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Summary Report No. 20

Simulation of soil moisture patterns using a topography-based model at different scales

D.-I. Müller-Wohlfeil, W. Lahmer, W. Cramer, V. Krysanova (July 1996)

This work focuses on the question whether and to what extent topography-based models like TOPMODEL provide a way to introduce lateral flow components into regional/global ecosystem models, where only vertical processes have been considered so far. The applicability and implications of the TOPMODEL approach were studied for regional and large scale investigations and for target mesh sizes up to 1 km. The main goal of the present work was to study scaling effects in TOPMODEL applications. Simulations performed for various German catchments and the German part of the Elbe drainage basin show that TOPMODEL can be used even in lowland regions when high resolution DEMs are available. However, special care must be taken in preparing distribution functions of the topographic index ln(a/tanß). The influence of catchment size, topographic conditions, spatial resolution, and channel position on the ln(a/tanß) distributions and the simulation results were analyzed. It could be shown that runoff estimations are scale dependent and that differences in discharge to a large extent can be compensated by changes in mean values of transmissivities T0, even for the 1000 m grid scale. After this compensation, spatially aggregated saturation patterns are more or less independent of scale. The similarity between the ln(a/tanß) distributions produced from high and low resolution DEMs (when corrected for scale dependent parameters) justifies the use of TOPMODEL at larger scales.

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