Water Balance for a Tropical Lake in the Volcanic Highlands: Lake Tana, Ethiopia

Round 1

Reviewer 1 Report

The paper is aimed to improve insight into water balance of Lake Tana, which is the largest lake in Ethiopia. Previous studies showed high uncertainty related to precipitation data and discharge from some part of the watershed. The Authors try to achieve a better accuracy of the lake water balance calculations by using i.a. data from newly established rainfall monitoring stations located on the lake and near its shoreline, which show underestimation of precipitation component if only station located on land are considered. They also confirmed that some subsurface flow events are of importance for this lake. The paper is generally well written, but some inconsistencies and gaps occur. I further provided specific and detailed comments and suggestions. English needs attention in many places which I tried to point out, but I omitted some errors just because of time and future English editing which will be surely made. 

In the manuscript I`ve received for review there were no lines` numbers so I`m giving my comments divided into chapters with original sentences in italic and my comments/suggestions after them.

General remarks

• Please provide consistent way of presenting units and figures (Fig. vs fig.; mm/a vs mm a-1 etc.).
• In the paper in the whole text please use unique and clear terms for water inflow coming from rivers which are gauged (maybe gauged rivers/catchments) and other rivers/catchments which are not observed. In many places in the text I needed to conclude which of them are actually being considered by the Authors.
• Some of the results were presented in the Discussion section, which is wrong. All the results should be placed in the Results section. Moreover, in the Discussion some of the methodological aspects were described as well (PED model). This construction is wrong. Please transfer this information into Materials and Methods section.

Introduction

• “This becomes even more important in light of the recent developments around Lake Tana where the 100,000 ha [13,39] (or 7% of the watershed area) is planned to be irrigated and can decrease inflow to Lake Tana by as much as 7% since the inflow per unit lake area of the watershed is approximately equal to the additional evaporation of the irrigated area during the dry monsoon phase.” – Please split this sentence into two shorter ones to make this paragraph easier to follow.
• “In all studies except Wale, et al. [24] and Rientjes, et al. [27], land-based rainfall gauging stations were used, installed at Dek Estifanos Island in the middle of Lake Tana. Wale, et al. [24] used a value of 1220 mm a-1 which is less than any of the land-based systems (Table 1). Rientjes and coworkers [28] reported an average lake rainfall of 1347 mm a-1” – I don`t understand what is the message here. Do You mean here, that there are no on-lake stations?
• Few times in the text “scenario 1” for the work of Dessie et al was mentioned. This “scenario 1” should be somehow explained in the text.
• “The closure term was usually small in Table 2 (…)”. Should this sentence refer to Table 2 or Table 1?
• “The accuracy of precipitation over the lake is affected by the density of the rainfall gauging stations used to calculate the lake precipitation [27,35- 38].” – Should it be “The accuracy of precipitation determination (…)”?
• “Omitting the water balances for a single year, average precipitation on the lake varied from 1220 mm a-1 [25] to 1451 mm a-1 [7]” - You mean here average annual precipitation? The same comment is valid for the next sentence where evaporation is reported.
• “Evaporation varied from 1248 mm a-1 by [25] to 1789 mm a-1 [24] with the majority above 1600 mm a-1 .” - You mean here, that typically, in most of the years, evaporation was higher than 1600 mm a-1? Please specify.
• “As expected the discharge from rivers (both gaged and ungauged) into the lake varied widely from 1162 [7] to 2829 mm a-1 for an 11-year average from 1995 to 2005 [24] and 2349 mm a-1 for the years 2012 and 2013 for scenario 1 of Dessie et al [31].” Do You mean here, that within the period of 1995-2005 annual discharge ranged from 1162 to 2829 mm???? Add “and accounted for” before “2349”.
• “The objective of this study is, therefore, to improve the accuracy of the spatially distributed rainfall estimation by installing additional gauging station and then with the improved water balance improve the calculations of the runoff from the ungauged watersheds and improve the understanding of the unaccounted losses in Lake Tana and its basin.”  I`m not sure if the goal of the paper can be formulated like this. As far as I understood, Your goal was to improve the accuracy of the lake water balance, including understanding of unaccounted water losses, thanks to enhanced number of rainfall stations and improved accuracy of river inflow determination. So I believe, higher resolution of rainfall data was a tool (and not a goal itself) to achieve higher accuracy in the water balance with regard to precipitation, river inflow and unaccounted losses.

