Reply on RC1

Comment 1.1: This paper presents a land surface model with an explicit representation of northern peatlands, ORCHIDEE-PCH4. The model simulations are compared to data from 14 wetlands. This paper focuses on methane emissions and refers to previously published work for peat carbon accumulation and carbon balance. The authors use the root mean square difference between simulated and observed methane emissions to optimize 7 model parameters. They first perform the optimization separately for each of the 14 sites, then perform a multi-site optimization.

Response: As we pointed out at the beginning of the discussion section L524-527 "Sensitivity analyses were previously performed to assess methane emission models responsiveness to parameters values (Meng et al., 2012;Riley et al., 2011;Spahni et al., 2011a;Wania et al., 2009;Zhu et al., 2014). These studies (Van Huissteden et al., 2009;Riley et al., 2011) suggested that temperature dependency of methanogenesis is the most influential parameter affecting methane production whereas methane emissions are mostly sensitive to oxidation and plant transport." Indeed, by definition (equation 3, L175-177) "the rate of methanogenesis (k i in s -1 ) depends on soil temperature and moisture according to the same function as for the heterotrophic respiration (Qiu et al., 2019)". And our results displayed in the manuscrit, Figure 2 to 5 and in supplementary document, Figure S3, show that methane emissions are correlated with optimum when both simulated soil temperature and moisture conditions are the highest.
Besides, in our model, as explained L493-495 "the simulated water table position is a prognostic variable defined by the cumulative amount of soil water content over the soil column ( Fig. S2 and Fig. S3)." We also demonstrate for two sites, at US-Los and DE-Spw, in the supplementary document Figure S4 and S5 and discussed in the manuscript L495-496 that above the simulated water table position, soil moisture is still higher than 80% which is sufficient for methanogenesis to occur in the model. This explains that the correlation between the simulated water table position and simulated methane emissions is not as strong as the one observed in the field.

Response:
As explained in the response to comment 1.2, we demonstrate in the manuscript (L175-177, L493-495, L524-527) and with additional figures in supplementary materials (Fig. S3) that our methane model is correlated with simulated soil temperature and moisture conditions. Indeed, Fig S3 shows a strong correlation at each site of methanogenesis maximum with both soil temperature and moisture maximum. We also discussed in the manuscript (L493-495) with additional figures in the supplementary (Fig.  S2, S4 and S5) that our prognostic water table position defined from simulated soil moisture content is not well correlated to the observation water table positions measured on sites. We added a few sentences in the conclusion to highlight these results L707: "Our results show that as in previous methane emissions models ( , simulated methanogenesis is strongly correlated to simulated soil temperature and moisture content whereas methane emissions are more strongly correlated to plant mediated fluxes and to soils methane oxidation proportion. We have to point out that in our model, a weak correlation has been established between the observed water table positions and the prognostic water table positions established from simulated soil moisture content. A correlation between soil moisture content and water table position in the field is needed to improve representation of the water table position in models." Comment 1.4: The second comment is related to the Dk-Nuf site. I happen to be familiar with this dataset and I noticed some imprecisions in the text (see specific comments), so I would ask each co-author responsible for a site to carefully proofread the manuscript.

Response:
We addressed the reviewer's concerns by addressing specific comments below (comment 1.5).

