Is Climate or Direct Human Influence Responsible for Discharge Decrease in the Tunisian Merguellil Basin?

Round 1

Reviewer 1 Report

The paper “Is Climate or Humans Responsible for Discharge Decrease in the Tunisian Merguellil Basin” authored by Khemiri et al. try to reveal the contributions of climate change and anthropogenic activities to the decrease of runoff in the Tunisian Merguellil Basin. This topic is interested and widely concerned. However some essential revisions should be done.

• The organization of paper should be improved. Such as the figure 1 and figure 4 could be merged. Some date process also should be clear, such as the mean value of rainfall in the long-tern and 10 years time scale? Why is 10 years? The figure 5 should be redrew with scientific drawing.
• The different trend for different time scale 10, 20 and 30 year should give much more clear description and deep discussion.
• Figures 2 and 3, the statistics information, such as SDEV for time or space mean value.
• The core problem in the paper, the effect of climate change and human activities to variation of runoff is partial, especially for the human activities, NDVI can cover the land use cover change and human activities. How about the irrigation, domestic water, industrial water, et al. on the decrease of runoff. What’s effect of ecological environment parameter, such as soil properties, et al. to the runoff.
• The mechanism in the decrease of runoff in this study area is unclear, much more effort should be inputted in this aspect.

Author Response

Response to Reviewer 1 Comments

 

We would like to express our sincere thanks to the editor and the reviewers for the valuable comments and time spent to improve the manuscript. We now firmly believe that with the implemented changes in text, the manuscript has improved greatly and would be of great interest to the readers of the Water journal. We have modified according to all comments made by the reviewer. Please find below the reviewer´s comments and our answer in red and changes in the text just below each comment. All the modifications are highlighted in yellow within the manuscript

 

1. The organization of paper should be improved. Such as the figure 1 and figure 4 could be merged.

We agree and have now tried to improve the organization of the paper. Among other things, we have merged Figure 1 and 4 as suggested (line 111).We have also added new information and figures to the text so as to further improve the organization.

 

2. Some date process also should be clear, such as the mean value of rainfall in the long-tern and 10 years time scale?

We agree and have now added more explanation and discussion of the presented mean values for different time steps. The use of long-term rainfall averages aims to assess the representativity of the study periods in relation to long-term rainfall conditions. We have as well added the term standard deviation to identify the standard deviation of the different mean (lines99-103).

 

3. Why is 10 years?

In Tunisia, the rainfall regime is often subject to a quasi-periodicity of about 10 years as shown by e.g., Benzarti (2003). The physical reasons for this are not clear in detail. In addition, since the 1950s, Tunisia has experienced a decrease of 5% per decade in annual rainfall (Verner 2013).

Benzarti, Z. Les ressources en eau de la Tunisie : contraintes du climat et pression anthropique in eau et environnement, ENS Éditions, 2003.

Verner, D. La Tunisie face aux changements climatiques: Évaluation et actions pour accroître la résilience et le développement. 2013. A World Bank study; Washington, DC: World Bank. https://openknowledge.worldbank.org/handle/10986/13114.

4. The figure 5 should be redrew with scientific drawing.

We agree and have now completely changed the Figure 5 to improve clarity.

5. The different trend for different time scale 10, 20 and 30 year should give much more clear description and deep discussion.

We agree that this was not very clear in the old text. We have now as suggested added a new section to give a clearer description and further discussion. The changes are seen from line 262-264, 268-269, and 348-350.

 

6. Figures 2 and 3, the statistics information, such as SDEV for time or space mean value.

We have now added more detail and information in the two figure legends.

 

7.1. The core problem in the paper, the effect of climate change and human activities to variation of runoff is partial, especially for the human activities, NDVI can cover the land use cover change and human activities.

How about the irrigation, Domestic water industrial water, et al. on the decrease of runoff.

 

We understand the reviewer´s concern. We have added further discussion on this. The area is mainly affected by water for irrigation. However, water for domestic, irrigation, industrial and tourist use is also being taken from deep groundwater for the specific case study in the Merguellil watershed. Consequently, the area´s water balance is affected by groundwater pumping but on larger time scales and a slow seepage from surface water to deeper groundwater. Information on this has now been added to the new text (lines 313-315).

