Testing different decoupling coefficients with measurements and models of contrasting canopies and soil water conditions

Abstract. Four different approaches for the calculation of the well established decoupling coefficient Ω are compared using measurements at three experimental sites (Tharandt – spruce forest, Grillenburg and Melpitz – grass) and simulations from the soil-vegetation boundary layer model HIRVAC. These investigations aimed to quantify differences between the calculation routines regarding their ability to describe the vegetation-atmosphere coupling of grass and forest with and without water stress.

The model HIRVAC used is a vertically highly resolved atmospheric boundary layer model, which includes vegetation. It is coupled with a single-leaf gas exchange model to simulate physiologically based reactions of different vegetation types to changing atmospheric conditions. A multilayer soil water module and a functional parameterisation are the base in order to link the stomata reaction of the gas exchange model to the change of soil water.

The omega factor was calculated for the basic formulation according to McNaughton and Jarvis (1983) and three modifications. To compare measurements and simulations for the above mentioned spruce and grass sites, the summer period 2007 as well as a dry period in June 2000 were used. Additionally a developing water stress situation for three forest canopies (spruce, pine and beech) and for a grass site was simulated. The results showed large differences between the different omega approaches which depend on the vegetation type and the soil moisture.

Between the omega values, which were calculated by the used approach, the ranking was always the same not only for the measurements but also for the adapted simulations. The lowest values came from the first modification including doubling factors and summands in all parts of omega equation in relation to the original approach. And the highest values were calculated with the second modification missing one doubling factor in the denominator of the omega equation.

For example, the averages of omega ranged in the summer period 2007 from 0.11 to 0.19 for the spruce site and moderate soil wetness and from 0.42 to 0.58 for the grass site and higher soil wetness. In the case of the simulated drying out of four different canopies the forest stands showed a similar change of omega from about 0.65 (moderate soil wetness) to 0.1 (low soil wetness). The absolute change of omega for the grass canopy was smaller than for the forest canopies (on average from 0.95 to 0.7). But the differences between the used omega approaches increased.

Especially the results from the longer period in summer 2007 demonstrate that the various modifications of the decoupling coefficient lead to a change in the long-term quantity of omega. This has, for example, consequences for the description of the coupling of heterogeneous landscapes.