Study area

• I would like to see on the Fig. 1 direction of water flow (rivers flowing in and out of the lake) together with dams and water gauging stations. Even if the figure would need to be much bigger in size, this would substantially improve understanding of the whole system. On the Fig. 1 It is not clear what “stations” are (are these precipitation monitoring stations? Or other?). It is given further in the text on page 5, but it would be much more convenient to have this explanation on the figure.
• “The average lake level is 1786.17 m a.s.l. for the recording period (1990 to 2007),” – Is should be “The average water level recorded in the period 1990- 2007was 1786.17 m a.s.l”.
• “The records of the other gauged rivers are only for shorter periods and reliability is not always good.” What does it mean? You have used these data anyway? Why? Have you corrected the data somehow or excluded some doubtful records? Please develop this question. Is this the reason why You used “simulated discharge data from the ungauged basis generated by Zimale, et al. [45].”? I would like to see here some short information about how this simulation was done.

Materials and methods

• “Daily precipitation data from 13 stations in and near the Lake Tana basin were collected from the Ethiopian National Meteorological Agency (NMA)”. On Fig. 2 and 3 data from 6 stations are provided (and not 13). Why only 6?
• “Also, the satellite-based Climate Hazards Group Infrared Precipitation version two (CHIRPSv2) that was created in collaboration of scientists at the US Geological Survey (USGS) and the Climate Hazards Group at the University of California, Santa Barbara (CHG, UCSB) was used to obtain rainfall data for Lake Tana.” This sentence is quite complicated and too long.  Please rephrase to something like “Moreover, to obtain missing rainfall data, we used the satellite-based Climate Hazards Group Infrared Precipitation version two (CHIRPSv2) that was created in collaboration of scientists at the US Geological Survey (USGS) and the Climate Hazards Group at the University of California, Santa Barbara (CHG, UCSB)”.
• “Previous studies in the Lake Tana area found a strong association between gauged rainfall and CHIRPS rainfall data estimate [46-48]” on page 5. I suggest “correlation” instead of “association”.
• “After correlating the CHRIPS data with the newly recorded rainfall data, CHIRPS data were used to extend the newly established rainfall data to match it with the study period.” – Please give here this “study period” again in brackets. I realized that I don`t fully understand, what the “study period” actually is. For the lake water balance it is clearly given (3.2.1.), but for the whole study itself it is not. Second comment here is that after You mentioned CHRISP and its correlating with precipitation data from new stations, You should refer to details, adding something like “Details on calculating missing precipitation data for the new stations based on CHRISP data, were described in section 3.2.2.”
• “The four major gauged tributary river of Lake Tana cover approximately 40% of the watershed” on page 5. Should be “rivers:”. Do You mean here, that catchment of these four rivers cover 40% of the lake`s catchment. In that case, for 60% of the lake`s catchment there are no reliable data on river discharge. Have I understood it correctly? Please specify.
• “Besides, the data from Hydroweb (www.legos.obs-mip.fr/fr/soa/hydrologie/hydroweb/StationsVirtuelles/SV_Lakes/Tana.html) were downloaded for 1992-2020 [49].”  - Why? You had already full data set for the Bahir Dar station. What is Hydroweb?
• I believe, the paragraph 3.1.5 “Storage characteristics of Lake Tana” should be included in the chapter “ 3.1. “Available long-term hydrological and meteorological data” as it is more about calculation methods so it should placed in the paragraph 3.2.
• “The bias-corrected CHIRPS data were used to extend the newly recorded data.” Please add here, from which period to which period the data were extended.
• “The calculated evaporation rate was 1426 mm a-1”. Is this annual average?
• I suggest starting section 3.2.3. with something like “Different variants of evaporation rates taken from literature were used in our study”. Moreover, as You used evaporation data calculated by other studies, I believe this section should be placed into paragraph 3.1.(“ Available long-term hydrological and meteorological data”).
• I`m very confused about section 3.2.4. First it was stated, that groundwater flow was neglected because of many reasons, whereas at the end of this sentence it was again repeated that some studies postulated significant contribution of groundwater. So why neglecting it?
• I`m also a bit confused about section 3.2.5. “Lake inflow from rivers” was calculated here. You mean here the rivers for which no MoWIE data were available??? Please specify and try to vbe specific in the whole paper when writing about river discharge.
• Paragraph 3.3. What is “first and second rainfall dataset”?
• There is one idea I got after I struggled through the Materials and methods section. It is advisable to set together in one table all the metadata for the data You used in the study. In the paragraphs 3.1.1-3.1.4 few parameters obtained from multiple sources and for sometimes different periods (why?) are described. If they would be given in the table (with column for period, source/method of obtaining these data), it would make it much more easy to follow the data set You used in Your study. I highly recommend such a solution.
• Paragraph “Methods” in unclear. As all the methods described here are calculations, I suggest to name this point “Calculation methods”.

