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Spring mounds of the Nefzaoua oases in Tunisia: Irreversible degradation of exceptional geomorphic structures
Abstract
The oases in the arid the Nefzaoua region in central-western Tunisia are associated with spring mounds. Spring mounds result from an exceptional succession of geomorphological, hydrogeological and climatic conditions and processes that follow each other in a specific order. They look like small volcanoes that range from 200 m to more than 2000 m in diameter and 3–30 m in height, which centres were once occupied by a pond fed by an artesian spring. They are at the origin of each oasis's irrigation system, which radiates from the mound's foot. Due to intense irrigation of intensive date palm plantations (Phoenix dactylifera) in recent decades, all artesian springs dried out. We established a complete inventory and typology (i.e. location, dimension, and types of degradation) of all spring mounds in the Nefzaoua region. Overall, in an area of 3000 km², 126 spring mounds of 7 types were identified, of which 112 (89%) were degraded. Due to the low salt content in spring mound sediments, they are excavated and used as a soil amendment to expand new palm plantations. Only 14 mounds (11%) are presently preserved. Their qualification as a geomorphosite could preserve them from complete destruction.
... In Tunisia, in Nefzaoua province alone, south of Djebel Tabagha in an area of 3000 km 2 , more than 126 spring mounds have been clearly identified (Raddadi, 2021;Raddadi et al., 2021). The size of only 88 mounds could be determined with a larger diameter ranging between 250 and 2430 m (average 877 ± 465 m) and an altitude ranging between 2 and 28 m (average 12 ± 5 m) (Raddadi & Podwojewski, 2022a). The mounds are associated with oases east of the Chott El Jerid ( Figure 1C). ...
... Spring mounds have lain at the centre of all permanent oasis settlements in Nefzaoua province for more than 2000 years, and all current oasis plantations there radiate around them ( Figure 2). Some mounds in Nefzaoua province have been mechanically excavated Raddadi & Podwojewski, 2022a), which completely revealed their internal structure in vertical sections F I G U R E 1 (a) Location of the Nefzaoua region (Governorate of Kebili in South West Tunisia) and the limit of the presaharian Tunisia. and thus provided a unique opportunity for the present study's stratigraphical and pedological analysis. ...
... increased from 5000 to 20 000 ha (Raddadi, 2021) and all artesian springs disappeared; in addition, the spring mounds have no longer been sustained by regular flooding since 1995 (Marlet et al., 2009;Zammouri et al., 2007). Following the decrease in the water table level, the spring mounds naturally silted up and eroded or were artificially levelled (Raddadi & Podwojewski, 2022a Raddadi & Podwojewski, 2022a). Their mechanical destruction has exposed their internal structure as vertical sections. ...
Spring mounds are specific geomorphological landforms in arid or semi‐arid environments associated with playas and artesian springs. Spring mounds are found worldwide, especially in the great Artesian Basin of Australia and in North American and Egyptian deserts. They result from an exceptional succession of climatic, geomorphological and hydrogeological conditions, with processes that follow each other in a specific order. In Tunisia, in the arid zone, ca. 126 spring mounds have been identified in Nefzaoua province alone, especially in the oasis east of Chott el Jerid. They have a conical shape that ranges from 200 to more than 2000 m in diameter and 3–30 m tall, and their centre is hollowed out by an artesian spring of fresh water. Palm groves (Phoenix dactylifera) spread out at the foot of each mound. The springs have dried up because of the proliferation of borehole wells. Because of the low electric conductivity (EC) of the sediments (<1 mS cm⁻¹) at their base, spring mounds have been excavated and used as a soil amendment to expand new palm plantations. This excavation allows for analysis of their internal structure, which has never been observed well. In the present study, fine analysis of sediment layers in four sections of two representative mounds showed that vegetation had trapped sediment at their base. Fine strata of variable texture alternating with variable calcium carbonate, gypsum or organic carbon contents suggest a clear limnic origin. Many redoximorphic features, sometimes associated with the presence of old roots, suggest variation in the water level in the centre of the mound. The mounds are capped by a thick layer of indurated gypsum, which helps them resist hydric and aeolian erosion. The origin of the sediment components is discussed.
