|This manuscript presents a new formulation of peatland hydrology implemented within the CLM land model. The model is parameterized to simulate the hydrology of a bog peatland in Minnesota that is part of the SPRUCE manipulation experiment. The model appears to do an excellent job of simulating water table fluctuations at the site, which closely match observed fluctuations. The model was then applied in a series of warming simulations with different levels of atmospheric humidity.|
I have read the comments from the two referees who reviewed the previous version of this manuscript, and I will try to place my comments in the context of those reviews. I agree with both other reviewers that the goal of improving the representation of peatland processes in earth system models is very important, given the large carbon pools contained in peatlands and the current poor representation of these ecosystems in global models. In that sense, this is important work and I am happy to see these processes being integrated into a widely-used, large-scale land model like CLM. However, as Reviewer #2 pointed out, the fact that this hydrological functionality was implemented into CLM does not necessarily make it novel or interesting to the broader community (outside of the CLM development community), if similar models already existed independently of CLM. As I see it, the real benefit of implementing functionality like this into a global model is that it allows the site-level simulations that are possible with smaller-scale models to be scaled up to the larger scales supported by the global modeling infrastructure of an earth system model component like CLM. Since this study did not attempt to scale the results up in that way, I’m not convinced that the model development portion is novel with respect to the broader biogeochemistry community, although it is a useful advance that scientists in the earth system modeling community will no doubt be very excited about.
Apart from the question of whether the model itself is a significant scientific advance, I think the actual model experiment and results are quite interesting. I think placing this modeling exercise in the context of a real-world warming experiment was a smart choice, and I appreciate the portion of the Discussion that points out specific areas where the model results may inform the interpretation of that experiment. If the simulated response of water table depth to warming holds true, it seems that there could be dramatic hydrological consequences to increasing temperature, and these results are certainly worth reporting. I do have some concerns about the validation of these results, however. The parameterization and validation of the model was focused on three years of water table depth measurements. The model does an excellent job predicting short-term variations in water table depth. However, I’m not convinced that the processes driving these short-term variations are the same processes that will drive the simulated response to rising temperature. I suspect that these short-term variations are primarily showing that the model can reproduce responses to precipitation events and subsequent drainage. Looking at the temperature time series, it does not appear that there were strong enough variations in temperature (outside of summer/winter seasonal variations) in the observed time period in order to separate out temperature effects on water table, and so it’s hard to say if the temperature response of the model was validated by this set of measurements. The model predicts very strong responses of evapotranspiration (ET) to warming, and these appear to be the main driver of the water table responses. In my opinion, the temperature response of the model has not really been validated for this site, and as a result it’s hard to have much confidence in the simulated warming responses. I think the results would be much more solid if they could be compared to some relevant measurements. Ideally, eddy covariance measurements of ET as a function of temperature or vapor pressure deficit (VPD) could be compared to similar response functions calculated from the model simulations, to see if the strong modeled ET response is consistent with observations in peatlands — i.e., is the percent response of ET to warming the same as the percent response that has been observed at other peatland sites? The manuscript does state that “previous studies have shown good correspondence between CLM predictions of latent heat flux and eddy covariance measurements”, but it’s not clear whether those previous measurements included peatlands. There is reason to believe that peatland measurements should be used here, since the high water table and prevalence of mosses would likely lead to different responses than forests or other ecosystems. In their response to reviewers, the authors say that eddy covariance measurements are not available for the SPRUCE site. I’m aware of a flux tower operated by the US Forest Service Northern Research Station in a northern Minnesota peatland