Anthropization of groundwater resources in the Mediterranean region: processes and challenges

A comprehensive overview is provided of processes and challenges related to Mediterranean groundwater resources and associated changes in recent decades. While most studies are focused thematically and/or geographically, this paper addresses different stages of groundwater exploitation in the region and their consequences. Examples emphasize the complex interactions between the physical and social dimensions of uses and evolution of groundwater. In natural conditions, Mediterranean groundwater resources represent a wide range of hydrogeological contexts, recharge conditions and rates of exploitation. They have been actively exploited for millennia but their pseudo-natural regimes have been considerably modified in the last 50 years, especially to satisfy agricultural demand (80% of total water consumption in North Africa), as well as for tourism and coastal cities. Climate variability affects groundwater dynamics but the various forms of anthropization are more important drivers of hydrological change, including changes in land use and vegetation, hydraulic works, and intense pumpings. These changes affect both the quantity and quality of groundwater at different scales, and modify the nature of hydrogeological processes, their location, timing, and intensity. The frequent cases of drastic overexploitation illustrate the fragility of Mediterranean groundwater resources and the limits of present forms of management. There is no easy way to maintain or recover sustainability, which is often threatened by short-term interests. To achieve this goal, a significant improvement in hydrogeological knowledge and closer collaboration between the various disciplines of water sciences are indispensable.     

Stochastic and deterministic multicloud parameterizations for tropical convection

Despite recent advances in supercomputing, current general circulation models poorly represent the variability associated with organized tropical convection. In a recent study, the authors have shown, in the context of a paradigm two baroclinic mode system, that a stochastic multicloud convective parameterization based on three cloud types (congestus, deep and stratiform) can be used to improve the variability and the dynamical structure of tropical convection. Here, the stochastic multicloud model is modified with a lag type stratiform closure and augmented with an explicit mechanism for congestus detrainment moistening. These modifications improve the representation of intermittent coherent structures such as synoptic and mesoscale convective systems. Moreover, the new stratiform-lag closure allows for increased robustness of the coherent features of the model with respect to the amount of stochastic noise and leading to a multi-scale organization of slowly moving waves envelopes in which short-lived and chaotic convective events persist. Congestus cloud decks dominate the suppressed-dry phase of the wave envelopes. The simulations with the new closure have a higher amount of stochastic noise and result in a Walker type circulation with realistic mean and coherent variability which surpasses results of previous deterministic and stochastic multicloud models in the same parameter regime. Further, deterministic mean field limit equations (DMFLE) for the stochastic multicloud model are considered. Aside from providing a link to the deterministic multicloud parameterization, the DMFLE allow a judicious way of determining the amount of deterministic and stochastic “chaos” in the system. It is shown that with the old stratiform heating closure, the stochastic process accounts for most of the chaotic behavior. The simulations with the new stratiform heating closure exhibit a mixture of stochastic and deterministic chaos. The highly chaotic dynamics in the simulations with congestus detrainment mechanism is due to the strongly nonlinear and numerically stiff deterministic dynamics. In the latter two cases, the DMFLE can be viewed as a “standalone” parameterization, which is capable of capturing some dynamical features of the stochastic parameterization. Furthermore, it is shown that, in spatially extended simulations, the stochastic multicloud model can capture qualitatively two local statistical features of the observations: long and short auto-correlation times of moisture and precipitation, respectively and the approximate power-law in the probability density of precipitation event size for large precipitation events. The latter feature is not reproduced in the column simulations. This fact underscores the importance of gravity waves and large scale moisture convergence.     

