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Evolution of salty surfaces on the Kerkennah archipelago between 1963 and 2010

Évolution des surfaces salées sur l’archipel de Kerkennah entre 1963 et 2010
Lucile Étienne, Abdelkarim Daoud and Gérard Beltrando
p. 39-44

Abstracts

The modification of the regional climate since the 70’s and the evolution of the standard of living on the Kerkennah archipelago (Gabes gulf, Tunisia) led on the perturbation of the archipelago environment and of the farming practices. The archipelago has a low topography that favors the sabkha development (low and salty lands). They overlay today 45% of the entire surface. In order to quantify the sabkha extension, a GIS has been made to compare a satellite image from 2010 and aerials photographs from 1963. It appears that the extension reach 465ha (or +18%) between both dates. A questionnaire has been proposed to the population (150 peoples) to evaluate the perception of salinization. It seems that the population understands the ongoing changes and the causes that are due to both climate evolution and anthropic activities in mutation.

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1

Recent climatic and social evolution led to higher salinization risk

2The Kerkennah archipelago (Fig. 1) is located in the Gabes Gulf in Tunisia, a semi-arid area shifting towards an arid climate since the 70’s (ALLEY et al., 2007; FIELD et al., 2012; DAHECH, 2007).

Fig. 1 - Localization and topography of the Kerkennah archipelago

Fig. 1 - Localization and topography of the Kerkennah archipelago

Source: R. R.

3This archipelago is a good laboratory to understand the consequences of the climate and society evolution. Measurements from the nearby meteorological station of Sfax (20 km west of Kerkennah) indeed show that the length of the summer period (May to September) and the frequency of heat waves have increased in the area between 1970 and 2000 (DAHECH, 2007). This evolution of the temperature may be the cause of the observed increase of summery hydric stress that could have significant consequences for the natural vegetation and for the agricultural crops. The wind speed has also increase slightly since the 70’s (DAHECH, 2007), reinforcing the phenomena of deflation. Finally, the sea level rise –likely caused by the elevation of sea temperature and the high subsidence in this part of the Mediterranean Sea (PASKOFF and SANLAVILLE, 1983; OUESLATI,  1994)– and generates inter alia groundwater salinization and rising of the water table (PASKOFF,  1998; CHANG et al., 2011).

4The conjunction of above-mentioned phenomena leads to the extension of sabkhas, low and salty surfaces characteristic of semi-arid areas, at the expense of arable lands (ÉTIENNE, 2014). The intra-annual cycle of the sabkhas highly depends on seasonality. When the system is not disturbed, in winter they are often flooded and are the receptacle of rainwater, runoff, marine submersion and capillarity lift. The water entering the sabkhas system is often salty. During summer, high temperatures allow water evaporation, thereby leaving a salty crust that is then eroded and displaced on the sabkhas margin by the wind. In these areas, the different levels of soil salinity are brought to light by a typical vegetation gradient (Fig. 2).

Fig. 2 - Plant succession from the sabkha to the palm grove

Fig. 2 - Plant succession from the sabkha to the palm grove

Personal photographs from 2011.

5The phenomena of evaporation and dissemination of salts by the wind has been accentuated by the climate evolution measured since the 70’s. This is particularly worrying because low surfaces are by far the majority in the islands –sabkhas represent today 45 % of the Kerkennah archipelago. The maximum altitude is only 13 m, the mean altitude 2.5 m, and the water table sometimes reaches only a few centimeters under the surface. This promotes the spread of salt but also brackish groundwater lifts.

6Since the 60’s, cropping practices evolved too with, among other things, the creation of irrigated perimeters in 1995. In those perimeters the water (pumped from wells) is brackish and is spread in agricultural parcels, part of which are not drained leaving the salt to accumulate in the soil.

7In this context of natural and social changes, we analyzed the evolution of sabkhas in the Kerkennah archipelago between 1963 and 2010 and the influence of human activities on this phenomenon. To better understand the perception of the population we also proposed a questionnaire to a sample of 139 people. Finally we present the case of the Essendouk sabkha as an example of dramatic extension.

