Peatlands are poorly represented in global Earth system modeling frameworks. Here we add a peatland-specific land surface hydrology module (PEAT-CLSM) to the Catchment Land Surface Model (CLSM) of the NASA Goddard Earth Observing System (GEOS) framework. The amended TOPMODEL approach of the original CLSM that uses topography characteristics to model catchment processes is discarded, and a peatland-specific model concept is realized in its place. To facilitate its utilization in operational GEOS efforts, PEAT-CLSM uses the basic structure of CLSM and the same global input data. Parameters used in PEAT-CLSM are based on literature data. A suite of CLSM and PEAT-CLSM simulations for peatland areas between 40°N and 75°N is presented and evaluated against a newly compiled data set of groundwater table depth and eddy covariance observations of latent and sensible heat fluxes in natural and seminatural peatlands. CLSM's simulated groundwater tables are too deep and variable, whereas PEAT-CLSM simulates a mean groundwater table depth of −0.20 m (snow-free unfrozen period) with moderate temporal fluctuations (standard deviation of 0.10 m), in significantly better agreement with in situ observations. Relative to an operational CLSM version that simply includes peat as a soil class, the temporal correlation coefficient is increased on average by 0.16 and reaches 0.64 for bogs and 0.66 for fens when driven with global atmospheric forcing data. In PEAT-CLSM, runoff is increased on average by 38% and evapotranspiration is reduced by 19%. The evapotranspiration reduction constitutes a significant improvement relative to eddy covariance measurements.Plain Language Summary Peatlands are wetlands in which plant matter has accumulated over thousands of years under almost permanently water-logged conditions. Alterations in these conditions as a result of global climate change can lead to the release of the huge peatland carbon pool as carbon dioxide over much shorter timescales than were required for accumulation. The additional emissions would amplify global warming. A better representation of the peatland hydrology in global Earth system models can help quantify how peatlands respond to a changing climate. In this paper, we add a peatland-specific land surface hydrology module to the land surface model used in NASA's GEOS Earth

Peat specific yield (SY) is an important parameter involved in many peatland hydrological functions such as flood attenuation, baseflow contribution to rivers, and maintaining groundwater levels in surficial aquifers. However, general knowledge on peatland water storage capacity is still very limited, due in part to the technical difficulties related to in situ measurements. The objectives of this study were to quantify vertical SY variations of water tables in peatlands using the water table fluctuation (WTF) method and to better understand the factors controlling peatland water storage capacity. The method was tested in five ombrotrophic peatlands located in the St. Lawrence Lowlands (southern Québec, Canada). In each peatland, water table wells were installed at three locations (up‐gradient, mid‐gradient, and down‐gradient). Near each well, a 1‐m long peat core (8 cm × 8 cm) was sampled, and subsamples were used to determine SY with standard gravitational drainage method. A larger peat sample (25 cm × 60 cm × 40 cm) was also collected in one peatland to estimate SY using a laboratory drainage method. In all sites, the mean water table depth ranged from 9 to 49 cm below the peat surface, with annual fluctuations varying between 15 and 29 cm for all locations. The WTF method produced similar results to the gravitational drainage experiments, with values ranging between 0.13 and 0.99 for the WTF method and between 0.01 and 0.95 for the gravitational drainage experiments. SY was found to rapidly decrease with depth within 20 cm, independently of the within‐site location and the mean annual water table depth. Dominant factors explaining SY variations were identified using analysis of variance. The most important factor was peatland site, followed by peat depth and seasonality. Variations in storage capacity considering site and seasonality followed regional effective growing degree days and evapotranspiration patterns. This work provides new data on spatial variations of peatland water storage capacity using an easily implemented method that requires only water table measurements and precipitation data.