... Hence, salt lakes are rare sources of water, flood and fishery, they serve as regulators of temperature and hydrology, and they support carbon fixation for environmental sustainability [84]. Moreover, they play a crucial role in maintaining biodiversity [85], serving as habitats for rare African species and birds in nearby oases [86][87][88] and habitats for halophilic species [89]. Such phenomena were revealed in studies on environmental monitoring that evaluated landscape changes in Tunisia. ...
This study documents the changes in the Land Use/Land Cover (LULC) in the region of saline lakes in north Tunisia, Sahara Desert. Remote sensing data are a valuable data source in monitoring LULC in lacustrine landscapes, because variations in the extent of lakes are visible from space and can be detected on the images. In this study, changes in LULC of the salt pans of Tunisia were evaluated using a series of 12 Landsat 8-9 Operational Land Imager (OLI) and Thermal Infrared (TIRS) images. The images were processed with the Geographic Resources Analysis Support System (GRASS) Geographic Information System (GIS) software. The study area included four salt lakes of north Tunisia in the two regions of the Gulf of Hammamet and Gulf of Gabès: (1) Sebkhet de Sidi el Hani (Sousse Governorate), (2) Sebkha de Moknine (Mahdia Governorate), (3) Sebkhet El Rharra and (4) Sebkhet en Noual (Sfax). A quantitative estimate of the areal extent analysed in this study is 182 km × 185 km for each Landsat scene in two study areas: Gulf of Hammamet and Gulf of Gabès. The images were analysed for the period 2017–2023 on months February, April and July for each year. Spatio-temporal changes in LULC and their climate–environmental driving forces were analysed. The results were interpreted and the highest changes were detected by accuracy assessment, computing the class separability matrices, evaluating the means and standard deviation for each band and plotting the reject probability maps. Multi-temporal changes in LULC classes are reported for each image. The results demonstrated that changes in salt lakes were determined for winter/spring/summer months as detected changes in water/land/salt/sand/vegetation areas. The accuracy of the classified images was evaluated using pixel rejection probability values, which were filtered out using the ‘r.mapcalc’ module of GRASS GIS. The confidence levels were computed and visualised with a series of maps along with the error matrix and measured convergence level of classified pixels. This paper contributes to the environmental monitoring of Tunisian landscapes and analysis of climate effects on LULC in landscapes of north Africa.
Les paysages des tertres oasiens du Nefzaoua sont disposés à l'est du Chott el Jerid en Tunisie aride. Les palmeraies s'étalent au pied des tertres, monticules de forme conique creusés au centre par une source artésienne d'eau douce. Ils ont été aménagés et ont été à la base du développement des oasis. Aujourd'hui, les sources sont taries par la prolifération des forages et les tertres, fruits d'une conjonction de conditions géomorphologiques, hydrogéologiques et climatiques sont à l'abandon, voire détruits pour étendre les plantations de palmiers dattiers. Ces édifices peuvent être considérés à la fois comme un géopatrimoine et un patrimoine culturel en péril et certains mériteraient d'être préservés.
Megalithic monuments are a distinctive archaeological feature of Saharan landscape,
as indicated by different systematic research projects undertaken so far. Starting from a
very low baseline of previous archaeological research, and as part of a comprehensive research
programme focussed on northern Sahara, we launched in 2015 a territorial investigation of
stone monuments of the pre-protohistory and early history of southern Tunisia. To do this,
we selected a sample study area east and south-east of the Chott el Jérid depression (Kebili region)
where to conduct field research. In this paper we present the research strategy adopted,
planned to address some issues such as the poor state of preservation of the monuments or
their uncertain chronology, also known from other parts of the Sahara. Our results, based on
the combination of remote sensing analysis, field survey and selected excavations, highlight
a dense occupation of this area of northern Sahara, where monumental buildings of possible
funerary function tentatively trace back to the late pre-protohistory up to the roman age. The
persistent use of the area across a long-time span corroborates its pivotal location in ancient
trans-Saharan connection routes.
