Burns et al. present a study on evaluating 15 different actual evapotranspiration methods within three conceptual hydrological models across seven diverse Australian basins. They establish a multi-objective calibration framework, in which observed streamflow and observed AET data from flux tower sites are used. The authors have established a comprehensive framework, and their current manuscript might be suitable for publication in HESS after addressing my comments listed below. The readability of some figures could be enhanced by increasing the font size of the labels. Tables are not well-designed, and it is often hard to identify the information to which row the appropriate information belongs. Please, revise. Overall, I value that the authors have considered more than one hydrological model structure and used a joint multi-objective calibration of the model’s parameters. Below you will find my three main remarks and further below, more minor suggestions for revisions:

1. I missed any discussion on the spatial transferability of the calibrated parameters. I understand that this methodology is presented as site-specific. IN the line of Klemes et al., 1986 (doi 10.1080/02626668609491024), to assess robustness of the most suitable AET formula, I would also definitely welcome an evaluation of the selected parameters in ungauges locations (apply the 1calibrated parameter set to the 6 remaining basins) and objectively assess, whether the selected AET formula also holds across other locations as well. This could enhance the temporal split-sample test, already presented.
2. When focusing on AET, the current formulation does not consider any uncertainty in potential evapotranspiration estimates. The authors should consider their in the discussion, see e.g. studies like Oudin et al, 2005 (JoH), Pimentel et al, 2023 (WRR); Thakur et al, 2025 (HESS); which presents sometime large differences of employed PET methods on hydrological components from different perspectives, besides uncertainty in the precipitation estimates, which is also a crucial element for the water balance closure.
3. It is not very clear, at this temporal resolution the study was done. Daily?  I missed information about the length of the calibration periods in the Methods. This should be provided around Lines 241-247, to provide more details on the split-sample testing.

Minor:

• Lines 28-29: be careful, soil moisture is not internal model’s flux, please, reformulate sentence accordingly, soil moisture is a state!
• First paragraph of Introduction: please, say more clearly, which kind of hydrological models you have in mind (conceptual, black box, spatially distributed, etc, there are many subcategories ).
• Equation (4) should be written in full in the Methods, providing a reference to original work is not enough here.
• Line 30: remove “perhaps” and rephrase.
• Line 31: instead of “culmination”, consider using “integral variable”
• Consider merging the last two paragraphs of Intro, and move the detailed MARRMOT descriptions to the Methods, where it fits better.
• Line 140: you don’t say, which methods was used to calculate PET. This is important. Also, in section 2.3, information on the discharge is missing. Additionally, here is the right place to introduce length of the timeseries, number of years, and say, how did you treat missing values in your analysis.
• Line 268: equation 19 should be connected to the AET definition, Current statements looks like it refers to eq. 19, which is not defined in the manuscript.
• Table 4: instead of having only categorical values for which AET method was best, it would be also valuable to have some qualitative category.
• Figure 4: Consider different colors for native equation: now, every model’s native equation is shown by black star, with overlaying colored circle. Considering coloring the star by model’s color. Also, I recommend not to use 4 columns, rather 3 columns and then 3 rows. This would allow your figures to become bigger.   
• Figure 6: I would recommend to keep same model’s colors as used earlier in Figure 4.

Review of “A systematic evaluation of 15 actual evapotranspiration formulations within conceptual hydrological models”

A very nice and well-structured manuscript. The study is indeed very systematic, and the results thoroughly analyzed and of general interest. I have a list of comments and suggestions below.

