Scenario-based groundwater modeling of a raised bog with Mike She

Water table modeling in peatlands is often done on the large scale and, consequently, based on coarsely resolved models. The models commonly used in literature are often either not capable of modelling the full water cycle or they are not purely physically based. In particular in Bavaria there is a high number of small isolated peatlands with a dense drainage network, therefore a coarse model is not feasible. For rewetting success and climate impact analysis the fully integrated and largely physically based Mike She modelling software by DHI was used in the KliMoBay Project.

The main goal was to achieve a temporally and spatially highly resolved model enabling water table investigations for different rewetting stages as well as associated vegetation and soil changes.

For this purpose, the partially rewetted raised bog Königsdorfer Weidfilz in Bavaria was monitored and replicated in Mike She. Active and partially rewetted drainage ditches were implemented in the hydrodynamic model Mike Hydro and coupled with the Mike She model. After calibration and validation on twelve automatic water level gauges, scenario analyses were conducted. Compared with the climatic reference period (1961 – 1990), the dry year 2018 and the average year 2020 were modeled for three different scenarios: 1. current state, 2. drainage ditches deactivated, 3. vegetation and soil property succession after rewetting. The influence on the water table was analyzed based on a reference depth of - 0.15 m which is considered as an average threshold for climate impact. For this purpose, seasonal and annual mean water table maps were created, as well as standard deviation maps to portray high water table dynamics within the respective mean season.   

As the model results show, it is possible to investigate even small peatland areas for their rewetting potential. Furthermore, we could show the positive impact of rewetting measurements on reducing climate active areas with water levels below - 0.15 m in raised bogs. Vegetation and thus soil property changes in the model – which are assumed to occur after sufficient rewetting along with active acrotelm growth – increase the effect even more. Although, the impact of dry seasons is still significant, the resilience of the peatland increases.

Using the example of the partially rewetted raised bog we were able to proof, that areas with different drainage states could be modeled. The areas rewetted in the respective model scenario react similar to the areas already rewetted in nature. Thus, we assume that the method is capable for planning stages. Consequently, it can offer a descriptive decision support tool. However, the process of model setup, calibration and validation is rather time consuming. Regarding fen peatland management, further models can be set up considering the capability of Mike Hydro to model controllable weirs.