ENVRI + visit

In the framework of ENVRI + , two scientists Prs. Thomas Giambelluca (University of Hawai'i)  and Alan Ziegler (University of Singapore) will be visiting La Réunion from 8th to 21st of oct 2017.

Scientific objectives:

The drainage basin “OSUR/Erorun” (OSU-R multiple site platform) in La Réunion is dedicated to study water and material flows within the critical zone (the area between the top of canopy and the base of soil). This site constitutes an excellent location to study tropical montane cloud forest (TMCF) ecohydrology, similar to climatic and geological situations in Hawaii. La Réunion exhibits also a very sharp and complex orography, which makes it specifically interesting to study the planetary boundary layer (PBL) dynamics.

The inversion in the trade wind regime of the tropics and subtropics is the result of the interaction between large-scale subsiding air from the upper troposphere and convection-driven rising air from lower levels. The occurrence and base height of the inversion are critical determinants of local climate in insular regions. The trade wind inversion (TWI) tops the planetary boundary layer (PBL), and understanding its dynamics is of importance for modeling the overall climatic system. In fact, the TWI drives the heights of clouds affecting the regular cycles of cloud formation thus cloud water resource for TMCFs.

We will investigate canopy water balance in the context of a TMCF and improve our understanding of the dynamical parameters driving the TWI. This TMCF presents specific hydrological functions linked to its ability to capture fog and affect the water balance specifically due to fog supply, interception and storage in the epiphytes considered as an upper soil compartment.

The canopy water balance approach supported by field measurements will be used to estimate the Cloud Water Interception (CWI). The canopy water balance can be described according to3:

RF+CWI=TF+SF+Ei+ ∆S

Where, RF: rainfall; TF: throughfall; SF: stemflow, Ei interception evaporation, and ΔS: change in the above ground water storage. All terms are in units mm.

Technical description of work to be performed

This field-instrumented site at “Plaine des Fougères” (1350 m), dedicated to the water balance monitoring, is already equipped with a weather station, soil water tension gauges and hydrometric station for runoff flows. New sensors will be added1,3 (TF and SF). Ongoing measurements of the cloud base and top height to study the development of the PBL will complement this project.

Fieldwork planned during the experts’ visit on site will be carried out over a short period (15 days) entirely dedicated to the installation of new sensors and alternative sensors for a later evaluation established over a period of 6 months including at least 3 intense precipitation events.

Meteorological variables measured at the site will be used to estimate the potential evaporation (PE), a variable used in the estimation of Ei. We will add recording TF and SF gauges to allow continuous observation of the precipitation (from rain and fog) reaching the ground. The TF gauges are tipping-bucket devices that capture water falling beneath the canopy in troughs1,3. The tipping mechanism is a scaled up version of the type found in tipping-bucket rain gauges and is equipped with a magnet-sensitive switch to signal tips. The signal is recorded using a Hobo Event Logger (Onset Computers, Inc.). Two TF gauges will be installed at each site and positioned to sample a representative range of canopy density; one SF gauge will be used to collect from multiple stems at each site.

A Juvik-type passive fog collector with rain shield will be mounted at each site (Juvik & Nullet, 1994) under and outside canopy. Water collected by the fog gauge will be funneled into a tipping bucket rain gauge to record the time series of fog water collection. Leaf wetness sensors will also be set up, to record dew and fog events under and outside the canopy. Valentin Wendling, Eric Delcher and Jean-Lambert Join, will particularly contribute to the building and set up of new sensors. Claudine Ah-Peng will be running the lysimeters experiment to measure the interception of cloud water and release by epiphytes (Ah-Peng et al. 2017).

A Vaïsala CT25K Ceilometer has been deployed on site to measure cloud base and top heights since may 2016 monitored by Valentin Duflot and Jimmy Leclair de Bellevue. Pierre Staménoff will assist technically and logistically the whole team.

Another aspect of the expert‘s visit will concern a conference and field training dedicated to the OSUR master students and technicians of the OSUR/Erorun and OSUR/FOREST stations.

Innovative aspects:

The innovative aspect concerns the qualification of an integrate measurement chain specifically dedicated to TMCFs. It will involve the use of new generation sensors and devices in need to be tested as leaf-wetness sensors and automatic digital cameras to improve fog gauge measurements. The use of the same standardized protocol1,3 will allow comparisons of canopy water balance established in other TMCFs (e.g. Hawaii). Novel aspects concern the characterization of the PBL and use of epiphytes to characterize canopy balance, providing insights into the understanding of meso and micro hydrological functioning of TMCFs.

Seminar at OSU-R: 8 october 2017, 9h30-12h30

ENVRI+ seminar

Seminar at the National Park headquarters: 18 october 2017, 10h

ENVRI+ seminar PNR