1 - The sabkhas extension exacerbated by human activities

1.1 - Quantification of the sabkhas extension

8To quantify the surface evolution of the sabkhas, a GIS (Geographic Information System) has been used for diachronic comparison of the surface between 1963 (33 aerial photographs) and 2010 (1 SPOT 5 image) with a RMS error of 1.1 meter. The study excludes the north-east part of the archipelago, where the 1963 aerial photographs could not be georeferenced precisely enough. The salty soils are not directly visible on the aerial photographs. We therefore used the vegetation as an indicator of salinity. With a resolution of 2.5 m, palm trees are the only plants that could be detected precisely. The limits of the palm grove shows the transition between an extremely salty soil and a soil where the trees can survive. It is this limit that we selected as a demarcation line for the sabkhas in this study, for both the 1963 aerial photographs and the 2010 SPOT 5 image. This limit is validated because no deforestation campaign by uprooting palm trees has been made during the last decades.

9The sabkhas of Kerkennah have expanded on average by 18 % in 47 years (+465 ha). This global tendency toward expanding suggests that regional scale factors –climatic or linked to the sea level rise–, or factors acting at the scale of the entire archipelago –like sociological evolutions– may have an impact. In detail, we see that the average variations between sabkhas hide different pattern of surfaces evolution. The sabkhas of the Gharbi island gained between 19.6 % and 50.2 % of their initial surface of 1963 (Fig. 3).

Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Source: Aerial photography from 1963 and a SPOT 5 image of 2010.

10The sabkha Henchir Salem (+19.6 %) is the one that gained the most in terms of absolute surface with a gain of +198.1 ha between 1963 and 2010, against +89.2 ha for the sabkha Essendouk (+50 % of its initial surface). In contrast, the sabkha in the Chergui Island have experience much smaller changes. The big sabkha Alif Ennkhal for instance gained only 90 ha between 1963 and 2010 which represents 10.2 % of its initial surface. The negative evolution measured in the sabkha of Ramla and in the West of Chergui do not means the long term decrease of soil salinity. Instead, those “retreats” are associated to (i) the advance of the sea on the land, or (ii) to human activity (development of gardens, field crops with leaching and soil amendments). The retreat of areas with no tree can occur but is a minor phenomenon and is, when not due to sea erosio74n, always linked to human activities. The local factors that could explain this difference of evolution between sabkha seems to be, at least in part, known by the population which perceives the retreat of the palm grove to the benefit the sabkhas.

11A questionnaire has been proposed in May 2012 to a sample of 150 inhabitants of Kerkennah aged over 25 years (11 have finally been excluded). A majority of men responded (132 men for 7 women) despite our attempts to interrogate women. The questionnaire was proposed where the people had time to answer (boat between Sfax and Kerkennah, coffee shop…). The questionnaire was proposed in both French and Arabic, the oral traduction (in Arabic) being always done by the same person. In absence of an exhaustive list of the population of the archipelago, we were unable to do a representative sampling. For that reason the results of this questionnaire are only qualitative and intend to give an idea of the perception of the population about risks such as soil salinity and in particular, its most visible effect, the death of the palm trees in the borders of sabkhas. The first task proposed was to rank 13 problems by their perceived severity, the second asked interviews people to assess the intensity of palm trees mortality on a scale from null to important, and the third was a multiple choice question asking about the causes of this mortality if observed.

12The two phenomena most cited as “major problems” were the retreat of the coastline (97/139 or 70 %) and the extension of sabkhas (93/139 or 67 % of the interrogated). Also frequently cited were the rise of the water table (75/139 or 54 %) and the soil salinization (70/139 or 50 %) (Fig. 4).

Fig. 4 - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Fig. 4 - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Source: Aerial photography from 1963 and a SPOT 5 image of 2010.

13Those four phenomena are also considered as worrying by the scientific community and the institutions working on the archipelago (OUESLATI, 1986; Agence de protection et d’aménagement du littoral, 2001; RHOUMA et al., 2005; FEHRI, 2011; ÉTIENNE et al., 2012).

14The others proposition seemed to be less problematic for the interrogated population. The environmental phenomena are the most worrying for our sample. The only directly visible indication of soil salinization is the perturbation of the vegetation which shows the transition between sabkha and palm grove. The interviewed have been interrogated about this.

15The large majority of the interrogated people (132/139 or 95 %) claimed that there is a problem of palm tree death in the archipelago. Among those 132 people, 60  (or 45.5 %) consider that the phenomenon is important, 46 (or 34.8 %) that it is medium and 26 (or 19.7 %) consider that it is weak (Table 1).