Results

• 1. “Lake rainfall” should be “Rainfall on the lake`s surface”?
• Please indicate on figures 2 and 3 which of the stations are land-based (on the legend). In the Fig. 2 there is not station “Anbesane” and on the Fig. 1 there is no station “A/Zemen”. I believe the set of stations should be the same for both figures…
• From the Fig. 3 it is not clear that for the station Gurer the precipitation is much higher than for the others, as it was stated in the text. On Fig. 3 Gurer is black line which is in summer lower than few other stations. Is black line actually Gurer? It looks like Zege has much higher rainfall in summer…. Is it correct? It looks like Zege was placed instead of Gurer… Moreover, rainfall for the station Jigrfa is only slightly higher as compared to few other stations (which are I suppose A/Zemen, Zege?). It is in fact greater, however, the difference is not that huge as compared to Gurer (if Gurer was by mistake named Zege, as I suppose)???
• Title of the figure 3 should be “Long term average monthly rainfall for 1990-2014 of the eight stations used in calculating the precipitation for the Lake Tana” (and not “of the Lake Tana”).
• “Thus, our rainfall amounts with the lake-based stations are similar to these studies”. Please add your rainfall value after “our rainfall amount”. Consider “from” instead of “with”.
• “The cumulative rainfall on the lake throughout 1990-2014 is 31.16 m for the land-based stations and 32.79 m when the lake-based station was included (Figure S2).” – It should be “The cumulative rainfall on the lake throughout 1990-2014 calculated based on data from land-based stations was …. whereas for lake-based stations it was ….”.
• 4. If yellow line and dots indicate land based and lake based stations together, on the legend it should be “land-based and lake-based” (and not “lake-based” as this is confusing’ “lake-based” suggest that only two lake-based stations were considered here). The same comment is valid for Fig. 9 and 11. Below Figure 2 You indicated, that all the stations together are called “lake-based stations”, but this term is confusing. Please provide consistent terms for lake- and land-based stations (in the text dash was used or not in these terms).
• “After 2007 the manually observed lake levels were high during the rain phase and indicating that portions of the lake surroundings would have been flooded and thus not realistic and are not shown in Figure 5.” Why not realistic? Do You suggest here, that these data are of poor quality? What is source of these data and their poor quality? English needs attention here.
• “Outflows in 2007 again changed significantly with peak outflows over 1000 m3sec-1 that had not been observed in any of the prior years (Figure 5)” – Why? Is this because You further state “The quality of the outflow discharge data, as reported by Gieske, et al. [23] was good from 2000 to 2006”? If so, these sentences should be somehow connected…
• “The monthly rate of flows into the lake and unaccounted water losses [??????? (?) + ?(?)] for the three evaporation rates calculated with Eq 9b are shown for the lake-based rainfall in Figure 6 (with units km3 month-1),” – According to this, evaporation was calculated using Eq. no 9b, which is not true J
• Title of Fig. 6 contains typing error “evaporation rated” – should be “rates”.
• On the Fig. 6 legend should be modified. Please specify, what data are presented for blue, purple and green lines (these are inflow from not gauged rivers and unaccounted losses calculated for different evaporation rates).
• “The gauged discharge is between half to two-thirds of inflow into the lake (Figures 6 and S3) while the area is only two-fifth of the watershed of Lake Tana (Table 2).” – do you mean here “while the area covered with gauging stations is only ….”????.
• What is the difference between Fig. S3 a and b? Please wrote this clear in the title of the figure (the information if provided in the text on page 12), but when the reader goes to the Suppl. Material it is not clear.
• “this means that water leaves from the lake (and its wetlands) other than through the outlet(s) and by evaporation.” – English needs attention here.
• “Besides the inflow to the lake, Figures 6 and S3 show that the total gauged flow from the four large watersheds.” - English needs attention here.
• Title of Fig. 9 should be “Water level predictions (yellow dots) FOR Lake Tana (…)”.
• “In regressing the data, we found that three distinct dates after which the relationship between gauged discharge and lake inflow changed “ – “that” is superfluous here.

Discussion

• “One of the reasons that the ungauged lower portion of the basin contributed less water to the

lake per unit area than the upper gauged watersheds is that it rains more on the higher elevations.

Another cause could be the abstraction of the water by irrigation [31]”. When  reading this sentence I realised that this is the first time when You mentioned, that ungauged part of the watershed is upper part. It would be critical to have this information provided earlier in the paper. I would also like to see somewhere on the map which part of the lakes watershed is gauged and which not.