Major comments:

1. Inclusion of AET and catchment correction factors pr. catchment Table 1.
2. More details on the objective function for discharge and weighting is needed
3. More discussions and potentially analysis of spatial parameter transferability and regionalization
4. Tables are not well organized

Specific comments:

• Line 9: correct spelling ”partitioniapendng”
• Lines 37-49: This section talks about dynamic vegetation modelling, but this is not covered in the manuscript, which is based on simple conceptual hydrological models. It seems disproportional and a bit misguiding to dedicate such attention to a topic, which is not part of the paper.
• Lines 118-127: These selection criteria seem a bit subjective. Here I had written a long section about the need to adjust the site flux to catchment water balances. However, you actually do that in section 2.4, which is great. I strongly suggest that you mention that catchment water balance adjustment already here in lines 118-127. In that way you can mention both careful site selection and the need for adjustment.
• Table 1: I strongly suggest that Table 1. Includes catchment mean daily Q and site mean daily AET from the flux-tower and the adjustment factors to illustrate the adjustments made. If the table gets large, I think you can skip the catchment code, PET, the state and High P Low P columns. Here I believe the AET and correction is more interesting than the PET. Also number of decimals are excessive, and flux tower sit description could be simplified.
• Line 179: Why not substitute the AET formulation across all storages? Why only for the storage that a priori is assumed to contribute the most?
• Line 219: I think we need some more details on this bias_penalising_log OF. I have not heard of it before, please write the equation. Also, it cannot be found in Trotter et al. (2022) (Eqn. 4), as stated.
• Table 2 is hard to read, please try to reorganize.
• Table 3 is not important, consider moving to supplement.
• Table 4, Why not in a similar format as S2 in the supplement, much easier to read.
• Lines: 225-227: The two OF’s are weighted equally, but how does that ensure that they contribute equally to the combined OF? What if the two OF’s have different magnitudes, e.g. the KGE OF will be below 1, what is the typical range of bias_penalising_log? From later plots e.g. Fig 4 and 7 it seems as if the OF’s are scaled to similar sizes, but please elaborate a bit on that.
• Lines 243-244: Great with such a split sample test, which I believe is important for this kind of exercise. However, why only do it for one AET equation? Would there be value in adding the split sample test for all equation in the supplement? I would be curious to see if there is a systematic difference between models and equations regarding overfitting and tradeoffs between Q and AET?
• Line 251: “In this figure, Simhyd was run with each of the 15 evapotranspiration (AET)”. Please make it clear if this is the result after recalibration to each AET equation.
• Lines 252: I assume it is the adjusted not the observed flux tower AET.
• Line 255: Also here, that depends on whether this is prior or post recalibration. I think that has to be very clear.
• Line 303: I think it is wise to place the other signature results in the supplement. However, I notice one thing for the Interannual variability signature Figure S4.2. It seems as if the pattern here is almost identical across all models indicating that this signature is largely a result of the precipitation data and not controlled by the models or equations.
• Line 335: I really appreciate the split-sample test and the split between Q and AET OF’s. However, I also feel that with this calibration to a single optimal parameterset, it can be difficult to interpret the possible overfitting. It seems as if Eq. 19 fits very well to AET, but performance drops for Q, could that indicate an overfitting to AET?  Perhaps in the discussion or under a description of limitations also mention that all results are based on one unique optimal parameter set and there will be a range of alternative parameter sets that would perform almost equally well, but with different transferability or tradeoffs. In principle the whole analysis could have been presented as results of an ensemble of parameter sets. I am not suggesting to change that, but perhaps worth noting something on this in the discussion.
• Line 337: for the analysis under “3.4 Seasonal timing of AET” you recalibrate to AET alone to test if the model has the capacity to match better. But would a recalibration including this seasonal timing OF (e.g. in combination with the original AET OF) not be a better test of that?
• Figure 8: I would suggest having the same y-axis scale on all three plots.
• 4.3 Implications: Overall, a very nice analysis and great that you take the time in the end to analyze eq. 19 in more depth. One remaining question, that I feel is not addressed, is the possibility to regionalize and thereby parametrize the two extra parameters in eq 19 beyond the current catchments. There is a large spread in those parameters (Table S6). So, there is not a unique parametrization across catchments. I think it would be great to address this, and in the discussion point in some direction regarding this, perhaps under future work. E.g. would the optimal values of p1 and p2 be spatially related to any mappable information? Also, I assume the PET used is a purely meteorologically driven PET, would a spatial scaling, using something like a high-resolution crop coefficient approach level out some of the variability between catchments, e.g. the need to reduce AET below PET also under high moisture availability?  And what is the impact of PET method used? Could some model or equations struggle with scaling a poor PET input?