Deriving Effective Soil Water Retention Characteristics from Shallow Water Table Fluctuations in Peatlands

Abstract

We have developed a novel and simple approach that can be used to derive effective in situ soil water retention characteristics from field monitoring time series in peatlands. The simplicity of the approach is given by the very limited data requirements, which comprise only precipitation, water table, and, if relevant, microrelief data. Our approach is built on two main assumptions: (i) for shallow groundwater systems, the soil moisture profile is always close to hydrostatic equilibrium; and (ii) during short time periods of high precipitation, the water storage change due to lateral fluxes is small compared with the precipitation input. Given these assumptions, the height of a water table rise due to a precipitation event mainly depends on the soil water retention characteristics, the precipitation amount, the initial water table depth, and, if present, the microrelief. In this study, this dependency was used to determine the effective van Genuchten parameters by Bayesian inversion assuming a uniform soil profile. We applied our concept to field data from a peatland with microrelief. Results indicated that observations of water table rises caused by precipitation events can contain sufficient information to constrain the soil water retention characteristics around monitoring wells in peatlands to plausible ranges. In principle, the approach should also be applicable to other shallow groundwater systems. Application limits and potential systematic errors are discussed.

Quantification of peatland water storage capacity using the water table fluctuation method

“…For example, the drainage experiments (small cubes and tank) measure drainage porosity, which is an approximation of S Y . Nevertheless, given the fact that the WTF method provided S Y values similar to those of the cube method and of the experimental tank and that the results obtained in this study are almost identical to the results obtained by Dettmann and Bechtold () using the same method, it is hypothesized that the assumption underlying the use of the WTF method are reasonable. In future research, the WTF method will need to be used in a wide variety of peatlands to fully constrain its validity.…”

“…These results are strong observational evidence of the sharp decrease of S Y with depth. Moreover, results obtained using the WTF method show almost identical ranges and patterns obtained by Dettmann and Bechtold ().…”

“…The S Y measurements and rates of decrease obtained in this study were generally consistent with the range of values reported for different types of wetlands (peatland, blanket peat, open water, constructed peatland, and cutover bog) and using different methods (e.g., gravity drainage, moisture retention measurements, infiltration rate experiment, WTF method, and tracer tests; Table ). Using the WTF method, Dettmann and Bechtold (), McLaughlin and Cohen (), and Moore et al () reported S Y values ranging from 0 to 1.1, from 0.13 to 1.05, and from 0 to 0.9 within the first 50, 65, and 45 cm, respectively. This was equivalent to rates ranging between ~0.007–0.013 cm −1 (Moore et al, ), 0.02 cm −1 (McLaughlin and Cohen, ), and ~0–0.08 cm −1 (Dettmann and Bechtold, ).…”

“…Using the WTF method, Dettmann and Bechtold (), McLaughlin and Cohen (), and Moore et al () reported S Y values ranging from 0 to 1.1, from 0.13 to 1.05, and from 0 to 0.9 within the first 50, 65, and 45 cm, respectively. This was equivalent to rates ranging between ~0.007–0.013 cm −1 (Moore et al, ), 0.02 cm −1 (McLaughlin and Cohen, ), and ~0–0.08 cm −1 (Dettmann and Bechtold, ). Using pressure chamber experiment measurements as quantitatively equivalent to S Y , Moore et al .…”

“…Peat S Y can vary by up to two orders of magnitude (0.01–1) within the first top 50 cm (Dettmann and Bechtold, ; Holden, ; Vorob'ev, ). This buffers peatlands against both inundation and excessive drying (Waddington et al .…”