Hydrologic cycle explains the evaporation paradox

Abstract

The evaporation of water, measured using evaporation pans, has been decreasing in the past few decades over large areas with different climates. The common interpretation is that the trend is related to increasing cloudiness, and that it provides an indication of decreasing potential evaporation and a decreasing terrestrial evaporation component in the hydrologic cycle. Here we show that, although these studies are valuable, pan evaporation has not been used correctly as an indicator of climate change.

Main

At first glance, reports of decreasing pan evaporation in European Russia, Siberia and the western and eastern United States¹, India² and Venezuela³ are paradoxical. They are hard to reconcile with well-substantiated increases in global precipitation and cloudiness⁴, which would normally require more surface evaporation as the only source of atmospheric water vapour, rather than less. They also run counter to predictions of increasing evaporation⁵, as one of the more robust outcomes of radiative forcing, resulting from increasing atmospheric CO₂in global circulation model calculations.

We resolve this paradox by demonstrating that, in non-humid environments, measured pan evaporation is not a good measure of potential evaporation; moreover, in many situations, decreasing pan evaporation actually provides a strong indication of increasing terrestrial evaporation.

The evaporation from a pan, E _pa, can be used as a good indicator of the evaporation, E, from the surrounding environment, but only when land-surface moisture is in ample supply; this normally involves multiplication by a ‘pan coefficient’ a of order one, depending mainly on pan type⁶. The evaporation from any large uniform land surface, with adequate moisture so available energy is the limiting factor, is usually referred to as potential evaporation, E ₀. The actual evaporation from a well-watered surface is E =E ₀=aE _pa. Whenever the land-surface moisture becomes limiting and insufficient to sustain E ₀, the actual evaporation, E, decreases below E ₀and the energy not used up by E manifests itself as an increase in sensible heat flux ΔH, such that E =E ₀- ΔH. This in turn causes aE _pato exceed E ₀, or aE _pa=E ₀+b ΔH, where b is another coefficient slightly larger than one, again depending mainly on pan type. Now aE _pano longer provides a direct measure of E ₀, so it is more appropriately called ‘apparent’ potential evaporation.

The main point is that E and E _paexhibit complementary rather than proportional behaviour; indeed, for instance in the extreme case of a desert environment, E is zero, whereas E _pais at its maximum. The idea of a complementary relationship between actual evaporation and apparent potential evaporation is not new⁷, and it has stimulated advances in the estimation of terrestrial evaporation⁸,¹⁰. In the case of a pan filled with water and placed in a region with less than adequate ground wetness to sustain E ₀, elimination of ΔH in the above yields E =[(1+b)E ₀- aE _pa]/b.

Because a and b are of order one, this equation indicates how the observed¹,³ decreases in pan evaporation, E _pa, can be interpreted as evidence for increasing terrestrial evaporation, E, in those regions. This is consistent with data⁴,¹¹ indicating an intensifying hydrologic cycle in large regions: increasing precipitation leads to increasing surface run-off and soil wetness, which in turn generates more evaporation, and so on.