 

7.2. What’s effect of ecological environment parameter, such as soil properties, et al. to the runoff.

 

Soil texture plays a big role in the generation of runoff. Indeed, upstream of the watershed (Skhira station), the bed of the Merguellil wadi is formed by hard rocks with a stable section. Downstream, at the Haffouz station, the Merguellil bed is very wide, formed by unstable sandy formations. It is then exposed to erosion phenomena, which means that the section changes shape almost at each passage of new floods (Bouzaiane & Lafforgue, 1986). This and further discussion have been added at lines 381-386. The new reference has also been added to the new text.

 

8. The mechanism in the decrease of runoff in this study area is unclear, much more effort should be inputted in this aspect.

 

We agree and have now added several sections according to the below. In the semi-arid region such as the Merguellil watershed, the hydrological processes are characterized by an extreme spatiotemporal variability, (e.g., Scanlon et al., 2006). This is displayed by a strong irregularity rainfall. The rainfall variability and high intensity have different negative impacts on the landscape such as erosion. To mitigate these negative effects, we particularly mention soil and water conservation practices, land use/land cover needs etc. Indeed, surface runoff is often generated by exceptional events which are often rare but devastating for erosion for example (Salem, 2012; Cudennac, 2007). Consequently, only extreme events produce significant runoff. As well, increasing groundwater abstraction has a long-term negative effecton streamflow. Lines 428-444.

 

The expansion of water and soil conservation practices has led to a marked decrease in runoff in the Merguellil watershed (MERGUSIE, 1998-2007). Many investigations have focused on the variability of the runoff coefficient (e.g., Kingumbi et al., 2007; Lacombe et al., 2008; Ogilvie et al., 2014). In this regard, by analyzing 114 comparable events over the period 1989-2010, Ogilvie et al. (2014) estimated a 40% reduction in the runoff coefficient following the expansion of water and soil conservation practices since 1996 with a decrease of 25% in annual inflows to the outlet. Lacombe et al. (2008) estimated that the sharp drop in flows arriving at the El Haouareb dam over the past 40 years is due to a reduction of the runoff generated by events below 40 mm from 28 to 32% and a reduction of 41-50% for all events over the period 1989-2005. Lines 434 - 442.

The new references were added to the new manuscript

Author Response File: Author Response.docx

Reviewer 2 Report

Please the comments and suggestions in the file attached 

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 2 Comments

 

We would like to express our sincere thanks to the editor and the reviewers for the valuable comments and time spent to improve the manuscript. We now firmly believe that with the implemented changes in text, the manuscript has improved greatly and would be of great interest to the readers of the Water journal. We have modified according to all comments made by the reviewer. Please find below the reviewer´s comments and our answer in red and changes in the text just below each comment. All the modifications are highlighted in yellow within the manuscript.

 

1. Line 93: Location of the Kairouan station is not shown on figures, the altitude of thestation should be also indicated.

 

The authors agree and have now added a new figure to show this as well as a new Table 1.

 

 

Figure 1. Location and topography of the Merguellil watershed in central Tunisia

 

Table.1 Characteristics of stations used

Station	Type	Longitude	Latitude	Altitude (m)
Kairouan SM	Meteorological	10°10'42"	35°67'59"	66
Haffouz  Telepherique	Hydrometric	07°19'19"	35°37'58"	250
Skhira	Hydrometric	07°02'49"	35°44'24"	600

 

Figure 1 and Table 1 were added within the manuscript.

 

2. Line 94-101: It is useful to provide basic descriptive statistics (e.g. Mean, Variance, Standard Deviation, Coefficient of variation, Std. err. of Mean, Skewness, Minimum & maximum, Autocorrelation r(1)) of precipitation (P),temperature (T), potential evapotranspiration (E), and flow (Q) time series observed/estimated within the Merguellil watershed for the study periods and make appropriate comments

 

We agree and have now added the below table.

 

Table 2 Descriptive statistics for data series observed/estimated in the Kairouan station for the study periods (1976-2017).