in Marcell Experimental Forest close to where the SPRUCE experiment is being conducted, and it may be worth contacting that group to see if any data are available for comparison with model results. Someone connected with that tower tells me that it has been running since 2006. The contact people for that site would be Randy Kolka, who works for the US Forest Service, and Tim Griffis, who is a professor at the University of Minnesota. If data from that site are not available, I think it might be worth doing a comparison with some of the longer-running peatland eddy covariance sites in the Ameriflux or Fluxnet networks. The Mer Bleue site in Ontario and the Lost Creek site in Wisconsin have fairly long records of evapotranspiration that probably contain enough temperature variations to compare with the model results (Lost Creek is a fen rather than a bog, but does have a hummock/hollow topography and a high water table). While every site is different, it should be possible to at least compare the percent change in ET with temperature between the model simulations and these other peatlands. At the very least, I think the dramatic simulated changes in ET should be placed in the context of previous measurements in the literature. Given how central the ET and water table responses to temperature are to the paper’s main scientific results, I think it would make sense to add a section to the Discussion placing these results in the context of previous observations. The part of the Discussion that addresses the temperature responses barely cites any literature at all, and there are certainly papers out there with measurements and analysis of ET and water table responses to temperature that could be used to improve confidence in the model results. Here are a few suggestions of papers that presented evapotranspiration data from peatlands:
Wu, J., Kutzbach, L., Jager, D., Wille, C., & Wilmking, M. (2010). Evapotranspiration dynamics in a boreal peatland and its impact on the water and energy balance. Journal of Geophysical Research, 115(G4). doi:10.1029/2009JG001075
Lafleur, P. M., Hember, R., Admiral, S. W., & Roulet, N. (2005). Annual and seasonal variability in evapotranspiration and water table at a shrub-covered bog in southern Ontario, Canada. Hydrological Processes, 19(18), 3533–3550.
Mackay, D. S., Ewers, B. E., Cook, B. D., & Davis, K. J. (2007). Environmental drivers of evapotranspiration in a shrub wetland and an upland forest in northern Wisconsin. Water Resources Research, 43(3), W03442. doi:10.1029/2006WR005149
Kellner, E. (2001). Surface energy fluxes and control of evapotranspiration from a Swedish Sphagnum mire. Agricultural and Forest Meteorology, 110(2), 101–123.
Sulman, B. N., Desai, A. R., Cook, B. D., Saliendra, N. Z., & Mackay, D. S. (2009). Contrasting carbon dioxide fluxes between a drying shrub wetland in Northern Wisconsin, USA, and nearby forests. Biogeosciences, 6, 1115–1126.
Humphreys, E. R., Lafleur, P. M., Flanagan, L. B., Hedstrom, N., Syed, K. H., Glenn, A. J., & Granger, R. (2006). Summer carbon dioxide and water vapor fluxes across a range of northern peatlands. Journal of Geophysical Research, 111(G04011). doi:10.1029/2005JG000111
Sonnentag, O., Kamp, G. V. D., Barr, A. G., & Chen, J. M. (2009). On the relationship between water table depth and water vapor and carbon dioxide ﬂuxes in a minerotrophic fen. Global Change Biology, 16(6), 1762–1776. doi:10.1111/j.1365-2486.2009.02032.x
I think the introduction is very well written and contains a very clear and useful summary of previously published peatland models.
Some other more minor comments:
Sonnentag et al (2008) is cited in the text but not listed in the references. The reference list needs to be checked against the text.
Lines 122-123: Ecosys has been applied in bog environments as well as fen environments (Dimitrov et al 2010, 2011) — actually, Dimitrov is also cited in the manuscript but not in the reference list.
Line 263-266: “subsurface drainage term becomes zero when the water table level drops to the local elevation of zlagg”: There should be some mechanistic explanation included for this. In the response to previous reviews the authors provided an interpretation having to do with the permeability of the glacial till layer: “The underlying assumption is that the glacial till acts as a barrier to drainage when the water table is lower than the lagg.” That explanation should be included in the actual manuscript. Also, the elevation of the lagg as a parameter seems to be very specific to the topography of this bog. It would really help make these results and the model more widely applicable if there were some discussion of whether this number is typical of bogs, or if these details of topography could be predicted in the context of larger-scale simulations.
Equation 2: Unless I’m interpreting this incorrectly, I think the equation should use “zw>zlagg”, not “zw<zlagg”. That is, qdrai varies when the water level (zw) is above (greater than) the zlagg level, and is zero when zw is below that level.