Deriving effective soil water retention characteristics from shallow water table fluctuations in peatlands
Peatlands are important storage locations for soil carbon and sinks for carbon dioxide. Peat is a type of soil that is primarily composed of partially decomposed plant residuals that exists in an oxygen-deficit environment. It’s the primary component of most wetlands of the world and thus holds an important place in water resources and hydrology. The soil moisture content of the peat soil determines the storage, transport, and release of carbon dioxide, and as a result, the soil hydraulic properties are very important in any climate change studies.
Authors of an article recently published in Vadose Zone Journal use a statistical inversion to determine the soil water retention characteristics using the assumptions of hydrostatic equilibrium and the fact that the lateral fluxes during precipitation are minor compared with the vertical fluxes during precipitation events. Traditional methods of determining the soil hydraulic properties in the laboratory are fundamentally flawed due to changes in the physical nature of a sample being disturbed when moved from its field location. Therefore, in situ field approaches are a better way to determine soil hydraulic properties compared with laboratory methods. In this article, the authors use the observations associated with the rise in the water table caused by precipitation to determine the soil water retention characteristics that can be used with established equations represented by the van Genuchten parameters.
The field data were derived from a near-natural ombrotrophic bog in Germany that has about 6 m of peat and three wells that monitor the water table. The topography and the vegetation of this region were well characterized. A number of events (greater than 100) were used in the algorithm to determine the soil water retention characteristics by Bayesian inversion.
For two of the three investigated monitoring wells, observations of water table rises caused by precipitation events contained sufficient information to constrain the soil water retention characteristics to plausible ranges. Differences of the soil water retention characteristics show small-scale heterogeneity within the field site. The comparison of the soil properties in the laboratory and the field indicated differences that might be related to different hydraulic behavior of the peat under lab and field conditions.
It was determined that there are numerous possible uncertainties in determining the water retention characteristics of the subsurface peat layer from water-level observations due to processes such as temporary non-equilibrium conditions and uncertain estimates of the infiltrating water, e.g., due to errors in the precipitation measurements and interception.