Institute of

Climate-Smart Agriculture

Ullrich Dettmann

Dr.-Ing. (13.05.2016)

Ullrich Dettmann with doctoral cap
Ullrich Dettmann with doctoral cap (© Thünen-Institut/AK)

On the 13th of May 2016, Ullrich Dettmann successfully defended his doctoral thesis “Improving the determination of soil hydraulic properties of peat soils at different scales” at the University of Rostock. The aim of his thesis was to improve the characterization of unsaturated hydraulic properties of peat soils, which are in turn fundamental to understand and accurately model soil moisture and hydrological dynamics as key control for the biogeochemical processes in peatlands. This was done with two major topics:

In the first major part, the applicability of commonly applied hydraulic functions to a broad range of different peat types and other organic soils was evaluated by transient laboratory evaporation experiments. The results demonstrate that peat soil moisture dynamics can be accurately modeled with these functions, if adequate model set-ups are chosen. Of particular importance is that, under wet conditions, macropores are taken into account. Considering the results can improve hydrological models of the transient flow processes in the unsaturated zone and practical applications like large scale simulations.

Hydraulic properties obtained in the laboratory may not be representative for field conditions due to spatial variability and scale dependency. Hence, in the second major part, soil water retention characteristics were determined for a Sphagnum bog by a new developed in situ method. The method is applicable to shallow groundwater systems and relies on the dependency between water level rise and precipitation. This dependency was used to estimate soil water retention characteristics by Bayesian inversion. However, water level rises of shallow groundwater systems are influenced by the microrelief due to partial inundation. To consider the microrelief effects on the water level rises, a new conceptual one-dimensional expression was derived in a separate study. Using the derived equation in situ soil water retention characteristics for the Sphagnum bog could be constrained to a plausible range for the low suction range. Results indicate that the approach is a promising tool to characterize field variability of soil water retention characteristics with commonly available data.

Overall, the thesis of Ullrich Dettmann improves the characterization of unsaturated hydraulic properties for different types of peat and other organic soils at the laboratory and the field scale. Furthermore, results improve the understanding of the hydrological processes in peatlands in general.