What causes cooling water temperature gradients in a forested stream reach?

Garner, G.; Malcolm, I. A.; Sadler, J. P.; Hannah, D. M.

doi:https://doi.org/10.5194/hess-18-5361-2014

Articles | Volume 18, issue 12

https://doi.org/10.5194/hess-18-5361-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-18-5361-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 18, issue 12

Research article

| Highlight paper

|

20 Dec 2014

Research article | Highlight paper |

| 20 Dec 2014

What causes cooling water temperature gradients in a forested stream reach?

G. Garner, I. A. Malcolm, J. P. Sadler, and D. M. Hannah

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Final revised paper (published on 20 Dec 2014)
Preprint (discussion started on 17 Jun 2014)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC C2137: 'Major revisions', Martijn Westhoff, 30 Jun 2014
- AC C2931: 'Response to Dr. M. Westhoff', Grace Garner, 05 Aug 2014
RC C2315: 'Review of HESSD-11-6441-2014', Anonymous Referee #2, 10 Jul 2014
- AC C2978: 'Response to Anonymous Referee #2', Grace Garner, 08 Aug 2014
RC C2696: 'Review of: What causes cooling water temperature gradients in forested stream reaches?', Anonymous Referee #3, 25 Jul 2014
- AC C3035: 'Response to Anonymous Referee #3', Grace Garner, 11 Aug 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (14 Aug 2014) by Hannah Cloke

AR by Grace Garner on behalf of the Authors (07 Oct 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Oct 2014) by Hannah Cloke

RR by Anonymous Referee #3 (26 Oct 2014)

RR by Anonymous Referee #2 (26 Oct 2014)

Suggestions for revision or reasons for rejection

It is obvious that the authors have made considerable efforts to revise the manuscript and I think this version is an improvement over the first. In particular, Section 5.3 is an important and welcome addition. Overall, I think the study is sound and the conclusions are reasonable. I have a few recommendations for making the manuscript more succinct and clarifying some ambiguities.

Recommendations:

Equation 1: define Qbhf.

Section 3.3.4: This is a limited sensitivity analysis and does not add much to the story. I understand that the only 'user-defined' parameter in the model is the threshold used in Gap Light Analyser. However, there is uncertainty in all the energy exchange estimates, the flow routing model, and the flow accretion surveys, even though the uncertainty may not be quantifiable. How sensitive are the main conclusions to these errors? I think Section 5.3 may be sufficient to highlight issues of model limitations and uncertainty if the authors add some discussion about how errors in the flow routing model and flow accretion surveys may impact their conclusions. If a more thorough sensitivity analysis is to be included, I suggest focusing on how sensitive the conclusions are to errors in net radiation modelling (not just the threshold value), the flow routing model, and flow accretion surveys.

Section 3.4.1: The analysis outlined in Section 3.4.1 could be removed. This analysis adds little to the story and only serves to create some confusion. As far as I can tell this analysis is only conducted in order to make a 'cleaner' Figure 6. Statistical inferences are not drawn from this analysis and implausible artifacts are created (e.g. negative canopy densities for the first few meters). If measured canopy density and corresponding modelled net radiation are variable in space time, it would be valuable to show this variability.

Section 5.2: When placing this work within the broader literature, the authors need to highlight that the negative longitudinal temperature gradients observed in this study were modest compared to those observed in other studies. Especially since the focus of the study was on the one week during the ~1.5 year study period with the largest instantaneous negative gradients. Also in this section (lines 6 to 9), it is not clear if the authors are suggesting that temperature gradients observed in previous studies were falsely attributed to cool groundwater inputs. Please clarify.

Tables 1 and 2: I think these can be removed in order to streamline the manuscript. The authors should mention the range of error statistics in the text, but I think the tables are unnecessary. They could be replaced with a simple time series graph similar to Figure 2f. The figure could compare observed temperatures at AWS_FDS and modelled temperatures at that site. This would give the readers a nice idea of how well the model is performing.

Figure 2f: If there was not a cooling mechanism (as stated on page 15, lines 38-39) would not the daily maximum stream temperature at AWS_FDS be the same as AWS_open? Instead the daily maximum stream temperature at AWS_FDS is consistently about 1 degree C lower than the daily maximum at AWS_open during clear sky days. This appears to be driven by a loss of energy at the stream surface (Fig 2d). The authors need to be careful to distinguish between diurnal warming and cooling patterns, and general statements made about longitudinal gradients (page 15, lines 38-39).

Figure 5: Why do the Gap Light Analyser grids not extend to the edges of the fisheye photos? Does the lens have a view angle greater than 180 degrees?

Figure 8: Could be presented as a 'heat-plot'?

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ED: Publish subject to minor revisions (Editor review) (28 Oct 2014) by Hannah Cloke

AR by Grace Garner on behalf of the Authors (12 Nov 2014) Author's response Manuscript

ED: Publish as is (21 Nov 2014) by Hannah Cloke

Short summary

This study demonstrates the processes by which instantaneous longitudinal water temperature gradients may be generated in a stream reach that transitions from moorland to semi-natural forest in the absence of substantial groundwater inflows. Water did not cool as it flowed downstream. Instead, temperature gradients were generated by a combination of reduced rates of heating in the forested reach and advection of cooler (overnight and early morning) water from the upstream moorland catchment.