In arid oases, soil salinity and a rise in shallow groundwater are serious threats to long-term sustainability of irrigated agriculture. Understanding the impacts of irrigation practices on soil salinity and shallow groundwater dynamics is critical for improving soil conditions and water efficiency. In this study, the impacts of excessive irrigation of date palms with low-quality water on soil salinization and shallow groundwater properties (depth and electrical conductivity) were evaluated for a 10-year period (2005–2015) in a Saharan Tunisian oasis. The study included three phases: (1) assessment of the suitability of groundwater for irrigating date palms; (2) quantification of the long-term water use of date palm plantations; and (3) quantification of dynamic patterns of soil electrical conductivity and shallow saline groundwater in the studied oasis for a 10-year period (2005–2015). Results of this study indicated that under high evapotranspiration conditions, the identified low-quality water resources (leading to high soil salinization risk) coupled with rapidly rising shallow groundwater at critical depths (<1.5 m) (resulting from the high water use) were key factors of the rapid increase in soil salinization within the oasis. The soil electrical conductivity built up to levels that exceeded the salt tolerance of date palms, estimated at 4 dS m−1. Although the excessive use of irrigation water dilutes the groundwater, and its salinity decreases, the risk of increasing soil salinity is greater because of rising water table.
The present study examines the geoarchaeological history of an oasis in Kharga Depression in central Egypt. El-Deir is renowned for its Ptolemaic temple and Roman fortress on the road from former Hibis (Kharga) to the Nile Valley. During the survey, spring mounds and irrigation soils belonging to an ancient agricultural zone were discovered, and further documented by ceramics found on the site. Our methodology combines the geomorphological interpretation of landforms (especially yardangs) with ceramics and 14C-dated charcoal to distinguish and date former agricultural areas in El-Deir. The results show that the oasis experienced several phases of soil accretion and destruction through time. Playa sediments were deposited in the humid early Holocene and severely eroded by deflation before the onset of irrigated agriculture between Pharaonic and Persian times. Very fast vertical soil accretion occurred in the Ptolemaic period, but irrigation soils were later destroyed during the Roman period by a combination of wind deflation and flash floods (second to fourth century A.D.), suggesting a period of climate instability. The case of El-Deir invites reevaluation of constructive agencies for the development of irrigated land and destructive agencies as limiting factors for the sustainability of agricultural practices in late antiquity.
The question of geomorphological heritage is truly put since the past twenty years and has led to international recognition of the concept of Geomorphosite. In the first part of the paper are discussed the concepts of natural heritage, geological heritage, geosite and geotope according to the Vade-mecum for the Inventaire National du Patrimoine Géologique (INPG). One wonders about the motives of the INPG in terms of objectives and conservation. Another difficulty is to define whether all parties share the same scale of values. The spatial dimensions of a geomorphosite have been recently discussed at length. However, it may be fruitful to compare the problem of spatial scales with the paradigm of landscape developed in the theory of Geosystem. In terms of time scale, it should be wherever possible to distinguish in every geomorphosite the deep time of Geology, the long period of Quaternary times, the distant past of Prehistory and postglacial environments, the near past of History and sub-actual environments, the current time of active geotopes where intervene the dynamic interaction of internal or external geodynamics, including anthropogenic factors. In terms of scale of values, the global value of a geomorphosite is defined by the sum of its scientific and additional values, which could be subdivided into cultural values and societal values. Taking better account of their specificities could help to include many geomorphosites to the INPG.
Extraction of groundwater for agriculture has resulted in the loss of springs across arid regions of the globe. The history and fate are recorded of the artesian springs of Egypt’s Western Desert, from ancient times to the present, spanning the rise and fall of the great civilisations from the Pharoanic dynasties to Persian, Greek and Roman conquests. The study area includes oases Kharga, Dakhla, Bahriya, Farafra and Siwa, and several outer and small oases around Siwa and the edge of the Qattara Depression. The region is hyper-arid, receiving 10 mm or less average annual precipitation and evaporation rates are in the vicinity of 3,000 mm/a. Groundwater in the oases is largely derived from bores discharging from the Nubian Sandstone Aquifer. Based on an extensive survey, conducted for the first time, attention is drawn to the rapid demise of springs as a result of modern irrigation schemes which continue to deplete groundwater supplies.
