『Abstract
In a globally warming climate, observed rates of atmospheric
evaporative demand have declined over recent decades. Several
recent studies have shown that declining rates of evaporative
demand are primarily governed by trends in the aerodynamic component
(primarily being the combination of the effects of wind speed
(u) and atmospheric humidity) and secondarily by changes in the
radiative component. A number of these studies also show that
declining rates of observed near-surface u (termed ‘stilling’)
is the primary factor contributing to declining rates of evaporative
demand. One objective of this paper was to review and synthesise
the literature to assess whether stilling is a globally widespread
phenomenon. We analysed 148 studies reporting terrestrial u trends
from across the globe (with uneven and incomplete spatial distribution
and differing periods of measurement) and found that the average
trend was -0.014 m s-1 a-1 for studies with
more than 30 sites observing data for more than 30 years, which
confirmed that stilling was widespread. Assuming a linear trend
this constitutes a -0.7 m s-1 change in u over 50 years.
A second objective was to confirm the declining rates of evaporative
demand by reviewing papers reporting trends in measured pan evaporation
(Epan) and estimated crop reference evapotranspiration
(STo); average trends were -3.19 mm a-2
(n=55) and -1.31 mm a2 )n=26), respectively. A third
objective was to assess the contribution to evaporative demand
trends that the four primary meteorological variables (being u;
atmospheric humidity; radiation; and air temperature) made. The
results from 36 studies highlighted the importance of u trends.
We also quantified the sensitivity of rates of evaporative demand
to changes in u and how the relative contributions of the aerodynamic
and radiative components change seasonally over the globe. Our
review: (i) shows that terrestrial stilling is widespread across
the globe; (ii) confirms declining rates of evaporative demand;
and (iii) highlights the contribution u has made to these declining
evaporative rates. Hence we advocate that assessing evaporative
demand trends requires consideration of all four primary meteorological
variables (being u, atmospheric humidity, radiation and air temperature).
This is particularly relevant for long-term water resource assessment
because changes in u exert greater influence on energy-limited
water-yielding catchments than water-limited ones.
Keywords: Climate change; Stilling; Evaporation paradox; Pan evaporation;
Reference evapotranspiration; Trends』
Contents
1. Introduction
2. Trends of near-surface terrestrial wind speed
2.1. Global terrestrial review
2.2. Global terrestrial meta-analysis
2.3. Global (terrestrial and oceanic) synthesis
2.4. Possible causes of stilling
2.5. Implications for other disciplines
3. Trends in evaporative demand
4. Importance of wind speed to the evaporative process
4.1. Sensitivity analysis
4.2. Relative importance of aerodynamics on evaporation trends
5. Conclusion
Acknowledgements
Appendix A. Supplementary material
References