Note from the Flux Site: Trends in Evaporation

To Be or Not to Be…that is the Question

One of the challenging questions facing bio-geoscientists is whether the hydrological cycle is changing in a warming world with more CO₂. There is speculation and evidence for both acceleration and a reduction in evaporation.

The answer to this question is mixed as it is based on indirect estimates of evaporation derived from sparse and short eddy flux data sets, models, remote sensing, or evaporation pans. One set of analyses of long-term records of pan evaporation suggests a reduction in pan evaporation (Golubev et al., 2001; Lawrimore and Peterson, 2000; Roderick and Farquhar, 2004). Some scientists have interpreted the negative trends in pan evaporation as an indicator of increasing actual evaporation (Brutsaert and Parlange, 1998; Golubev et al., 2001). This pan paradox has been explained in terms of Bouchet’s complementary theory which argues that a decrease in pan evaporation can correspond with an increase in actual evaporation; yet we caution the reader as there are alternative views that dismiss the application of Bouchet’s complementary theory (McNaughton and Spriggs, 1989). Other scientists have claimed that the negative trends in pan evaporation follow a global reduction in the wind (stilling) and a decline in solar radiation (due to pollution) (dimming) (Roderick and Farquhar, 2002).

Another early study inferred that rising CO₂ is causing stomatal closure, reducing actual evaporation, and increasing runoff (Gedney et al., 2006). Some recent syntheses using relatively short time series from networks of flux data are showing a small rise in evaporation (Ueyama et al., 2020), a regional decline in evaporation (Wang et al., 2021), or an increase in water use efficiency (Keenan et al., 2013)

Many of us in the Ameriflux community have been reluctant to tackle this intriguing problem, with direct measurements of trends in evaporation, because our time series had been perceived as being too short. But with more sites reaching 2 decades, it can be time for members of our community to assess this problem with our growing flux dataset.

What does theory tell us to expect? On paper, colleagues with a physiological background tend to advocate that warming should increase evaporation (E) because the saturation vapor pressure of the surface increases exponentially with temperature, yielding a stronger moisture gradient between the surface and atmosphere. In other words, rising temperature is expected to increase the supply of moisture to the atmosphere.  We can see this behavior if we express latent heat exchange using Ohm’s Law resistance analog, where q is the mixing ratio of water vapor, R_w is the resistance for water vapor exchange.

λE = ρλ  (( q_s ( T_s ) – q_a ) / R_w )

But this response is only part of the story. We also need to consider need the demand for evaporation, as set by the surface energy balance (Monteith, 1981). In a warmer world, there may be less energy available to drive latent heat exchange. In this case, warming can reduce evaporation because more longwave energy and sensible heat are lost by a warmer surface.

λE = ( 1 – ρ) R^↓ + εL^↓ – εσT_s⁴ – G – ρC_p (( T_s – T_a ) / R_H )

As we may infer from the following figure, actual evaporation may occur at the conditions where the supply and demand curves meet in a warming world. In practice, these opposing feedbacks may act to mute the potential rise in evaporation with warming.

The intersection between evaporative supply predicted by Ohm’s Law and the demand, limited by the energy balance. Computations of the energy balance assume incoming shortwave and longwave radiations were 800 and 450 W m-2, respectively. The physiological evaporative flux was based on air temperature at 20°C and atmospheric vapor pressure of 850 Pa. Computations were performed for a range of surface temperatures and surface resistances. Adapted from John Monteith (Monteith, 1981).

This muting of evaporation trends seems to be consistent with some preliminary results from our 20-year evaporation records in California, using both eddy covariance measurements over an oak savanna and annual grassland and applying the BESS model across the state of California. For our Mediterranean climate, we have not detected a trend in evaporation over the past 20 years at the field and statewide scales, despite a 1.6°C rise in air temperature over long-term base temperatures.

Will this flat trend in evaporation continue to hold into the future? Does it represent what may be occurring in more temperate and wetter environments? We don’t know until we extend our Ameriflux measurements into the future and evaluate what we are collecting so far from other sites. Then we will be able to answer the question ‘will it be or not be’.

References

Brutsaert, W. and Parlange, M.B., 1998. Hydrologic cycle explains the evaporation paradox. Nature, 396(6706): 30.

Gedney, N. et al., 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature, 439(7078): 835-8.

Golubev, V.S. et al., 2001. Evaporation changes over the contiguous United States and the former USSR: A reassessment. Geophys. Res. Lett., 28(13): 2665-2668.

Keenan, T.F. et al., 2013. Increase in forest water-use efficiency as atmospheric carbon dioxide concentrations rise. Nature, 499: 324.

Lawrimore, J.H. and Peterson, T.C., 2000. Pan evaporation trends in dry and humid regions of the United States. J. Hydrometeorol., 1(6): 543-546.

McNaughton, K.G. and Spriggs, T.W., 1989. An evaluation of the Priestley and Taylor equation and the complementary relationship using results from a mixed-layer model of the convective boundary layer. In: T.A. Black (Editor), Estimation of areal evapotranspiration, pp. 89-104.

Monteith, J.L., 1981. Evaporation and Surface-Temperature. Q. J. R. Meteorol. Soc., 107(451): 1-27.

Roderick, M.L. and Farquhar, G.D., 2002. The Cause of Decreased Pan Evaporation over the Past 50 Years. Science, 298(5597): 1410-1411.

Roderick, M.L. and Farquhar, G.D., 2004. Changes in Australian pan evaporation from 1970 to 2002. Int. J. Climatol., 24(9): 1077-1090.

Ueyama, M. et al., 2020. Inferring CO2 fertilization effect based on global monitoring land-atmosphere exchange with a theoretical model. Environ. Res. Lett., 15(8): 084009.

Wang, R. et al., 2021. Long-term relative decline in evapotranspiration with increasing runoff on fractional land surfaces. Hydrol. Earth Syst. Sci., 25(7): 3805-3818.