干旱区生态环境知识资源中心

The productivity of the vegetation layer and its consumption by cattle, goats and sheep are important topics in characterizing the ecologic conditions in the North-western Namibian rangeland. Using a mesoscale biosphere model the calculation of above ground phytomass (= biomass) and their seasonal productivity based on satellite data is of specific interest. The investigation area, Kaokoveld (north western Namibia), is characterized by a strong hydro climatic gradient with an annual precipitation range from 380mm/a in the north eastern part of the research area to 50 mm/a at the border of the Namib Desert. Small scale vegetation patterns with fractions of savannahs, woody savannahs, open and closed shrub lands and grasslands are the manifestation of this climatic gradient and the heterogeneous relief. The study area is partly used by local herders of the Himba as pasture ground for their livestock. This usage causes problems such as overgrazing and degradation of the vegetation. Together with the impact of climate change the known ecological gradient has strengthened during the last decade.

With the remote sensing based regional biosphere model (RBM Kaokoveld) quantitative information about biomass changes and pasture ecology can be determined. Growth and reduction of biomass can be observed by using the theory of Monteith(1) and Running et al.(2) Biomass production can be derived from the combination of incoming solar radiation, NDVI, resulting from MODIS data and a biophysical conversion factor. This factor describes the ability of plants to produce net primary production (NPP). The regional biosphere model allows extracting detailed information from an area-wide biomass balance by using remote sensing. This balance describes the production as well as the consumption of biomass by cattle, game and natural decomposition. The modelling approach runs on medium temporal and spatial scale with a decadal time step and spatial resolution of 1 km. These temporal and spatial resolutions may allow in future the integration of retrospective NOAA AVHRR time series. At the moment the model uses a four years time series of MODIS data from 2000 to 2005, with biomass changes and degradation areas as results. First results of the modelling outcome shown that the influence of overgrazing and the process of temporal vegetation degradation within a yearly cycle are mostly driven by sufficient production of biomass during the raining season.