Hydrochemical data from a total of 104 groundwater samples were used to investigate the main factors and mechanisms that control the chemistry of groundwaters in the Complexe Terminal (CT) aquifer of Chott region in southern Tunisia. Multivariate statistical techniques combining Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) were applied to the dataset of 12 physicochemical parameters (i.e. pH, T°, depth, Na+, K+, Ca2+, Mg2+, HCO3−, NO3−, Cl−, SO42−, and TDS). The HCA using Ward’s method and squared Euclidean distance classified the parameters into four clusters based on their dissimilarities. The application of PCA resulted in two factors explaining 64.25% variance. Geochemical methods combined with HCA and PCA confirm that groundwater chemistry is not controlled by the different lithological facies of the aquifer, but by the presence of evaporates randomly distributed in the basin.
Although stable isotope data (2H and 18O) of some groundwater are consistent with a slight modern recharge, CT groundwaters are mostly depleted bearing witness to the recharge of the aquifer system occurred under different climatic conditions than present. The stable isotopic composition of these paleowaters which all lie to the right of the local meteoric water line also indicates more pronounced evaporation of rainfall during the recharge process than during present-day conditions.
Some fundamental concepts on geosites and particularly on geomorphological assets (geomorphosites) are reviewed and a methodology
for their survey and assessment is presented and pointed out. Also, a case study in the province of Modena is presented.
Keywordsgeomorphosite-geomorphological asset-geosite-Modena-Italy
Groundwater pumped from the semi-confined Complexe Terminal (CT) aquifer is an important production factor in irrigated oases agriculture in southern Tunisia. A rise in the groundwater salinity has been observed as a consequence of increasing abstraction from the aquifer during the last few decades. All sources of contamination were investigated using hydrochemical data available from the 1980s. Water samples were taken from drains and observation wells tapping both the CT and the phreatic aquifers and analyzed with regard to chemistry, temperature, isotopes and other environmental tracers. Local salinization mechanisms are suggested, i.e. the upwelling of saline water from the underlying, confined Continental Intercalaire (CI) aquifer, as well as backflow of agricultural drainage water. At this stage, the main salt pan, the Chott el Djerid, is not a contamination source. A finite difference model was also developed to simulate groundwater flow and contaminant transport in the oases. Calibration for the period 1950-2000 was carried out in order to adjust geological and chemical system parameters. The simulation of planned extraction projects predicts a worsening of the present situation. Maintenance of the present abstraction regime will not reduce or stop the salinity increase.
The question of geomorphological heritage is truly put since the past twenty years and has led to international recognition of the concept of Geomorphosite. In the first part of the paper are discussed the concepts of natural heritage, geological heritage, geosite and geotope according to the Vade-mecum for the Inventaire National du Patrimoine Géologique (INPG). One wonders about the motives of the INPG in terms of objectives and conservation. Another difficulty is to define whether all parties share the same scale of values. The spatial dimensions of a geomorphosite have been recently discussed at length. However, it may be fruitful to compare the problem of spatial scales with the paradigm of landscape developed in the theory of Geosystem. In terms of time scale, it should be wherever possible to distinguish in every geomorphosite the deep time of Geology, the long period of Quaternary times, the distant past of Prehistory and postglacial environments, the near past of History and sub-actual environments, the current time of active geotopes where intervene the dynamic interaction of internal or external geodynamics, including anthropogenic factors. In terms of scale of values, the global value of a geomorphosite is defined by the sum of its scientific and additional values, which could be subdivided into cultural values and societal values. Taking better account of their specificities could help to include many geomorphosites to the INPG.
