Research

Coupling optical and thermal directional radiative transfer to biophysical processes in vegetated canopies

PhD thesis (2.09 MB)

On the basis of every remote sensing algorithm, like estimation evapotranspiration and drought monitoring, the retrieval of essential parameters is required. The retrieval of these parameters is highly coupled to the transfer of radiation on the ground surface. The modeling of the radiative transfer is complicated because of the resolution of the satellite sensors. Satellite observations typically have resolutions ranging from 30 meters to 3 km. The retrieval of information from such a pixel is a delicate procedure. The retrieval of these parameters therefore is mostly based on the difference in absorption for different wavelengths.  This multi-spectral approach in most cases is limited to the optical spectrum of the radiation, and produces reasonable results. However with increasing complexity of the remote sensing algorithms the requirements of such data (amount and accuracy) is increasing.

In order to meet these requirements investigation needs to be performed to increase the number of biophysical parameters that can be retrieved from space, and the accuracy of such retrieval schemes. Increasing the spectral resolutions of the retrieval schemes will not be sufficient to meet those requirements, where increasing the number of observation angles will.

At the moment most satellites look straight down to the earth. The amount and accuracy of the parameters from these satellites are therefore limited by this constraint.  Using multiple view angles more parameters can be retrieved, and with better accuracy. For example: traditional movies provide a 2D sensation to the audience, however using special glasses (enabling dual view capacity) the audience is now treated with an extra sensation: depth (or 3D).

The objectives of this research is to investigate the retrieving of biophysical parameters and implement these into a model that can estimate evapotranspiration.

Specific tasks are:

• The multi-directional observation of reflected (optical) and emitted (thermal) radiation, concurrently with the measurement of the biophysical parameters (LAI). 
• The retrieval of these biophysical parameters and validation.
• The implementation of these biophysical parameters into an ecological radiative transfer model (SCOPE). 

Pictures of data acquisition

  
Figures 1 and 2: Setup of a goniometer for multi-directional measurements.

  
Figures 3 and 4: A standard meteorological tower with a scintillometer system to measure fluxes, and
an eddy-covariance tower measuring three-dimensional windspeeds.

  
Figures 5 and 6: SEN2FLEX field campaign area. We participated in many ESA extensive field campaigns. This ensured that all the biophysical parameters were measured on the ground, space and in the air.

  
Figures 7a+b and 8: Vegetation height data from a terrestrial laser scanner data. The laser scanner data was modeled using the L-systems approach.

Pictures of measurement output

Figure 9: Multi-directional mono-spectral thermal radiative measurements of a vineyard. The measurements were performed with a broadband Irisys thermal camera. Concurrently a radiometer was attached to the goniometer.

Figures 10-14: Multi-directional multi-spectral measurements.

Pictures of implementation of data into a coupled model

Figure 15: The SCOPE model flowchart. The SCOPE model couples radiative fluxes to sensible heat fluxes and latent heat fluxes.

Figures 16 and 17: Directional simulated radiation from the SCOPE model.

Figures 18-21: Biophysical fluxes simulated by the SCOPE model. The blue line denotes the fluxes during the overpass time of the AATSR sensor onboard of the ENVISAT satellite.