 

Statistic	River flow (m3/s)		Precipitation (mm/year)		Temperature (°C/year)	Potential evapotranspiration (mm/year)
	Skhira	Haffouz	Skhira	Haffouz	Kairouan	Kairouan
Mean	0.28	0.53	395.1	289.5	20.3	982.6
Variance	0.04	0.11	24402	7904	2.71	30086
Standard deviation	0.21	0.34	156.2	88.91	1.65	173.5
Coefficient of variation	0.75	0.64	0.40	0.31	0.08	0.18
Std. err. of mean	0.03	0.05	23.8	13.5	0.25	26.5
Skewness(Pearson)	0.60	0.41	0.69	0.39	0.42	0.86
Minimum	0.01	0.05	116.0	128.7	17.53	780.3
Maximum	1.06	1.40	844.2	524.2	24.80	1591.6
Autocorrelation Coefficient r(1)	0.51	0.16	0.25	0.18	0.78	0.81

 

Table 2 shows descriptive statistics of the river flow, rainfall, evapotranspiration, and temperature series. The rainfall upstream in the basin (Skhira station) is significantly larger compared to downstream (Haffouz station) due to orographic effects [43]. Figure 5 shows double mass curves for annual precipitation and river flow at the Skhira and Haffouz stations and approximate homogeneity of the data [44-46].

 

3. Line 107, 108: Please show samples of intra-annual variations in P, T, E0, and Q within

the Merguellil watershed

We agree and have added the Figure.4 of intra-annual variation in P, T, E0, and Q within the Merguellil watershed in the new manuscript

 

4. Line 130: Have the authors analyzed the statistical homogeneity of the study time series? If so, by what means? What are the results?

 

The effect of homogeneity in the present study is controlled by the double mass curve (Figure 4). Please see the new figure below.

 

 

(a)	(b)

Figure 4. Double mass curves for (a) precipitation and (b) river flow

 

The results show that the series are approximately homogeneous since there is no break or deviation from the general mass curve. Certainly, the double mass curve for precipitation follows the trend curve, while it is slightly tilted for discharge. This is probably due to the variation in the runoff response to climate change [44-46]. This part was added at lines 150-152.

 

5.1.Line 143: Please justify the use of the ITA method for runoff trend analysis.

 

We agree and have added the following text. The ITA method is used in this study since its comparison with conventional methods shows certain advantages [26]. It has been used in the detection of trends for several hydro-meteorological variables such as evaporation, river, flow, and precipitation in different regions [26-29]. The method can indicate potential future impacts of climate change, management, and design of hydraulic structures [24]. This part was added to the manuscript at lines 75-79.

 

5.2.Are there considerable disadvantages of the ITA from the point of view of results obtained?Does the method take into account autocorrelation in the time series? Please refer to [Serinaldi, F., Chebana, F. & Kilsby, C.G. Dissecting innovative trend analysis. Stoch Environ Res Risk Assess 34, 733–754(2020). https://doi.org/10.1007/s00477-020-01797-x ]

 

We corrected the results of the trend analysis while considering the disadvantages of the ITA.

The ITA method detects trends over a time series across two divided subseries of equal lengths. In addition, the standard deviation of the trend slope deviation formula assumes that variation is constant. This assumption is not valid for asymmetric and dependent series [49,50]. Therefore, ITA detects trends on a main time series with equal sub-series length [51]. Thus, we divided the 42-year study period into three periods 20 years (1976-1995; 1996-2017), 30 years (1976-2005), and 40 years (1976-2017), and we verified that the series are not auto-correlated. Thus, we have corrected the results of the present study. This was added at the section 2.3 lines 177-183.

 

6. Line 182: Please explain how the F(?,ϕ) value has been calculated

 

F(?,ϕ) = (L/12)(N/30)

Where N is the number of days in the month being calculated and L is the average day length (hours) of the month being calculated. This was added at line 218.

 

7. Line 194: Equation (14) should be corrected: please insert “plus” instead of “minus”

 

The Equation (14) was corrected, line 228.

 

8. Table 1, Table 2:Are only the values written in bald statistically significant?

 

We have corrected the results, the values of the slopes in bald show the significant trend at a confidence interval of 95%.

 

9. Table 3: How have the authors computed precipitation and potential evapotranspiration rates over the two study watersheds in different time periods, using data from the only Kairouan station, taking account of a big variety (see Fig.1) of basin surface altitudes and landscapes? Please explain in detail. Such areal estimations of P, T and E look to be unreliable enough

 

We have only one case study which is the Merguellil watershed. We used the precipitation measured upstream (Skhira station) and downstream (Haffouz station). For lack of data acquisition, we used meteorological data (T and E) from the Regional Meteorological Institute at Kairouan.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

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