Salt balance methods are generally applied in the root-zone and at local scales but do not provide relevant information for salinity management at irrigation scheme scales, where there are methodological impediments. A simple salt balance model was developed at irrigation scheme and yearly time scales and applied in Fatnassa oasis (Nefzaoua, Tunisia). It accounts for input by irrigation, export by drainage and groundwater flow, and provides novel computation of the influence of biogeochemical processes and variations in the resident amount of salt for each chemical component in the soil and shallow groundwater. Impediments were overcome by limiting the depth of the system so that the resident amount of salt that remained was of the same order of magnitude as salt inputs and allowed indirect and reliable estimation of groundwater flow. Sensitivity analyses as partial derivatives of groundwater salinity were carried out according to non-reactive salt balance under steady-state assumption. These analyses enabled the magnitude of the salinization process to be foreseen as a function of hydrological changes linked to irrigation, drainage, groundwater flow and extension of the irrigated area. From a salt input of 39Mgha-1year-1 by irrigation, 21Mgha-1year-1 (54%) and 10Mgha-1year-1 (26%) were exported by groundwater flow and drainage, respectively. 7Mgha-1year-1 (18%) were removed from groundwater by geochemical processes, while a non-significant 2Mgha-1year-1 were estimated to have been stored in the soil and shallow groundwater where the residence time was only 2.7 years. The leaching efficiency of drainage was estimated at 0.77. With a water supply of 1360mm by irrigation and 90mm by rainfall, drainage, groundwater flow and actual evapotranspiration were 130, 230, and 1090mm, respectively. The current extension of date palm plantations and salinization of groundwater resources are expected to significantly increase the salinity hazard while the degradation of the drainage system is expected to be of lesser impact. The approach was successfully implemented in Fatnassa oasis and proved to be particularly relevant in small or medium irrigation schemes where groundwater fluxes are significant.
Geological resources are basic elements that have shaped both the course of human history and characteristics of human society. Protection of their imprint on our landscapes underlies the field of nature conservation known as geoconservation. Explicitly from 1991—when the term “geoheritage” was used at the First International Symposium on the Conservation of our Geological Heritage—a large body of literature has been produced with multiple conceptualizations. In response to challenges in finding a common definition of geoconservation, a systematic mapping of the scientific literature was undertaken to define key concepts supported by scientific evidence. The aim of this study was understanding the evolution of the research field between 2000 and 2019 to improve conceptual clarity. Relevant databases such as Google Scholar, Science Direct, and Scopus were searched, resulting in the identification of a total of 2478 studies. A method of systematic mapping with supervised keyword acquisition was used to assess geoheritage conservation documents. The analysis uncovered four recurrent concepts, each one describing a very different scope for geoheritage conservation: i) geoscience focus; ii) aligned conservation methods for geo- and biodiversity; iii) the concept of geomorphosites, as a leading resource for geoparks; iv) emphasis on community involvement for sustainability. For characterization of the concept groups, 70 findings were identified as primary articles following criteria implemented through direct reading. These criteria were derived from the identified keywords of each concept group. The information gained by the process allowed us to characterize and list all the major standpoints present in the scientific literature.
Examples of springs in Egypt deal only with examples of natural springs producing potable water. None of the natural springs producing highly mineralized thermal water for therapeutical are considered. No water from natural springs in Egypt is bottled. Egyptian standards state that the total dissolved solids in potable water should not exceed 1000 ppm, except in Siwa, where the only available source for water for human consumption is from springs that have water containing more than 2000 ppm TDS. Six natural springs in Egypt provide typical examples for the Sinai and the Western Desert: Ain Furtaga in the southern pre-Cambrian province of Sinai Peninsula; Ain El Gudeirat in the sedimentary plateau of North Sinai; and Ain El Bishmo, Ain El Bousa, and Ain El Gabal in the Western Desert Oases of Bahariya, Kharga, and Dakhla. They discharge from the Nubian Sandstone aquifer system. The sixth spring, Ain El Arayes, is a spring in Siwa Oasis.
Link: https://geomorphologie.revues.org/11499
The Western Desert of Egypt is one of the most arid areas of the world. Because of the extreme scarcity of the rainfall (less than 1 mm of annual mean rainfall in some places) and because of the amount of available sediment, aeolian processes are the major morphogenetic factors. The studies of the palaeoenvironments in this area (Wendorf et Schild, 1980; Sanlaville, 1997; Brookes, 2003; Bubenzer et Riemer, 2007) show that the climate has gradually become hyperarid between 5800 BP and 4500 BP. So, the ascendancy of aeolian processes isn't new. During the Holocene, the aeolian processes and landforms has acted as a resource (moisture of the deposits, arable lands) and as a stress (sanding and silting up, erosion) at the same time. The geomorphological and geoarchaeological survey of the Dush area, located in the South of Kharga basin has highlighted the existence of fine-grained sediment which are neither spring deposits (as described by Bousquet, 1996) nor playa deposits (as described by Embabi, 1972, 1999, 2004). The archaeological data produced since 1976 by the IFAO (Institut Français d'Archéologie Orientale) mission and the sedimentological analyses indicates that the sediment of these deposits had been windblown and trapped by vegetation (fields of the ancient oases). The cultivation of the fields lasted from 2500 BP to 1500 BP, allowing the height of the deposits to reach several meters. The drop of the water table and the uplift of the level of the fields forced the farmers to adapt themselves, up to the abandonment of most of the sites (around 1500 BP). After 1500 BP, the deposits have been eroded by wind, involving the formation of yardangs. The current landscape of the studied hills is the result of the succession of a period characterized by the deposition of windblown sediment linked to the cultivated fields and a period of aeolian erosion linked to the end of agricultural activities.
