Robust Inverse Modeling of Growing Season Net Ecosystem Exchange in a Mountainous Peatland: Influence of Distributional Assumptions on Estimated Parameters and Total Carbon Fluxes

Abstract

While boreal lowland bogs have been extensively studied using the eddy‐covariance (EC) technique, less knowledge exists on mountainous peatlands. Hence, half‐hourly CO2 fluxes of an ombrotrophic peat bog in the Harz Mountains, Germany, were measured with the EC technique during a growing season with exceptionally dry weather spells. A common biophysical process model for net ecosystem exchange was used to describe measured CO2 fluxes and to fill data gaps. Model parameters and uncertainties were estimated by robust inverse modelling in a Bayesian framework using a population‐based Markov Chain Monte Carlo sampler. The focus of this study was on the correct statistical description of error, i.e. the differences between the measured and simulated carbon fluxes, and the influence of distributional assumptions on parameter estimates, cumulative carbon fluxes, and uncertainties. We tested the Gaussian, Laplace, and Student's t distribution as error models. The t‐distribution was identified as best error model by the deviance information criterion. Its use led to markedly different parameter estimates, a reduction of parameter uncertainty by about 40%, and, most importantly, to a 5% higher estimated cumulative CO2 uptake as compared to the commonly assumed Gaussian error distribution. As open‐path measurement systems have larger measurement error at high humidity, the standard deviation of the error was modeled as a function of measured vapor pressure deficit. Overall, this paper demonstrates the importance of critically assessing the influence of distributional assumptions on estimated model parameters and cumulative carbon fluxes between the land surface and the atmosphere.

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

“…Broder and Biester (2015) provided information on the geochemical composition of the substrate and pore waters which supports this. Weber et al (2017b) give a detailed account of the carbon, water, and energy fluxes of the site during the vegetation period of 2013, a year with exceptionally dry summertime conditions in comparison to the 136-year long term record measured by the meteorological station of the German Weather Service (DWD) at nearby Braunlage.…”

Eddy covariance based surface‐atmosphere exchange and crop coefficient determination in a mountainous peatland

“…1.3% of the data). This finding is corroborated by the fact that the measured and simulated CO 2 fluxes were also not dependent on water content (Weber et al, ). As a consequence, ET at OM was not controlled by water availability during the observation period and that the determined K c* is a valid estimate for well‐watered conditions.…”

“…In September and October, maximal flux uptake reached~8 μmol m −2 s −1 and 4 μmol m −2 s −1 , respectively. A reduction of CO 2 uptake as a result of low WTD in the extended dry period of July is not visible in the data; this indicates that water did not limit the vegetation's photosynthesis, which is supported by a F CO2 modelling study from this site (Weber et al, 2018). Derived from the median diurnal course of F CO2 , the median daily CO 2 exchange was determined.…”

“…Peat depth is between 3 and 5 m in the central part with a maximum of 6.6 m (Beug, Henrion, & Schmüser, 1999). Recent studies have further characterized the study site including the effective soil hydraulic properties (Weber et al, 2017b;Weber, Iden, & Durner, 2017a), CO 2 exchange (Weber et al, 2018), and DOC export characteristics and controls (Birkel, Broder, & Biester, 2017;Broder & Biester, 2015). and 5-cm depths using 5TM temperature and moisture content sensors recorded with an EM50 logger (Decagon Devices, Inc., Pullman, Washington, USA).…”

Peatland bog pedogenesis is reflected in unsaturated hydraulic properties

“…Broder and Biester (2015) provide information on geochemical composition of the substrate and pore waters. Weber et al (2017b) and Gerling et al (2017) give a detailed account of the carbon, water and energy fluxes of the site during the vegetation period of 2013, a year with exceptionally dry summer time conditions in comparison to the 136 year long term record measured by the meteorological station of the German Weather Service (DWD) nearby Braunlage. 5…”