WP2 - Evapotranspiration

Objectives
1. To measure evapotranspiration and energy fluxes at different scales and for different surface types using eddy correlation, scintillometry and remote sensing.

2. To analyze the effect of scale on estimation of evapotranspiration by combining multi-scale measurements, from ground measurements to airborne and satellite remote sensing sensors, including the land surface state variables that play a role in the energy fluxes (albedo, leaf area index, etc)

3. To increase the understanding of the linkage between evapotranspiration and climate change.

4. To estimate the evaporative flux over the catchment area.

Work content
Task 1: Eddy correlation measurements
Three micrometeorological eddy covariances flux stations were established to obtain the most accurate measure of evapotranspiration and energy balance for three sites representing the main land-use types. Measurements have been running since they were established in 2008, thus providing information on both the geographical and interannual variability and will to an increasing extent be used for modeling and scaling of exapotranspiration to the entire catchment.

Data are processed using the Alteddy software (Wageningen University and Research Centre, The Netherlands).

Task 2: Vegetation specific potential evapotranspiration
The most appropriate methods to assess crop specific evapotranspiration will be studied and tested at different scales. Project-data as well as standard meteorological data and data on surface characteristic will be utilized. ‘Transfer functions’, which relate meteorological data to net radiation and reference evapotranspiration to crop specific potential evapotranspiration, will be developed.

Task 3: Remote sensing based estimation of evapotranspiration
Sensible and latent heat fluxes (evapotranspiration) will be estimated by means of different remote sensing dataset. The models used are based on the physically based Two Source Energy Balance Model (TSEB), which partitions the fluxes into soil and canopy (soil evaporation and canopy transpiration).

Data from different satellite sensors with varying viewing and spectral characteristics will be compared and evaluated for the evapotranspiration modeling. The different approaches will exploit different remote sensing platforms, such as TERRA and AQUA MODIS, for the assessment of the morning temperature rise, and ENVISAT AATSR to assess directional effects on Land Surface Temperature and the retrieval of canopy and soil temperatures.

The results will be related to the approaches used for remote sensing of soil moisture described in WP3 and will be essential for the regionalization of evapotranspiration. The data and results from the eddy correlation masts and the scintillometer are essential for the validation of the remote sensing algorithms and serve as a basis for improving the remote sensing methods.

Task 4: Regionalization of evapotranspiration
Upscaling of evapotranspiration from plot, field to regional scale, will be done based on remote sensing data in a multi-platform approach. Data from high spatial resolution as airborne data and Landsat and ASTER to coarse resolution (MODIS and AATSR) will be utilized to assess not only the estimated fluxes, but also the retrieved surface state variables, such as surface albedo, leaf area index, soil moisture or land surface temperature and emissivity.

It will be addressed how the different surface types affect evapotranspiration and the effect of the sensor resolution will be analyzed. By using the information from the three eddy-correlation masts together with ground remote sensing sensor, a study of the evapotranspiration from three different surface types will feed into a general framework for assessment of area integrated evapotranspiration based on standard meteorological observations. The satellite derived evapotranspiration can support up-scaling to the entire catchment.

Task 5: Partitioning of evapotranspiration
Direct measurements of the partitioning of forest evapotranspiration into transpiration, interception evaporation, and moss evaporation. Identification of strengths and weaknesses in sub-process description in current hydrological surface models by comparison with measurements. Please refer to the publication list for literature on the detailed results.

Task 6: Field work activities
To support all the above mentioned tasks, intensive field campaigns will be systematically carried out during the growing season as well as coincident with specific airborne remote sensing campaigns. Both destructive and non-destructive sampling will be carried out to obtain field information about leaf area index, plant height, fraction of green vegetation, vegetation water content, as well as chlorophyll content and photosynthetic activity, in coordination with WorkPackage 6 (Greenhouse gasses).