Many springs associated with the Great Artesian Basin have unusual geomorphological features and contain rare flora and endemic invertebrates and fish. Because of their ecological simplicity, they have considerable potential for evolutionary and ecological studies. Many springs have become extinct in the last hundred years and most of those remaining are under threat of degradation.
Due to the renewed interest for sites of geological interest, both in nature conservation and tourist sectors, numerous regional and national geosite inventories have been carried out in several countries during the last two decades. Several research groups discussed methodological issues, in particular concerning the assessment, and proposed various assessment methods. The University of Lausanne developed a method for assessing the scientific and additional values of geomorphosites in 2007. The method was widely used in regional inventories in Switzerland and abroad. This paper presents a new version of the method and discusses four main issues: (1) the assessment methodology is included in a larger process, from the selection of potential geomorphosites to their use and evaluation by stakeholders; (2) a particular attention is put on the selection of potential geomorphosites that will then be assessed and a simple approach crossing a spatial selection (the selected geomorphosites should be representative of the regional geomorphological processes) and a temporal one (the selected geomorphosites should cover the whole temporal stages of the regional morphogenesis, with both relict and active landforms) is proposed; (3) a new part is added in the assessment method; it concerns the use characteristics and fills a gap in the existing method; (4) finally, the paper discusses mapping and representation issues: specific maps are produced at two levels: simple geomorphological maps accompany the assessment of each geomorphosite and synthetic maps, covering the whole study area, are produced for the communication of results to stakeholders. For this, four types of representation (qualitative, univariate, bivariate, multivariate) are proposed. The method is illustrated by three examples of inventories carried out in Western Switzerland and Haute-Savoie (France). © 2015 The European Association for Conservation of the Geological Heritage
In attempting to understand the specific mode of water distribution in the presaharian oases during the Roman era, as from the disparate archeological data and the rare ancient documents, it has been necessary to investigate examples where the traditional system was valid up to a recent past.
The case of Tozeur where the sharing of water attributed to Ibn Chabbath may be considered as referential, exhibits a modulated breakdown of flow through a regular set of dams (khachba), followed by set shared periods of watering time along the terminal feeder-channels. Time-units are determined in diurnal sub-divisions or by the mean of various water-clocks (gadûs) formerly used from Ghadames to the moutain oases.
The graduated dam remains still observable in these localities, the mensurated time-units, testified to by Pliny the Elder at Tacapae (Gabès) and by the Lamasba inscription, added to the latin origin of the term gadûs itself, suggests that beneath the surface diversity particular to each case, the same fundamental practices were already customary in berber communities settled around perennial springs of the Roman frontier zone.
Society and Hydraulic System in a Tunisian Oasis.
This article is devoted to the study of the social and political organization of four villages whose separate communal territories form one continuous palm oasis. A single spring irrigates the oasis. The water supply is divided in halves, each half being shared by two villages belonging to the same çoff. Aspects of the hydraulic system as such are here considered to be indicative of a type of social organization. Insofar as there is no higher authority to control the use of water, the ways in which it is allotted directly reveal the nature of the social equilibrium and the relationships between the groups. A permanent bi-partition of the water flow has replaced the former system of rotation in time, and the author here considers the effects of this mutation on the oasis society. Within each community, the modalities (the diversity of which is emphasized) of measure, distribution and appropriation of the water supply may be considered as socially pertinent features. Thus the analysis of one village's water allotment system reveals the progressive decline of the lineage order. The article concludes with an analysis of power : the control of water is both its expression and its stake.
