Simulation of Heat Flow in a Synthetic Watershed: The Role of the Unsaturated Zone
Round 1
Reviewer 1 Report
The paper investigates the numerical modelling of heat transport in the subsurface unsaturated zone and its influence on groundwater temperature changes under atmospheric temperature increase. Three different synthetic numerical models that simulate or neglect the presence of the unsaturated zone, also varying its thickness, are implemented through the numerical code MT3D-USGS. The study applies a methodology adopted for one-dimensional models in a previous work, but extends it to three-dimensional watershed scale models, also considering warming conditions due to climate change.
The procedure and stages applied are well explained and justified, the limitations of the study are also duly presented and discussed. I really appreciate the paper and I think it represents a useful advancement about how forecast the impact of global warming on groundwater resources
Below are some minor comments:
Line 132: here is cited the 8.55° temperature but you never explained before how is calculated. It’s clear for me that is the average temperature at the end of the spin-up period but I would say that at lines 110-113 in order to help the reader
Line 141-173: in this paragraph is explained how the GW model is implemented, but I can hardly find information about the following items:
· GHB- inflow temperature is kept constant during the warming period?; it would have sense because you want to focus on the influence of the recharge in the model domain, if yes I think it would be useful to declare it. Or is it the one in line 284, Table 2, listed in the GHB as 8.55°C and then increased during the warming period? But in this case it would mean that at the BC’s the temperature increase of 0.9°C in 30 years, than how is possible that in LAY8 the T rise less than 0.5 in all the models? The groundwater flow is so low to justify such behavior? I don’t think because in Figure 4 I can’t see a T front moving from north to south
· SFR- how much is the inflow entering in the model for the 2 streams coming from outside the model domain? Is that constant? How much is the temperature? Is that constant during the warming period?
Maybe you can add this information in Table
Line 250-267: is the LAY 4 thickness the same in all the model domain and also in the 3 models? I suppose yes, but please specify it. If not this would influence the temperature represented in the figures because the water coming from UZ would mix with a different volume of groundwater depending on LAY4 thickness.
Line 351: I can’t understand why is written “ particular layer”, maybe is a paste mistake and the correct word is particular year? Because in the following discussion there is nothing concerning layer temperature distribution
Line 357-358: I don’t know if is correct to speak about “mix with warmer water in the UZ”. Is it really a question of mixing or is more a question of delay? i.e. the water that infiltrate push below the colder water that had infiltrated in previous months. Maybe you can add some more lines in order to better explain
Line 379: I would suggest to modify the sentence as “…overall, but mostly under….”
Line 397-398: is not the opposite? Looking the figure 5 (black line) it seems that MID_UZ_THK has 0.3° more that HI_UZ_THK
Line 399-400: why this behavior ? Is it because there is an upward flow caused by the drainage of the river? Than higher inflow of cold water from deep layers?
Line 493-498: If the SFR has an inflow and a T° entering in the domain I suppose that this can influence the resulting temperature in the streams (please see my question about line 141-173). In the mass entering at the SFR upper reach is relevant compared to the water volume drained this can strongly influence the resulting temperature of the streams. This is even more influencing if the SFR inflow is kept constant at 8.55° during the warming period. Please rephrase considering those aspects
Line 591: considering the sensitivity analysis presented in the previous pages I would suggest to specify that flow parameters have a modest influence
GENERAL QUESTIONS
It seems that the influence of external air temperature on the first few meters depth of the unsaturated zone has not been considered, do you think it is important to point this out? What would considering this aspect meaning for the results?
Correctly, in the paper (Line 631) is indicated that the lake energy budget representation is “inadequate”: if I understand the direct radiation on the lake is not even considered
FIGURES AND TABLES
Line 284, Table2: missing table caption
Lines 329-331 (Figure 4): firstly, I recommend increasing the size of the legends to the right of each map because they are difficult to read even when zooming in on the document. I also believe that it would make the next paragraph (lines 338-386) easier to read if Figure 1 would be placed in 1 single page. Figures H and K are cut along the west side
Line 152 (Figure 2): I can hardly see the position of the monitoring location, please use bigger symbols; furthermore think in this map it would be useful to represent also an average head distribution (for example obtained at the end of the spin-up period) because no one is presented in the paper. I know the flow direction is not the main subject of the paper but I think it would help the reader in understanding the synthetic watershed behavior.
Line 405 (Figure 5) I suggest to add 1 more graph for each monitoring point (alternatively in the Supplementary material) where you represent the LAY 4, 6 and 8 overlapped temperature of the 3 different models in order to easily appreciate the differences in groundwater
Line 490 the table number and the caption is missing. I suppose the third column is indicating a % of the model domain, in this case I would use % as in the 4th column
Author Response
The manuscript has undergone significant revision based on internal USGS reviews that go beyond what the journal-selected reviewer has identified, though we have taken steps to address the reviewer's concerns. For example, tables and figures have been overhauled or re-worked (in many cases). Detailed responses to concerns are in a version of the manuscript that includes track changes (a Microsoft Word feature) and comments in the margin ("Article_II_Draft_9_post_review"). A clean version is also provided ("Article_II_Draft_9_post_review_clean").
Reviewer 2 Report
The research showed the importance of explicitly representing UZ heat transport processes in watershed-scale models and with good writing. The work also extend the method presented by the others , such as Norway. At last, the authors explore temperature patterns and trends within the UZ, at the water table, within the groundwater (saturated) system, and along the stream network as the synthetic watershed warms. This is a good article and does a lot of work. I think it should be accepted.
Author Response
No concerns highlighted for us to respond to here.
Reviewer 3 Report
It is important to consider the role of unsaturated zone in heat flow simulation. The research content of the article will attract readers' interest. I suggest to consider publishing in this journal.
Author Response
This review has not listed any concerns for us to respond to.
