Tree transpiration


Thanks in part to the contribution of UCL researchers, the amount of water that returns to the atmosphere from a given tract of forest can now be measured. This is a major advance for the field of resource management.

‘Too bad there’s no Nobel Prize for environmental science’, quips Marnik Vanclooster, professor and researcher at UCL’s Earth and Life Institute, a laboratory for the study of water and soil phenomena. Indeed, its research in collaboration with the Royal Observatory of Belgium and other Belgian and French partners is particularly innovative for hydrology, as it enables scientists to precisely measure variations in gravity caused by tree transpiration. It’s a world first: never before has it been possible to measure this phenomenon on a such a scale. And yet the technique is essential to water resource and ecosystem management and climate modelling.

‘Water is a major concern for the entire planet, and evapotranspiration plays a crucial role’, Prof. Vanclooster explains. ‘In some regions, fresh water is lacking, which causes major economic and political problems and migratory flows that can’t be ignored. The measurement system that we’ve implemented has enormous potential. It could provide a methodology for reducing uncertainty in quantifying water resources. This can help us understand how water circulates through the porous mass of the earth’s crust and varies in places, and thus be able to more precisely describe hydraulic flow. More broadly, we’ll have a better understanding of available resources.’

Before and after: changing mass

On sunny days, through evaporation and transpiration, soil and trees release an enormous amount of water into the atmosphere. This evapotranspiration increases with the sun’s rise, peaking at mid-day, and decreases with its descent. The process can be quantified by measuring gravity.

‘Measuring evapotranspiration is a complex process’, Prof. Vanclooster says. ‘It’s easy to do with a bottle of water, just weigh it before and after. If it weighs less, that means evapotranspiration has taken place. But doing the same on a large scale is virtually impossible.’

Hydrologists and climatologists are well-acquainted with evapotranspiration, but until now most attempts to measure it were based on models or satellite images – indirect, imprecise methods that could only lead to hypotheses. ‘Our method makes it possible to calibrate these hypotheses. We worked toward a system for measuring the earth’s mass, by monitoring gravimetric signals, which is generally done for tectonic analysis, but we studied the data in a way that enabled us to analyse variations in mass caused by evapotranspiration.’

Under the Membach forest

Every tree releases hundreds of litres of water into the atmosphere. It’s this loss of mass researchers were able to measure using a superconducting gravimeter at the Membach geophysical station, near Eupen, in eastern Belgium. ‘The gravimeter is in the Vesdre river valley, in a tunnel 50 meters below a forest. It’s a sensor that for years has measured tectonic activity in the Ardennes, but we wanted to analyse the data differently. We were able to measure the mass of the earth’s surface above the sensor using variations in gravity.’ These variations in the acceleration of gravity are less than one nanometre per second squared, or in other words one tenth of a billionth of a gram (given that a gram’s gravity is equal to 9.81 m/s²).

By analysing the data, the researchers found that on average the Membach forest releases into the atmosphere 1.7 litters of water per square meter every day in June.

The study was the fruit of collaboration between the Royal Observatory of Belgium, the Université catholique de Louvain, the universities of Liège, Mons, La Rochelle and Paris Diderot, and the Institut National de l'Information Géographique et Forestière français (IGN). It was the subject of an article in the renowned American journal Geophysical Research Letters.

The research began a dozen years ago and will continue with the benefit of additional sensors, including a gravimeter in the Rochefort caves. In parallel, a researcher based at UCL will continue to work on the topic in order to improve data interpretation.


Anne-Catherine De Bast


A glance at Marnik Vanclooster's bio

Marnik Vanclooster is a professor and researcher at the Université catholique de Louvain. He earned a doctorate in soil physics and develops research projects in the fields of agricultural hydrology and unsaturated zone hydrology. His team’s research addresses the characterisation and modelling of the flow of water and accompanying material in unsaturated geological zones. He has 20 years of experience in coordinating and leading projects at the national, European and international levels, particularly in the Maghreb and Central Africa. He chairs the Environmental Sciences Department of the UCL Earth and Life Institute. He was elected president of the Vadose Zone Sciences Division of the European Geophysical Union (2011-13) and of the Belgian Commission for UNESCO’s International Hydrology Programme. He was a member of the editorial committees of the Journal of Hydrology and the Vadose Zone Journal, and is currently a member of the editorial committees of Agricultural Water Management and the Hydrology and Earth Sciences Systems Journal (HESS).

Published on January 15, 2017