The North‐Western Sahara Aquifer System (NWSAS), one of the world's largest groundwater systems, shows an overall piezometric decline associated with increasing withdrawals. Estimating the recharge rate in such a semiarid system is challenging but crucial for sustainable water development. In this paper, the recharge of the NWSAS is estimated using a regional water budget based on GRACE terrestrial water storage monthly records, soil moisture from the GLDAS (a land data system that assimilates hydrological information), and groundwater pumping rates. A cumulated natural recharge rate of 1.40 ± 0.90 km3 yr−1is estimated for the two main aquifers. Our results suggest a renewal rate of about 40% which partly contradicts the premise that recharge in this area should be very low or even null. Aquifer depletion inferred from our analysis is consistent with observed piezometric head decline in the two main aquifers in the region. Annual recharge variations were also estimated and vary between 0 and 4.40 km3 yr−1for the period 2003–2010. These values correspond to a recharge between 0 and 6.75 mm yr−1 on the 650,000 km2of outcropping areas of the aquifers, which is consistent with the expected weak and sporadic recharge in this semiarid environment. These variations are also in line with annual rainfall variation with a lag time of about 1 year.
Salinization caused by irrigation mismanagement is a major cause of desertification. Monitoring of land degradation caused by salinization and other processes has been subject to international scientific debates leading to the commitment by global decision-makers to address these threats collectively through the UNCCD. This paper discusses the experience of monitoring land and water degradation in the salinity-prone irrigated arid environments of the Northern Sahara in light of current international scientific developments affecting both conceptual and methodological approaches.
The paper integrates a range of simple and accessible methods to achieve a multidisciplinary analysis including remote sensing, use of national research archives, interviews with decision-makers and direct surveys of cultivators. Revised assessments of the extent and ecological processes of salinization emerge from the analysis. As irrigated areas expand globally, the new conceptual and methodological techniques in dryland development science have the potential to enable scientists in affected areas to contribute to global efforts to monitor degradation effects caused by desiccation, salinity and human responses.
A geochemical study using hydrochemical techniques and multivariate statistical analysis were undertaken to characterize shallow waters in southwestern Tunisia.
Hydrogeochemical investigations demonstrated that groundwater can be classified into different water facies either for the oasis or the Saharan shallow aquifers. The Kebili oasis shallow aquifers showed a Ca–SO4 water type. The Douz and Djerid oasis shallow aquifers indicated a Na–Ca–Cl water type, whereas the Saharan shallow aquifers showed a Ca–HCO3–NO3 water type representative of the shallow waters in the piedmont of Dahar chain and a Na–Ca–SO4–Cl water type representative of Saharan shallow waters with a TDS >2.2 g/l.
Compared to 2.2 g/l TDS limit, Q-mode hierarchical cluster analysis grouped 52 groundwater samples in two main groups: (1) the relatively less saline water (Group 1) and (2) the highly mineralized water (Group 2). The application of factor analysis to the datasets resulted in two factors, explaining 61.89% and 15.65% of the total variance. Those factors indicated that groundwater quality depended mainly on the dissolution of some evaporite minerals in the oasis shallow aquifers and the Saharan shallow aquifers with TDS >2.2 g/l. In addition, nitrate contamination (natural or anthropogenic) appeared to be a secondary process controlling the mineralization of the shallow aquifers located in the piedmont of Dahar chain. This pollution is local and related to the infiltration of livestock wastes and/or nitrification under reduced aquifer conditions. These waters were also characterized by the dissolution of carbonates and the cation exchange. The suitability of waters indicates that waters situated in the piedmont of Dahar could be used for drinking after precautions on nitrate contents. These waters associated with waters of the south west of Douz region could be used for the irrigation of date palms.
The Chotts Trough of southern Tunisia is a sedimentary basin that lies immediately north of the Saharan platform. Within this basin there exist several structurally controlled depocentres, which were created by Miocene to Early Pleistocene compression associated with the Atlas orogeny. One of these depocentres, the Chott Djerid basin, has an average Quaternary subsidence rate of 0.01–0.27 mm yr1. Additional structural features within the Chotts Trough include east-west trending anticlines and synclines, at least some of which are controlled by east-west striking subsurface faults and northwest striking right-lateral strike-slip faults. Present movement along the northwest striking faults is a direct consequence of the Africa-Europe collision. The northwest striking faults and the east-west striking faults have cut the Chotts Trough into numerous smaller structural blocks, which experience different rates of subsidence. For the Quaternary, subsidence is caused by local extension associated with movement along the various strike-slip faults.
