Bachelor Thesis
Untersuchung des Bowen-Verhältnisses über verschiedenen Landnutzungstypen während des COPS-Experimentes
Katrin Kohnert (04/2010-08/2010)
Support: Rafael Eigenmann, Thomas Foken, Johannes Lüers
This study aims to find local correlations between vegetation and the atmosphere by means of the Bowen ratio and to relate the results to the height of the sites. It is based on data from eight measuring sites of the COPS (Convective and Orographically-induced Precipitation Study) campaign that took place in South Western Germany and Eastern France in the sum-mer of 2007. The area of COPS is special for its heterogeneity regarding its orography as well as the occurring types of land use. The relevant measuring sites for this study are located in the Rhine valley, in the lee of the Black Forest, in the Kinzig valley and on top of the Hornis-grinde, the highest mountain in the Northern Black Forest. At the sites measurements of tur-bulent fluxes and measurements of moisture were performed. As this study focuses on fluxes that represent single types of land use as well as on fluxes that are energetically relevant, it is shown that the selection of the data has to follow strict criterions and that it is indispensable to reject the majority of the data. In this study 83 per cent of the data available are rejected. Based on the selected data the Bowen ratio is determined by different methods, such as a regression analysis and the determination of the average value. The validity of these methods and the obtained results are discussed. In order to find reasons for the variances, the course of the Bowen ratio for the whole measuring time of the COPS campaign is shown. Especially for the measurements above grassland high variations can be noticed that are related to the dates of mowing. With the transpiration decreasing after mowing, the Bowen ratio rises. It was found that precipitation has an influence on the extent of this increase. It is explained that it is necessary to exclude data that result out of human activities in order to determine the mean Bowen ratio that is representative for an area. Based on the dataset no clear correlation be-tween the Bowen ratio and the type of land use could be detected. However, the Bowen ratio seems to depend on the height of the measuring site, as by far the highest Bowen ratio was determined on the Hornisgrinde. In addition, it is found that the splitting up of the residuum to the latent and sensible heat flux according to the Bowen ratio does not lead to significant changes in the Bowen ratio.