I have a strong interest in evapotranspiration measurement and theory (Cuxart et al. 2019). However, it has always been a concern of mine that the Penman-Monteith method for calculating evapotranspiration misses several crucial terms, notably (i) evaporation from inundated water and (ii) hillslope processes.

Why is this a problem? In areas of the world covered for example by oil palm grown on slopes, or crops grown under inundated conditions such as rice, it seems obvious that Penman-Monteith cannot be used without modification to calculate ET. However, land surface models such as JULES do not have any modifications to P-M, so doesn't that mean the models will struggle to calculate ET from rice and oil palm plantations?

Following this logic, there is an unrecognised problem here for land surface modellers: of the 1560 million ha total cropland in the world, 162 million ha are rice paddies (2019) and 20 million ha are oil palm plantations (2018). Can it really be the case that we cannot trust ET estimates from 12% of global croplands because we know there are missing fluxes there?

[For me, this should start as a review paper: see my email to ER Feb 2019 pasted into my ppt from the GEWEX workshop Feb 2021].

In October 2019 we organised a GHP cross-cutting workshop Determining Evaporation led by Prof. Joan Cuxart in Sydney, Australia. See here. I presented a talk Evapotranspiration in the JULES land surface model.

The 2nd GEWEX Evapotranspiration workshop was hosted by Univ. Wageningen online in February 2021.

Prof. Henk de Bruin visited CEH Wallingford on 8th March 2019 and gave a wide-ranging talk entitled Evapo(transpi)ration: How to model it? How to measure it? A critical review of the state of the art and a call for novel research. The seminar attracted several senior hydrologists and micro-meteorologists from pre-CEH days. CEH article in The Grapevine is here and the slides and audio are here (including the energy balance closure blues!).

Photo: L-R: Chris Taylor, John Gash, Colin Lloyd, Jim Shuttleworth, Eleanor Blyth, Henk de Bruin, Richard Harding, Anne Verhoef , Jim Wallace, Jonathan Evans.