1. Research Centre Jülich; Inst. of Bio- and Geosciences, Agrosphere (IBG 3); Jülich 52425 Germany
2. Bulgarian Academy of Sciences; Inst. of Electronics; Sofia 1784 Bulgaria
3. KU Leuven; Dep. of Earth and Environmental Sciences; Leuven 3000 Belgium
4. Univ. of Bonn, Inst. of Agricultural Engineering Bonn; Bonn 53115 Germany