1. Plot scale processes
Jan Vanderborght: Soil Physics, modelling in terrestrial systems
Nicolas Brüggemann: Stable Isotopes
Mathieu Javaux: Soil Physics, modelling soil-root interactions
Harry Vereecken: Hydrogeophysics and modelling
Jan Van der Kruk: Geophysics

2. Sources and isotopic signatures of transpiration
Source: Yakir & Sternberg, Oecologia 123, 297–311, 2000.
Source: Pate and Dawson, Agroforestry Systems, –275, 1999.
General pattern:
Soil evaporation is isotopically much more depleted than transpiration; hence, a differentiation is possible
Isotopic signature of transpiration water equals that of stem (=xylem) & source water at isotopic equilibrium
Questions:
How do the different sources of plant water (surface water/precipitation, deep water/groundwater) shift with changing environmental conditions?
How does water stress affect the isotopic enrichment of leaf water?

3. Experimental approach for water isotopes
Profile measurements of water vapor and its isotopic composition (δ18O, δD) in air and soil
Plot analysis of the isotopic composition of the air water vapour source
Determination of the isotopic composition of soil water with high time resolution
Modified from Campbell TGA100A manual
Gaspermeable tubing

4. Combination of ground-based remote sensing, geophysics, flux measurements, soil moisture sensor networks
13.5 m TDR Trench Temperature, soil water content, CO2, Matric potential
L-Band Radiometer (soil moisture)
EC Station (CO2 & H2O)
ERT (soil moisture)
IR camera
(canopy T)
climatic station
H2O isotope analyzer
Field scanner (GPR, IR-Sensor,soil roughness)

5. Research Questions
Relation between soil water content and water stress.
Interpretation of isotope measurements to detect (water) stresses, investigate water uptake, and water transport in plants.
Approach:
Combination of different methods to observe the system
geophysical methods  subsurface water content distributions
ground based remote sensors  canopy status
isotope techniques  characterization of plant water sources, plant water uptake, differentiation between evaporation and transpiration, plant water stress
Mechanistic soil-plant-atmosphere models
link different data sources.

6. Model components Observations
Canopy surface temperature
(IR camera)
Energy balance
Photosynthesis
Root water uptake, stomatal conductance, transpiration
18O, D
Soil water content (3D)
(TDR, ERT, GPR)
Root water uptake
Soil water flow

7. Objectives for first period
Relations between d18O and dD in transpiration fluxes and water stress
Integrated monitoring of different variables (soil moisture, canopy temperature)
Integration of data using modelling
Identification of advantegeous plant properties to avoid water stress using modelling: root architecture, root growth parameters, water use efficiency