteorologie, 21: 441-452.

Thornthwaite, C.W., 1948. An approach toward a rational classification of climate. Geographical Review, 38(1): 55-94.

## Known publications[edit]

- Oldekop, E., 1915. Addition to the correlation theory for computing minimum night temperatures for natural soil surfaces (in Russian). Department of land improvement, Hydrometeorological Station in Turkestan, Tachkent.

- Oldekop, E., 1915. Instructions for observations on evaporation from water (in Russian), Department of land improvement, Hydrometeorological Station in Turkestan, Tachkent.

- Oldekop, E., 1916. Instructions for installation of mountain raingages and related calculations (in Russian), Department of land improvement, Hydrometeorological Station in Turkestan, Tachkent.

- Oldekop, E., 1917. Graphical method for evaluation of hygrometric deficit based on actual observations of absolute and relative air humidity (in Russian), Department of land improvement, Hydrometeorological Station in Turkestan, Tachkent.

- Oldekop, E., 1917. Some comments on relative methods of data analysis on floods using ombrographs (in Russian). Meteorological Herald(1): 1-7.

## Reference Material[edit]

Andréassian, V., Ü. Mander & T. Pae. 2016. The Budyko hypothesis before Budyko: The hydrological legacy of Evald Oldekop. Journal of Hydrology, 535: 386–391. doi:10.1016/j.jhydrol.2016.02.002.