Explaining and forecasting interannual variability in the flow of the Nile River

Siam, M. S.; Eltahir, E. A. B.

doi:https://doi.org/10.5194/hess-19-1181-2015

Articles | Volume 19, issue 3

https://doi.org/10.5194/hess-19-1181-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-19-1181-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 19, issue 3

Research article

|

03 Mar 2015

Research article |

| 03 Mar 2015

Explaining and forecasting interannual variability in the flow of the Nile River

M. S. Siam and E. A. B. Eltahir

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Final revised paper (published on 03 Mar 2015)
Preprint (discussion started on 14 May 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C2584: 'Suggested revisions and corrections to "EXPLAINING AND FORECASTING INTERANNUAL VARIABILITY IN THE FLOW OF THE NILE RIVER M. S. Siam and E. A. B Eltahir"', Anonymous Referee #1, 18 Jul 2014
- AC C4254: 'Reply to Anonymous Referee #1', Mohamed Siam, 03 Oct 2014
RC C3815: 'Explaining and forecasting interannual variability in the flow of the Nile River', Anonymous Referee #2, 15 Sep 2014
- AC C4259: 'Rely to Anonymous Referee #2', Mohamed Siam, 03 Oct 2014

Peer-review completion

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

ED: Reconsider after major revisions (28 Oct 2014) by Thomas Kjeldsen

AR by Mohamed Siam on behalf of the Authors (08 Dec 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Dec 2014) by Thomas Kjeldsen

RR by Gabriel Senay (30 Dec 2014)

Suggestions for revision or reasons for rejection

The study identified a new region of the Southern Indian Ocean to develop an SST index-based hybrid algorithm, that appears to be independent of the ENSO, and thus combined with it explain 84% of the variability in the Nile Flow for “rare/extreme” flow conditions (years) that cause drought and flood hazards. Although the forecast skill depends on the accuracy of the global SST forecast, the method appears to provide a lead time of 3 to 6 months.

The paper is well written, with a good level of literature review, organization and analysis. The finding is both interesting and useful; it will make good contribution to advance knowledge in forecasting potential drought and flooding years in the Nile Basin, with a particular focus to the Upper Blue Nile system-the main contributor of the annual and peak Nile flows.

The following suggestions and comments shall improve the quality of the manuscript.

Important Concerns:
1) Clarify the role of westerly winds for rainfall in Africa. From the literature review (introduction) it was clearly indicated that westerly winds over east Africa enhance rainfall in the Upper Blue Nile (UBN). But it seems strong westerly winds over Western Indian Ocean reduce rainfall over UBN. Details on the physical interaction between these two statements will clarify the seemingly contradicting statements.

2) Similarly, a clearer statement is required on the physical explanation why UBN rainfall reduces when SIO index is positive. It is clearer when ENSO is positive. Then how/why the two occurring at the same time depress/or enhance the rainfall needs more explanation. I understand this is mainly a correlational study, but the attempted explanation requires more supportive physical understanding. A connection/disruption to the seal level air pressure during the Indian monsoon and airflow from the Congo to UBN to would be most helpful.

3) There appears to be a mix-up of numbers between the literature and the current study. For example, this study shows R2 of 0.3 for ENSO while the literature citation shows 0.25. Because of this mix-up, the proportion of ENSO vs SIO is confused. Instead of being 30% ENSO and 28% SIO for a combined 44% (when both ENSO and SIO are used). The text shows 44% combined, 25% ENSO (literature) and 20% SIO (44-25 = rounded to 20???). NOT CLEAR why this mix-up is chosen? Why not use the results from this study for all numbers?

4. I think the abstract should clearly separate the contribution of each index for explaining annual variability vs “rare” case variability when the two are non-neutral. For example, ENSO = 30% and SIO% = 28%, combined at 44% for explaining inter annual variability and ENSO = 64%, SIO = 60% and combined at 84% on rare years (~20% of time, put rarity level) when both conditions are non-neutral.

5) It would be helpful to show the multiple regression equation that relates Q with ENSO and SIO.

Other matters:

1) Good to discuss why the analysis was not extended to 2013? Unable to find data at Dongola other than the global runoff dataset?
2) Table 1: Explain what the variance stands for, flow in percent?

3) Table 5: Check for typo for “1” for condition of ENSO Colds/SIO Normal

4) Figure 3: caption: I do not see a blue line on map…
It would be helpful to show the entire world map, especially to also show the connection between the Atlantic, Congo and Nile
5) Figure 4: MCM/day scale appears in error? Spell in caption what MCM is, if million meter cubic, then 400 MCM per day will yield > 146 km3/year which is far more than the annual Nile Flow at Dongola?
Spell axis label in caption…
The headings for the three figures is not uniform about the use of SST with Index…
6) Figure 5: Show “n” in each case
7) same problem as #5 above
Check if per month value is correct? 10 km3/month which suggests a flow of 120 km3/year??
Figure 2 clearly shows a monthly value of less than 10 km3/month in any of the years. So how these figures happen to be more than 10 or > 15 km3 per month is not clear…
8) Figure 7: will improve if years were shown in place of symbols (or as labels next two markers) so we can see the correspondence of ENSO and SIO values in a given year. Or some kind of color coding to reduce the clutter….the point is when both are non-neutral do they tend to have the same sign (+/-) despite being independent? Or a tabular summary for this information?
9) Figure 8c: y-axis, label uniformly
10) Including a separate section for the methods after the “data” section will improve the readability. The method and results appear to be all combined.

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RR by Anonymous Referee #2 (16 Jan 2015)

ED: Publish subject to minor revisions (Editor review) (16 Jan 2015) by Thomas Kjeldsen

AR by Mohamed Siam on behalf of the Authors (26 Jan 2015) Author's response Manuscript

ED: Publish as is (02 Feb 2015) by Thomas Kjeldsen

AR by Mohamed Siam on behalf of the Authors (09 Feb 2015) Manuscript

Short summary

This paper explains the different natural modes of interannual variability in the flow of the Nile River and also presents a new index based on the sea surface temperature (SST) over the southern Indian Ocean to forecast the flow of the Nile River. It also presents a new hybrid forecasting algorithm that can be used to predict the Nile flow based on indices of the SST in the eastern Pacific and southern Indian oceans.