[1] We used the carbon isotope composition ( 14 C and d 13 C) to measure the source and age of DOC, POC, dissolved CO 2 and CH 4 (d 13 C only) released from three natural peat pipes and the downstream catchment outlet of a small peatland in northern England. Sampling under different hydrological extremes (high flows associated with storm events and low flows before or after storms) was used to explore variability in C sources as flow paths change over short periods of time. The d 13 C composition of organic C differed (d 13 C-DOC À28.6‰ to À27.6‰; d 13 C-POC À28.1‰ to À26.1‰) from that of the dissolved gases (d 13 C-CO 2 À20.5‰ to +1.1‰; d 13 C-CH 4 À67.7‰ to À42.0‰) and showed that C leaving the catchment was a mixture of shallow/deep pipe and non-pipe sources. The isotopic composition of the dissolved gases was more variable than DOC and POC, with individual pipes either showing 13 C enrichment or depletion during a storm event. The 14 C age of DOC was consistently modern at all sites; POC varied from modern to 653 years BP and evasion CO 2 from modern to 996 years BP. Differences in the isotopic composition of evasion CO 2 at pipe outlets do not explain the variability in d 13 C and 14 C at the catchment outlet and suggest that overland flow is likely to be an important source of CO 2 . Our results also show that the sources of CO 2 and CH 4 are significantly more variable and dynamic than DOC and POC and that natural pipes vent old, deep peat CO 2 and POC (but not DOC) to the atmosphere.

Introduction Background and RationalePeatlands are organic-rich wetlands that provide important ecosystem services at a range of spatial scales (Kimmel & Mander, 2010). Local hydrological setting is of central importance in determining the characteristics and functions of these ecosystems (Siegel & Glaser, 2006). Peatlands are characterized by waterlogged, anoxic conditions that suppress microbial decomposition, causing carbon to accumulate slowly but persistently over thousands of years in the form of partially decomposed plant detritus (Yu et al., 2010). Peatlands cover less than 3% of the Earth's land surface (Xu et al., 2018b) yet they are thought to store between approximately 500 and 600 Gt (5-6 × 10 17 g) of carbon (Müller & Joos, 2020;Page et al., 2011;Yu, 2011Yu, , 2012, equivalent to between approximately one sixth and one third of global soil carbon (Scharlemann et al., 2014). As well as being long-term carbon sinks, peatlands also emit greenhouse gases, particularly carbon dioxide (CO 2 ) and methane. Peatland greenhouse gas budgets are highly sensitive to surface wetness, and even modest changes in water-table depths can cause peatlands to switch between being net sinks and sources of greenhouse gases when measured in CO 2 -equivalent units (Evans et al., 2021;Günther et al., 2020). In some locations, water that drains from peat

[1] Peatland pipes are large natural macropores that contribute significantly to streamflow and represent a potentially important transport pathway between terrestrial and aquatic/ atmospheric systems. Our study aimed to estimate the contribution of pipe flow to catchment-scale greenhouse gas (GHG) losses (CO 2 , CH 4 , and N 2 O) in a British peatland using a combination of fortnightly spot and continuous sensor measurements. Interpipe variability was high for all GHGs. Mean pipe water concentrations ranged from 0.70 to 6.51 mg C L , and 0.36 to 1.36 mg N L −1 for CO 2 , CH 4 , and N 2 O, respectively. High-resolution CO 2 data showed temporal changes in the connectivity between pipes and the surrounding peat, with connectivity greatest when water table was high and lowest at low water table depths when discharge was associated with deeper, CO 2 -enriched sources. Total downstream export from the eight studied pipes represented 3%, 38%, and 3% of CO 2 , CH 4 , and N 2 O export at the catchment outlet, while contributing only ∼2% of total catchment runoff. Direct degassing of CO 2 and CH 4 to the atmosphere was evident from an intensively monitored pipe outlet. Upscaling evasion estimates from the pipe outlets gave conservative catchment-scale emission rates of 7.08 g CO 2 -eq m −2 yr −1 and 50.2 g CO 2 -eq m −2 yr −1 for CO 2 and CH 4 , respectively. Although the catchment-scale estimates contain significant uncertainty, they highlight the potential importance of pipes as a pathway for the release of terrestrially produced GHGs to the atmosphere.