Specific comments:
Comment 1.5: L 243: Dk-Nuf : The methane emissions are measured by automatic chambers on this site. There is a flux tower but it only measured CO2 fluxes (besides turbulent energy and radiative fluxes). Also, the water table depth is not available at this site.  Table 1. Sites ecological characteristics summary. Sites identification includes the country initials and the short three letters name of each site, locations of the sites are provided by the country, latitude (Lat) and longitude (Lon) values. Hydrological characteristics are distinguished by the type of ecosystem, fen, bog, tundra and marsh. Y and N indicate presence and absence of snow cover in winter, permafrost soil, forest above the peat. Temporary drawdown of the water table level is specified by presence and absence indicators Y or N. Grey color highlight groups of peatlands organized by amount of methane emissions in ranges 0-10, 10-150, 150-400, 400-600 mg m -2 d -1 . The authors say at line 270 that they impose site level meteorological forcings. They also seem to indicate that the spin-up to reach close to observed peat carbon content and depth was done by using the site specific meteorological data. So is it the texture that is at 0.5 degree or the hydrology? If it is the hydrology to calculate the peatland fraction then what is used to force this calculation? A gridded meteorological forcing or the site specific one ? If a gridded meteorological forcing was used then it should be mentionned. This is a bit confusing.
Response: to improve the description of simulation setup, we modified paragraphs L266: "Each peatland site is a sub-grid area embedded in the 0.5°x 0.5° grid cells whose extent is determined by a fraction of grid area as defined in Table 2  Response: Optimization simulations were performed over site-specific observation periods as defined in Table 1. We added these precisions in the sentences L305 "Two types of simulations are performed over the site-specific observation period defined in Table 1: single site (SS) experiment for which parameters are optimized for each site and a multi-site (MS) experiment that aims at refining one set of parameters considering all sites together." The timestep of the model is half-hourly but the model output is the mean monthly value. We choose to have the same timescale for the outputs throughout the sites following measurements timescale that we all converted to monthly timescale because it was the timescale of the chamber measurements. We added measurement timescale L284-288: "These sites are a subset of the 30 peatlands sites collected for the calibration of ORCHIDEE-PEAT (Qiu et al., 2018) for which, in addition of eddy-covariance data and physical variables (water table, snow depth, soil temperature), methane emissions were measured by eddy-covariance at daily time scale at US-Los, hourly timescale at DK-Nuf and otherwise at half-hourly timescale or chamber measurements at monthly timescale for FR-Lag and RU-Che. All methane emissions data were monthly average." Comment 1.9: L326-327: this is very strange. If zroot is increased to 0.75m then, if I am not mistaken, froot=0 (I am assuming zsoil=0.75m since this is the peat depth), so that fpmt=0. If zsoil is not equal to 0.75m, increasing zroot decreases fpmt (in absolute value). Is this wanted?
Similarly, why increase the rate of methanotrophy to get higher methane net emissions?
Response: Indeed, as in Walter and Heimann (2000), froot is decreasing with depth meaning that proportion of root decreases with depth. Nevertheless, since the plant mediated transport scheme is limited to layers containing roots by increasing zroot we increase the number of layers that will be involved in methane transport. Concerning the rate of methanotrophy, when the rate is higher, a lower proportion of methane is oxidized which leads to higher content of methane in the soil that will eventually be emitted. We modified the sentence L324: "Three parameter ranges were modified for DK-Nuf, the minimum value of q MG was lowered to 7.0, z root maximum is increased to the maximum peat depth at 0.75m in order to consider plant mediated transport in all the peat layers, the maximum value of T veg was increased to 40.0 and the maximum rate of methanotrophy k MT was enlarged up to 8 d -1 to decrease the methane oxidation and to obtain in the simulation methane emissions higher than 150 mg CH 4 m -2 d -1 . " Comment 1.10: Another question: what did the authors do with missing data for methane emissions? There are very few winter measurements of methane emissions at Dk-Nuf. Did the authors gap-fill the data and then optimized their parameters on this? It should be clearly stated.

Response:
To clearly explain data availability and timescale, we added fews sentences L243 : "For the optimization simulations, at DE-Sfn, DE-Hmm, FI-Lom, PL-Kpt, PL-Wet, and US-Wpt, year-round data were available and zero values were filled for the first and the last month of years at the beginning and the end of the observation period. Otherwise, winter months were filled with zero and during spring, summer and fall months missing data were filled gapped using a linear regression." Comment 1.11: L327: to be coherent with the rest I believe Table 4 should have values in parenthesis for qMG at PL-Wet site (4 is outside the range given in

Response:
The simulated water table position is defined as the accumulation of water content height in each soil layer. When a soil layer, or part of it, is frozen water infiltration is reduced involving reduction of soil moisture in the deepest soil layers and a lower water content height and consequently a lower water table position. We modified the sentence L458: "Indeed, both sites are underlaid with permafrost which limit water infiltration to the deepest soil layers and can explain these deeper simulated water table positions." Comment 1.14: L 562: I couldn't agree more with the authors comment on the need for more data on vascular plants in peatland Response: This sentence is: "While a significant number of studies provide insight on gas exchanges through vascular plants, densities of vascular plants with aerenchyma in peatlands is poorly characterized." Comment 1.15: L 590-591: I am not sure the authors really showed that these 2 sites were limited in methane substrate. It is likely the case in the model, but is it the case in reality ? Because it seems that this model result might be related to the partitioning between active, slow and passive C pools.

Response:
We agree with the reviewer that for these 2 sites methane substrate is limited in our model. We modified the sentence to specify this L590: "Only two values have been defined above 10 at US-Wpt and DK-Nuf which are two sites that are limited in methane substrates in the model which explains these high values of T veg . " Comment 1.16: L 727: I couldn't agree more with this last sentence.
Response: This sentence is: "This demonstrates the complexity of interactions of the methane cycle with environmental conditions considered at various scales and the need for more detailed on-site studies."