Chotts Djerid and Fedjadj lie in the arid zone of southern Tunisia, and are chemically active, artesian environments which give rise not only to extensive emergent groundwater gypsum and salt deposits, but also to landforms of more complex architecture. In some geological situations belts of spring mounds up to 25 or 30 m high straddle former high-level late Pleistocene lake shorelines. Spring mounds are circular, gypsiferous or calcareous, volcano-like structures fed by point sources of artesian water rising from aquifers in the Continental Intercalaire and Complexe Terminal aquifer series. Similar mounds have been identified around playas in the USA, Australia, Egypt, Iraq and elsewhere. In some mounds a central well is occupied by a small pond with water conduits in the base, whilst others are dry and degraded. In these latter, water is only present at depth and wind erosion and redeposition are moulding the landforms. The morphology of spring mounds is determined by the balance between aquifer pressure head, output, chemical content, net evaporation, aeolian activity, and plant colonization. Their stratigraphy comprises alternations of crystalline materials, wind-blown gypsiferous sand, wash and splash deposits, and organic layers arranged in overlapping sequences which dip outwards from the water source. Such deposits offer a potentially sensitive index of environmental changes in the late Quaternary and Holocene.
Dakhla Oasis (25.5°N, 29°E) occupies a structurally localized depression at 90–140 m above sea level, ∼1200 km2 in area, below a 300 m escarpment bordering the Libyan Plateau, Western Desert of Egypt. Semi-arid intervals of the Quaternary period generated eight sedimentary formations, separated by erosion during hyper-arid intervals. Sediments comprise three generations of colluvial/fluvial fanglomerates, two generations of tabular spring-laid clastic and chemical sediments, two generations of mound springs and basinal sediments. of fluvio-lacustrine, evaporitic, pluvio-aeolian and aeolian origins. Discussion of these sediments is organized according to geomorphic region, from north to south, plateau, scarp and piedmont, lowland and cuesta plain.Chronological evidence is restricted to many radiocarbon dates of Holocene cultural material associated with playa sediments (9−4.5 ka), a isochron age of ∼62.0 ±7.6 ka for basinal evaporites, and two ages of ∼176 and ∼170 ka for a boulder of derived travertine. The regional Quaternary sequence is reconstructed from stratigraphic and geomorphic relationships of the sediments and erosion surfaces. It is broadly similar to sequences earlier reconstructed in the topographically similar Kharga Oasis region 150 km to the east, and Kurkur Oasis, 400 km to the south-southeast. Speculations on chronology and driving mechanism are offered in conclusion.
Iron-rich sediments in Dakhleh Oasis, Western Desert of Egypt, have been recognized as spring mounds and as archaeological sites where Paleolithic materials have been recovered. Detailed sedimentologic investigation and mapping of these features reveal that spring mound sediments were deposited in a shallow vegetated wetland formed by the discharge of iron-rich Nubian Aquifer waters along the southern margin of the oasis, controlled largely by localized faulting and the variable paleotopography of the basin floor. The spring sediments represent peri-lacustrine or lake independent features and can be differentiated from fully lacustrine deposits on the basis of their sedimentary characteristics as well as the presence of goethite and jarosite in a region where authigenic deposition during Pleistocene pluvial activity principally resulted in tufas and lacustrine marls. Spring mound formation incorporated sediment through the ponding of surface water, aeolian entrapment by local vegetation, and the formation of iron precipitates in a low-energy, oxidative and acidic environment that was not dependent upon surface water inputs into the Dakhleh basin. The potentially interpluvial nature of this water resource makes it an important sedimentary archive for archaeological investigations in the basin. The Dakhleh spring mounds record a unique groundwater controlled paleoenvironment, providing the first evidence of a bog iron in Egypt and one of the few occurrences of iron-rich wetland remnants in the modern Sahara.
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