How over 100 years of climate variability may affect estimates of potential evaporation

“Hydrological modeling frameworks require an accurate representation of evaporation
fluxes for appropriate quantification of e.g. the soil moisture budget, droughts,
recharge and groundwater processes. Many frameworks have used the concept of
potential evaporation, often estimated for different vegetation classes by multiplying
the evaporation from a reference surface (“reference evaporation”) with crop specific
scaling factors (“crop factors”). Though this two-step potential evaporation approach
undoubtedly has practical advantages, the empirical nature of both reference
evaporation methods and crop factors limits its usability in extrapolations and non
stationary climatic conditions. In this paper we assess the sensitivity of potential
evaporation estimates for different vegetation classes using the two-step approach
when calibrated using a non-stationary climate. We used the past century’s time series
of observed climate, containing non-stationary signals of multi-decadal atmospheric
oscillations, global warming, and global dimming/brightening, to evaluate the sensitivity
of potential evaporation estimates to the choice and length of the calibration period.
We show that using empirical coefficients outside their calibration range may lead to
systematic differences between process-based and empirical reference evaporation
methods, and systematic errors in estimated potential evaporation components. Such
extrapolations of time-variant model parameters are not only relevant for the calculation
of potential evaporation, but also for hydrological modeling in general, and they may
limit the temporal robustness of hydrological models.”
(Citaat: Bartholomeus, R.P., Stagge, J.H., et al. – How over 100 years of climate variability may affect estimates of potential evaporation – Hydrology and Earth System Sciences 11(2014), p.10787-10828)