• ” To investigate whether rainfall could be responsible for the decrease in contribution per unit area we employed the simulated discharges into Lake Tana with the PED model [45].” – Do You mean here “to investigate whether differences in precipitation between upper and lower part of the watershed can be responsible for the low contribution of the gauged (lower) part to the lake`s water supply….”???
• “Thus in addition to precipitation, water from the uplands contributed via the many springs that can be found the ungauged watershed to the flow in the rivers.” – English needs attention here.
• “The lake-based rainfall is more accurate than the lake based rainfall depth” - I don`t understand this sentence.
• “This is almost equal to the unaccounted losses in this study.” – which is….?
• “The study of Dessie, et al. [13] that measured the flows of the rivers of a large portion of the basin over two years shows losses in losses 27 cm even with a high evaporation rate of 1789 mm a-1 (Table 2, scenario 1 [13]).” – English needs attention here.
• “Finally, as Mamo et al. [33] suggest” – it should be “suggested”.
• “This might happen again if rainfall would decrease and irrigation in the Lake Tana basin increased beyond that is currently planned” – should You not mention here increase in evaporation as well?

Conclusions

• “The exact amount was difficult to estimate because the evaporative flux of lake Tana is not known exactly, but it is in the order of 0.6 km3 a-1 .” – Is 0.6 km3a-1 the “exact amount” or “evaporative flux”? Please rephrase to avoid confusion.
• Would it be also reasonable to improve accuracy of determination of evaporation? If evaporation is calculated based on temperature, humidity and so on taken from land-based station in Bahr Dar, it is likely that the actual evaporation from the lakes surface differs from that on land… Temperature and wind speed in Bahir Dar are surely affected by the city…. Wind speed on the lake is surely higher than on land as well…

Some more sentences where English needs to be reconsidered:

“In addition, many smaller lakes are extremely sensitive to any decrease or increase in rainfall more so that land-use changes [6].” – Do You mean here “more sensitive to rainfall (…) than to land use changes”?

 “Subsequent water balances of Lake Tana were made by Kebede, et al. [7] from 1960 through 1992;” on page 2. Should be “for the period 1960-1992” instead of “from 1960 through 1992”.

“For Lake Tana, Grabham [16] investigated the very first storage capacity of Lake Tana in the early 1920s.” on page 2. “For the Lake Tana” is superfluous here.

“Except for the outflow data of the lake from 1921-1933 [18,19], published streamflow data are not available until the United States Bureau of Reclamation carried out a major survey of the land and water resources of the Blue Nile basin between 1958 and 1963 [20] and even that data cannot be obtained easily.” On page 2. It should be “were not available” instead of “are not available”.

 “The relationships were similar to the survey results by other surveys Ayana [50] and Wale, et al. [24].” On page 6. I suggest “… were similar to results obtained by Ayana [50] and Wale, et al. [24].”

“The evaporation rate of [8] was in the same order as several other studies [24,25,29] (Table 1).”
I suggest “…… was in the same order as reported by other studies….”

“The outflow of the Lake Tana to the Blue Nile as measured with the gauging station just downstream of the outlet increased from an average 1207 mm a-1 from 1921 to 1933 to around 1600 mm a-1 for the most recent years (Table 1).” on page 3. Should be “,,from an average 1207… for the period 1921–1933 to around….”?

“Dessie, et al. [31] measured the discharge in a sizable portion of the basin could not account for 0.74 km3 a-1 (equivalent to 273 mm a-1) in his scenario 1.” on page 3. Something is missing in this sentence.

“They attributed the abstractions to irrigation and evaporation from the flood plain as reasons for unaccounted losses.” On page 3. I suggest “They suggested water abstractions for irrigation ….”.

I suggest „river stretches” or „river sections” instead of „river portion” (page 4 at the end of 3^rd paragraph)

“The flood plains were inundated at this level and affected the settlements around the lake” (page 4) – should it be “The flood plains were inundated at this level which affected the settlements around the lake” ?

“Currently, most of the water that flows once through the Blue Nile is now diverted to the Beles basin to generate 460 MW of electricity” – is “now” not superfluous here?

Comments for author File: Comments.pdf

Author Response

Dear Reviewer

Thank you so much for your positive evaluation and the many comments. It greatly aided in improving the revised manuscript. 

In the attached pdf we have repeated the comments and immediately following it, both our response and the improved text of the revised manuscript. At the end of the responses, the marked-up manuscript is appended, The revised text is in red. We omitted the deleted text for ease of reading.

Thank you again.  

Muluken, Abeyou, Fasikaw and Tammo

Author Response File: Author Response.pdf

Reviewer 2 Report

Very nice work. See attached comments.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer

Thank you for your excellent comments. In the attached pdf we have repeated the comments and immediately following it, both our response and the improved text of the revised manuscript. At the end of the responses, the marked-up revised manuscript is appended, The revised text is in red. We omitted the deleted text for ease of reading.

Thank you again.

Tammo, Moluken, Abeyou and Fasikaw

Author Response File: Author Response.pdf