EXPERIMENTAL FIELD SITE : DESCRIPTION OF THE O3HP PLATFORM

mardi 5 février 2013
popularité : 100%

At the O3HP field site, special emphasis is put on the investigation of effects related to climate change. The site is equipped with a precipitation management system to simulate drier climate. A device is installed above 300 m² of canopy, for dynamically excluding a defined fraction of precipitation by extending automated covers only during rain episodes, to limit both greenhouse effect and shading. It also allows to re-irrigate part of the excluded water using a a sprinkler system attached to the metal structure in order to fine-tune the rain exclusion. The system manages a reduction in precipitation by 40 % using temperature-derived functions based on 50-year records of local meteorological data. This results in a scenario of 500 mm annual precipitation corresponding to about 2°C temperature increase, which is in line with climate predictions for the Mediterranean region (Giorgi & Lionello, 2008). During leaf devel-opment in spring, exclusion of precipitation events is performed at night-time to avoid photomorphogenesis disturbance.

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A system of dateways

They are organized as a cross, each arm is 10m long and located in two levels of heights : 0.80 m and 3.5 m.

A precipitation management system

A system to simulate drier climate. A device is installed above 300 m² of canopy, that dy-namically excludes a defined fraction of precipitation (rain & hail) by extending automated covers, and which allows to re-irrigate part of the excluded water (Fig. 2). The system manages a reduction in precipitation by 40 %.This results in a scenario of 500 mm annual precipitation corresponding to about 2°C temperature increase, which is in line with climate predictions for the Mediterranean region (Giorni & Lionello 2008) (see below). Intercepted precipitation will be evacuated to a temporary reservoir. An irrigation (sprinkler) system attached to the metal structure will use this water to fine-tune the rain exclusion.

Meteorological Data defining the functioning of the Precipitation Management System

The driving of the precipitation treatment was based on local climate data 1960-2010 (WMO standard, temperature and precipitation, 1960-2003 St Michel l’Observatoire/ Meteo France 04192001 ; 2003-2010 Dauphine/ Meteo France 04068001 - on and close to the property of the Observatoire de Haute Provence OHP, respectively).
An increase in mean annual temperature had been observed and was linearly interpolated between 2000 and 2100, which results in a mean annual temperature of 12.4 and 14.8°C respectively. The relationship between mean annual temperature and the sum of annual precipitation explained 12% (R²) of the variation. This (negative) relation improved considerably to 19% when beginning the interval for the mean annual temperature was set to the dry season (15th August instead of 1st January). However, this also increased the negative slope of relationship (Fig. 3c). The sum of precipitation was (linearly) interpolated to year 2100 and the mean result of the two approaches was 500 mm per year for a 2°C temperature difference. Compared to the reference period 1967-2000 this corresponds to a 40% reduction in annual precipitation.

Piloting of the Precipitation Management System

The piloting of the precipitation treatment is dynamic in the annual course, which is based on the 10 driest years in the dataset (1964, 1966, 1980, 1988, 1989, 2001, 2004-2007, all < 600 mm annual precipitation, Fig. 3d). A normalised curve was derived from the cumulative precipitation. A linear relationship with zero offset is fitted from Julian day 1 to Julian day 365. Two characteristic deviations (minima of the residuals, dry periods) from this linear relationship are approximated with Gaussian functions. The standard deviation is fitted with a 3rd order polynomial function, and is used as a decision tool, to better interpret deviations of the current precipitations in relation to the mean.

Impact of the Precipitation Management System on the vegetation

Great care was taken to reduce the impact on the vegetation when putting the device in place. No heavy machines were allowed on the experimental site and walking is only al-lowed on marked paths or the scaffolding. Foundations are reduced to a minimum size of 0.8 m x 0.8 m x 0.8 m, and a plastic cover surrounds the foundation as to avoid exchanges of the concrete with the environment. The structure is made of steel and is protected with a water-based paint. The maximum spacing was approached for the crossbars in agreement with the specifications of the automatic covers. In the pho-tomorphogenetic period of leaf development, rain exclusion is restricted to night-time (<50 µmol quanta m-2 s-1, limit shown for Quercus robur foliage, IM Reiter pers. comm.) to avoid shading. However, some inevitable shading persists. The effect of the shading was integrated using an optical leaf area index measurement (LAI-2000, Li-Cor, NE, USA). The shading due to the structure is comparable to an additional leaf area above the canopy of 0.15±0.02. This effect is negligible, seeing that light is not a very limiting factor in Mediterranean forests. No changes in the temperature profiles (Table 1) have become evident due to the installation of the precipitation management system device.


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