Distributive rainfall–runoff modelling to understand runoff-to-baseflow proportioning and its impact on   the determination  of reserve requirements of the  Verlorenvlei  estuarine lake,    west coast, South Africa

Watson, Andrew; Miller, Jodie; Fink, Manfred; Kralisch, Sven; Fleischer, Melanie; de Clercq, Willem

doi:https://doi.org/10.5194/hess-23-2679-2019

Articles | Volume 23, issue 6

https://doi.org/10.5194/hess-23-2679-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-23-2679-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 6

Research article

|

24 Jun 2019

Research article |

| 24 Jun 2019

Distributive rainfall–runoff modelling to understand runoff-to-baseflow proportioning and its impact on the determination of reserve requirements of the Verlorenvlei estuarine lake, west coast, South Africa

Andrew Watson, Jodie Miller, Manfred Fink, Sven Kralisch, Melanie Fleischer, and Willem de Clercq

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Final revised paper (published on 24 Jun 2019)
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Preprint (discussion started on 21 Sep 2018)
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Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 27 Sep 2018
- AC1: 'Anonymous reviewer (#1) comments', Andrew Watson, 02 Oct 2018
RC2: 'Review of hess-2018-459 “Distributive rainfall/runoff modelling to determine runoff to baseflow proportioning and its impact on the determination of the ecological reserve” by Andrew Watson et al.', Dimitrios Stampoulis, 30 Oct 2018
- AC2: 'Response to review and short comment', Andrew Watson, 07 Dec 2018
SC1: 'Review', Sara Andersson, 09 Nov 2018
- AC2: 'Response to review and short comment', Andrew Watson, 07 Dec 2018

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (06 Jan 2019) by Nunzio Romano

AR by Andrew Watson on behalf of the Authors (17 Feb 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (03 Mar 2019) by Nunzio Romano

RR by Dimitrios Stampoulis (10 Apr 2019)

RR by Anonymous Referee #3 (15 Apr 2019)

Suggestions for revision or reasons for rejection

The authors have modified the J2000 model to understand the contributions of the different flow components to the Verlorenvlei lake, South Africa from the various contributing tributaries. Overall, the study presents a significant body of work which adds to the available literature in this area. Furthermore, the work is of value to South African hydrology, where studies such of this that determine groundwater contributions together with surface water contributions are rare.
The manuscript in the revised form, is well written, the methodology clear and the conclusions generally supported by the findings. The authors have to a large extent addressed the comments of the previous reviewers and significantly improved the paper. The figures and tables that have been included in the revised version have improved the clarity of the paper.
It is my opinion that the paper still only touches on the ecological reserve aspects. The paper is a sound contribution to modelling and the determination of the components of flow, by attempting to relate to the ecological reserve the authors have detracted from the work as they have not adequately addressed it.
The authors have used MODFLOW to validate the recharge estimates for the J2000 model (Section 3.7.3; figure 7). Models should not be validated against each other, at most their outputs can be compared. I would encourage the authors to use another dataset for the calibration and validation, or to better document the pitfalls and uncertainties introduced with this approach and why it was the only one possible.
The flows (Figure 9) from all tributaries are significantly higher during the last wet period (2007 – 2017). Please note that the average rainfall values used in the text and those in added in Figure 9 are not the same and should be corrected. The findings show higher flows in the latter period that are far greater than what would be anticipated from the higher rainfall. A 30 mm change in rainfall resulted in a more than doubling of the average baseflow response. It is during the latter period that the authors state that irrigation in the catchment has been expanding and that there is this growing threat of agriculture expansion to the water resources, but this has not been accounted for in the model. Have the authors considered the cause of the marked higher streamflow response – is it a change in the nature of the rainfall distribution in the latter period, a change in the timing of the rainfall?

Specific comments:
Introduction, Pg 3, Line 54/55 – insert “were” between “problems thought”
Caption Fig 1. Isohyets spelt incorrectly
Section 2, Pg 8, Line 153 – The sentence “Where rainfall was less than 50 % of the MAP (1965-1969 and 2015-2017), concerns over the amount of streamflow required to support the lake have been raised.” Consider rephrasing this sentence. I would presume that the concerns mentioned are now for the recent past not the 1960’s. You have stated that agric expansion has been a more recent phenomena in the catchment and is probably a driver of the concerns.
Table 2, what are the units of AVE?

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ED: Publish subject to revisions (further review by editor and referees) (24 Apr 2019) by Nunzio Romano

AR by Andrew Watson on behalf of the Authors (03 May 2019) Author's response Manuscript

ED: Publish as is (28 May 2019) by Nunzio Romano

AR by Andrew Watson on behalf of the Authors (29 May 2019) Manuscript

Short summary

River systems that support high biodiversity profiles are conservation priorities worldwide. Understanding river ecosystem thresholds to low-flow conditions is important for conservation practices. In this study, the groundwater components for a hydrological model were distributed to provide daily baseflow and streamflow estimates needed for reserve determination. The modelling approach was applied to a RAMSAR lake system under threat by agricultural expansion and climatic fluctuations.