Abstract
Abstract. Evaporation from open water is among the most rigorously studied problems in hydrology. Robert E. Horton, unbeknownst to most investigators on the subject, studied it in great detail by conducting experiments and heuristically relating his observations to physical laws. His work furthered known theories of lake evaporation, but it appears that it was dismissed as simply empirical. This is unfortunate because Horton's century-old insights on the topic, which we summarize here, seem relevant for contemporary climate-change-era problems. In rediscovering his overlooked lake evaporation works, in this paper we (1) examine several of his publications in the period 1915–1944 and identify his theory sources for evaporation physics among scientists of the late 1800s, (2) illustrate his lake evaporation formulae, which require several equations, tables, thresholds, and conditions based on physical factors and assumptions, and (3) assess his evaporation results over the continental U.S. and analyze the performance of his formula in a subarctic Canadian catchment by comparing it with five other calibrated (aerodynamic and mass transfer) evaporation formulae of varying complexity. We find that Horton's method, due to its unique variable vapor pressure deficit (VVPD) term, outperforms all other methods by ∼3 %–15 % of R2 consistently across timescales (days to months) and at an order of magnitude higher at subdaily scales (we assessed up to 30 min). Surprisingly, when his method uses input vapor pressure disaggregated from reanalysis data, it still outperforms other methods which use local measurements. This indicates that the vapor pressure deficit (VPD) term currently used in all other evaporation methods is not as good an independent control for lake evaporation as Horton's VVPD. Therefore, Horton's evaporation formula is held to be a major improvement in lake evaporation theory which, in part, may (A) supplant or improve existing evaporation formulae, including the aerodynamic part of the combination (Penman) method, (B) point to new directions in lake evaporation physics, as it leads to a “constant” and a nondimensional ratio (the former is due to Horton, John Dalton (1802), and Gustav Schübler (1831) and the latter to Jožef Štefan (1881) and Horton), and (C) offer better insights behind the physics of the evaporation paradox (i.e., globally, decreasing trends in pan evaporation are unanimously observed, while the opposite is expected due to global warming). Curiously, Horton's rare observations of convective vapor plumes from lakes may also help to explain the mythical origins of the Greek deity Venus and the dancing Nereids.
Highlights
The problem of accurate lake or open water evaporation estimation has been a subject of scientific inquiry, in the modern sense of combined experimental and theoretical study, for the past 4 centuries
High-latitude lakes are quite important in the context of accelerated Arctic warming (Smith et al, 2005), as the region is besprinkled with numerous tiny lakes, where the mean evaporation for each lake may vary appreciably due to the variability in the vapor blanket thickness (Eqs. 5–8), which means that the role of the vapor blanket cannot be ignored
It follows that the methods of midlatitudes cannot be directly applied, though Horton believed that his evaporation method is generalizable for sub-zero conditions and condensation
Summary
The problem of accurate lake or open water evaporation estimation has been a subject of scientific inquiry, in the modern sense of combined experimental and theoretical study, for the past 4 centuries. Horton’s lake evaporation formulae provides an overview of the concepts that have evolved from antiquity. Since the 1700s, key contributions have included those of Johann and Daniel Bernoulli (1700s), John Dalton, Rudolf Clausius, and Osborne Reynolds (1800s), who began the celebrated voyage through turbulence theory (Davidson et al, 2011) from European, American, and Russian schools, among others, especially as the data of field experiments on surface winds and diffusion became increasingly crucial for chemical warfare efforts over the course of the 20th century (Sutton, 1953). More recent developments include the recognition of the complementary principle of evaporation in the late 1900s (Bouchet, 1963; Morton, 1994; Brutsaert, 1982) and the evaporation paradox (Roderick and Farquhar, 2002), which have large implications in climate change debates
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