Tab. 1 - Perception of the phenomenon of death of the palm trees by the population of the Kerkennah Archipelago

Tab. 1 - Perception of the phenomenon of death of the palm trees by the population of the Kerkennah Archipelago

Results of a questionnaire on 139 people realized in May 2012.

16We also assessed the perception of the population by asking people to list the causes of the mortality. Three answers were proposed (soil salinization, exploitation of the palm tree milk (Legmi) and sea level rise) and the list could be freely completed. The most cited cause was soil salinization (82/139 or 59 %), second was the exploitation of the palm tree milk (65/139 or 46.8 %) and third was the sea level rise (48/139 or 34.5 %). 80 % of the respondents gave several causes and 47 % added causes other than those proposed. This fact let us think that the population is aware of the multifactorial nature of the causes that lead to a high palm trees mortality in the archipelago and that those mortality causes are both natural and anthropic. In addition two mortality causes in particular have been cited spontaneously: the agricultural changes (from palm grove to olive grove) and the abandonment of the palm grove (cited by 27 and 25 people respectively). The causes spontaneously cited by the interrogated people are directly linked to the recent mutation of agricultural practices. All of those causes are documented in the literature (RHOUMA et al., 2005; FEHRI, 2011) which suggest again that the local population is aware of the multifactorial nature of the phenomenon with an association of natural and anthropic aspects even if the results of the questionnaire must be taken with caution.

1.2 - Principal causes of the soil salinization in the Kerkennah Archipelago

17The soil salinization is due to the complex combination of climatic influence and sea level rise with the new agricultural practices during the last decades (Fig. 5)

Fig. - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Fig. - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010

Source: Aerial photography from 1963 and a SPOT 5 image of 2010.

18The progressive transition to an irrigated agriculture is due on the one hand to many decades of drought and on the other hand to a political will from the former president Ben Ali to develop the archipelago. Associated to the evolution of the fishing practices, those social and climatic evolutions led to a paradigm change in agriculture by a progressive abandon of palm trees in favor of olive trees and irrigation. Another phenomenon led to the perturbation of the natural behavior of sabkha: it is the closure of the outlet to the sea of the sabkhas to build road on the dikes. Those dikes have 2 effects: they prevent small water inputs (normally brought by weak storm and by high tides), and block the large water inputs (brought by strong storm and dikes overflow) inside the system. The salts are blocked into the land and so favor their accelerated salinization.

2 - Weight of human actions in the extension process of the Essendouk sabkha

19The case of the very important extension of the Essendouk sabkha (Fig. 3) highlights the interaction that led to the extension and allows us to make two hypotheses about its evolution. The first is of natural order when the second is more about anthropogenic order.

2.1 - The faults network of the archipelago

20The faults in Kerkennah are numerous. One of them follows the south-west south-east coast of the archipelago while the others are perpendicular to it and have a north‑west south-east direction. The thrust-faulting allowed the topographical organization of the archipelago in a succession of Horst and Graben. One of those faults is located east to the sabkha Henchir Salem and marks a relief with the sabkha on its east side and on the west the highest point of the archipelago. The vertical movements of this fault could have caused the subsidence of the eastern part of the Gharbi Island and so could have induced the relative sea level rise and the extension of the Essendouk sabkha. However, thishypothesis remains untested as no recent study has measured the vertical movements within the archipelago.

2.2 - The influence of the irrigation on soil salinization

21Between 1963 and 2010, agricultural practices evolved in the archipelago. In 1963 family farming was based on the palm grove. In 2010 arable lands were parceled because of (i) the increase of the land value and (ii) the family heritage which conducted to the land division. Moreover two irrigated an drained areas have been installed in 1995 which lead to (i) the progressive abandonment of the palm grove and (ii) the olive grove development, more profitable especially when irrigated. Thus the land use and the farming practices are very different between 1963 and 2010 (Fig. 6).

