|Review of hess-2021-89 (2)|
Title: Low hydrological connectivity during summer drought inhibits DOC export in a forested headwater catchment, by Blaurock et al.
Blaurock et al. sent a revised version of their previously submitted manuscript, together with their replies to reviewer comments. In this study, they investigated the mobilization of DOC during storm events in two topographically contrasting forest catchments and concluded that event size, antecedent wetness conditions, and topography are major determinants of DOC mobilization and export.
I carefully read the response letter and the revised manuscript, which I enjoyed. I thank the authors for addressing all my comments and making the pertinent changes in the manuscript or explaining why they didn’t in those cases where they disagreed with my points. For the most part, I am satisfied with the responses and the changes made in the manuscript, which I believe have helped increasing its clarity. I do however have a few further comments that follow up on a few of the discussed topics and that I would like to see considered/clarified before a final version of the manuscript can be accepted.
I understand now that the groundwater tables plotted in Figure 2 are from three deep wells and that they are used as a proxy for some kind of overall wetness conditions in the catchment that follow seasonal patterns rather than event-dynamics. However, the data from these deep wells should not be confused with groundwater table variations occurring in the shallow soil, which do show small-temporal scale dynamics responding to events and which you also discuss at times (e.g. L. 323-327 and L. 335-339). The distinction between these two types of groundwater tables has to be more clearly made, for example by explicitly differentiating between deep groundwater tables (with seasonal dynamics) versus shallow groundwater tables (which responds to precipitation inputs and relate to water and solute delivery during events). The way groundwater tables are presented now leads to confusion. Additionally, I wonder if a further distinction can be made by describing the water storage structure in the catchment. The way you describe things makes me think that the catchment has a perched shallow aquifer that overlies a saturated deep aquifer with varying degrees of connectivity to the stream. Is this the case? If so, please specify it and link it to the deep versus shallow groundwater table distinction.
While I tend to believe that in-stream DOC mineralization might not be significant during rainfall events that produce high water velocities and lead to short water residence times, this paradigm might not be as universal as previously thought as there are recent studies suggesting the opposite might be true in certain settings (e.g. Bernal, S., Lupon, A., Wollheim, W. M., Sabater, F., Poblador, S., & Martí, E. (2019). Supply, Demand, and In-Stream Retention of Dissolved Organic Carbon and Nitrate During Storms in Mediterranean Forested Headwater Streams. Frontiers in Environmental Science, 7, 14. doi:10.3389/fenvs.2019.00060). Therefore, consider this information and back up your statement in L. 499-500 with some references.
L. 91. Do you mean “hydrological conditions” here, or rather “(antecedent) wetness conditions”?
L. 101-102. For coherence, I think it is better to write “antecedent wetness conditions” here.
L. 163. Please, remove “continuously”.
L. 178-179 and L. 218. Please, correct figure numbers.
L. 235. What is “HQ1”? Please, consider removing if not relevant.
L. 500-503. Check this sentence, it includes the same information twice.