Fig. 6 - Land use evolution on the West art of the Gharbi Island, land division, sabkha extension and irrigation

Fig. 6 - Land use evolution on the West art of the Gharbi Island, land division, sabkha extension and irrigation

22When we examine the irrigated area of Melitta, it appears that the water is brackish and come from a well. This water is, of course, used inside irrigated area but also outside, on the land that are not drained. To estimate the surface concerned by this non-adapted irrigation we performed a remote sensing study using a Landsat TM 5 image from the 6th of June 2011 (ÉTIENNE et al., 2012). Here we show that a large area of the Gharbi Island could be concerned by this irrigation and so by a faster soil salinization (Fig. 6). This could explain the very important salinity increase by surface or subsurface runoff of brackish waters in the direction of the lowest areas, towards the sabkhas.

Conclusion

23The recent climate and sea level evolution leads to large scale changes that influence the islands of the Kerkennah archipelago at fine scale. The actual evolutions lead to a higher vulnerability of the islands in terms of soil salinization and extension of sabkha. In the context of social, political and economic evolution since the middle of the twentieth century in Tunisia, the decisions taken at different scale, from country scale to personal decisions, influence the evolution of land use. In the case of Kerkennah the effects of the evolution of the climate and sea level were aggravated by the important evolution of agricultural practices like the negative effects of the use of brackish water in undrained fields or the progressive abandonment of the palm grove. This example of positive feedback has led to early soil salinization. It also induced a significant vulnerability (i) of land bordering sabkhas that may become saltier by the effects of rising sea levels and the abandonment of old farming practices but also (ii) of undrained farmland due to mismanagement of irrigation water. Recently the authorities in charge of irrigated areas have set up a new water policy that will allow a more rational use of brackish water for irrigation which could slow or even stop the soil salinization process.

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Bibliography

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List of illustrations

Title Fig. 1 - Localization and topography of the Kerkennah archipelago
Credits Source: R. R.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-1.jpg
File image/jpeg, 88k
Title Fig. 2 - Plant succession from the sabkha to the palm grove
Credits Personal photographs from 2011.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-2.jpg
File image/jpeg, 224k
Title Extension of sabkha grouped (identified in dotted line) between 1963 and 2010
Credits Source: Aerial photography from 1963 and a SPOT 5 image of 2010.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-3.jpg
File image/jpeg, 128k
Title Fig. 4 - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010
Credits Source: Aerial photography from 1963 and a SPOT 5 image of 2010.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-4.jpg
File image/jpeg, 76k
Title Tab. 1 - Perception of the phenomenon of death of the palm trees by the population of the Kerkennah Archipelago
Credits Results of a questionnaire on 139 people realized in May 2012.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-5.jpg
File image/jpeg, 36k
Title Fig. - Extension of sabkha grouped (identified in dotted line) between 1963 and 2010
Credits Source: Aerial photography from 1963 and a SPOT 5 image of 2010.
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-6.jpg
File image/jpeg, 140k
Title Fig. 6 - Land use evolution on the West art of the Gharbi Island, land division, sabkha extension and irrigation
URL http://journals.openedition.org/mediterranee/docannexe/image/8559/img-7.jpg
File image/jpeg, 110k
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References

Bibliographical reference

Lucile Étienne, Abdelkarim Daoud and Gérard Beltrando, « Evolution of salty surfaces on the Kerkennah archipelago between 1963 and 2010 », Méditerranée, 128 | 2017, 39-44.

Electronic reference

Lucile Étienne, Abdelkarim Daoud and Gérard Beltrando, « Evolution of salty surfaces on the Kerkennah archipelago between 1963 and 2010 », Méditerranée [Online], 128 | 2017, Online since 01 November 2018, connection on 24 September 2021. URL : http://journals.openedition.org/mediterranee/8559 ; DOI : https://doi.org/10.4000/mediterranee.8559

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About the authors

Lucile Étienne

Université Diderot, Sorbonne‑Paris‑Cité, UMR PRODIG du CNRS, (c.c. 7001) 75205 Paris, France, lucileetienne1985@gmail.com

Abdelkarim Daoud

Université de Sfax, Faculté des lettres et sciences humaines, Département de géographie, Laboratoire Eau, Énergie, Environnement, B.P.553 Sfax 3000, Tunisie, daoudabdelkarim@yahoo.fr

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Gérard Beltrando

Université Diderot, Sorbonne‑Paris‑Cité, UMR PRODIG du CNRS, (c.c. 7001), 75205 Paris

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