Model multi-cloud parameterizations for convectively coupled waves: Detailed nonlinear wave evolution

Recent observational analysis reveals the central role of three cloud types, congestus, stratiform, and deep-convective cumulus clouds, in the dynamics of large scale convectively coupled Kelvin waves, westward propagating 2-day waves, and the Madden–Julian oscillation. Recently, a systematic model convective parametrization highlighting the dynamic role of the three cloud types has been developed by the authors involving two baroclinic modes of vertical structure: a deep-convective heating mode and a second mode with low level heating and cooling corresponding, respectively, to congestus and stratiform clouds. The model includes a systematic moisture equation where the lower troposphere moisture increases through detrainment of shallow cumulus clouds, evaporation of stratiform rain, and moisture convergence and decreases through deep-convective precipitation and also a nonlinear switch which favors either deep or congestus convection depending on the relative dryness of the middle troposphere. The detailed nonlinear evolution of large scale convectively coupled waves in the model parametrization is studied here in a chaotic intermittent regime of the nonlinear dynamics associated with weaker mean radiative cooling where such waves are isolated in space and time. This regime is utilized to elucidate in a clean fashion several novel features of the model parametrization. In particular, four stages of nonlinear wave evolution occur: in the preconditioning and birth stages, the role of congestus moistening and second baroclinic convergence are crucial while in the dying stage of the large scale convectively coupled wave, the role of the nonlinear switch, and the drying of the troposphere are essential. In the mature phase, the large scale features of the convectively coupled waves resemble those in observations of convectively coupled Kelvin waves including the propagation speed, wave tilt, temperature, heating, and velocity structure.     

Possible impact of equatorially trapped waves on the tropical cyclone drift

The effect of equatorially trapped waves on the movement of tropical cyclones (TC) is studied numerically based on a two-dimensional barotropic model in a beta-plane approximation. According to recent studies, equatorially trapped waves contribute to the genesis of TCs. It is thus natural to assume that these waves affect also the movement of the TC. The effect of three types of equatorially trapped waves, namely Kelvin, Rossby, and n = 0 eastward inertio-Gravity (EIG) waves, on the TC trajectory is investigated with a focus on the sensitivity on some key physical parameters such as the wavenumber and wavespeed. Using a simple barotropic model forced by a prescribed baroclinic flow, the barotropic response to equatorially trapped waves is simulated for a period of 50 days, under various wave parameter configurations. This response is then used as a background flow where TC’s can evolve and propagate. TC-like flows are injected into this wavefield background at arbitrary times during the simulation, and the TC trajectories are tracked and recorded for 48h after the injection time. The resulting TC trajectory patterns with respect to the injection times and wave parameters appear to be stochastic and the mean paths and the associated standard deviations are calculated and reported here. The statistics are different for different wave types. Kelvin waves make shorter length of TC trajectories and small divergence of direction. On the contrary, Rossby waves cause rather dramatic changes in the TC path and yield longer trajectories. Meanwhile, TCs in EIG waves maintain fairly the same direction and typically have longer trajectories though less dramatic. A robustness test using a random forcing instead has also been conducted.

     

Timimoun’s foggara (Algeria): an heritage in danger

Timimoun is located in the hyper-arid zone of the Sahara, with an annual rainfall not exceeding 100 mm/year. To fill the shortage of rainfall, the oasis has developed a traditional means for mobilization of the groundwater. In the foggara with this hydraulic system, oases have been developed throughout the region of Timimoun over ten centuries. Today, we witness the gradual disappearance of foggaras; we lose about one to two foggaras per year for over half a century. The discharge of the 250 functional foggaras shows a significant drop: 850 l/s in 1960 and 355 l/s in 2001, enough to irrigate 350 ha. The causes of declining foggaras are technical, social, and environmental. The surveys that we have led nearly with the population in the ksour during two missions in 2007 and 2008 in the oasis of Timimoun reveal that socio-economic problems (Heritage and depopulation) are common to both types of foggara (foggara of the Erg and foggara Albian). Environmental problems (silting and flooding of the galleries by wild plants) are the main causes of the disappearance of foggaras of Erg. Technical problems (collapse of tunnels) are mainly the causes of degradation to Albian’s foggaras. Attempts to introduce changes to foggaras are temporary solutions. The modern collection of water (pump and drilling) is gradually replacing the traditional hydraulic system.     

Better constrained selection of the Paleozoic West Gondwana (South America) paleomagnetic poles for the APWP determination

The reliability of an Apparent Polar Wander Path (APWP) obviously depends on the paleomagnetic poles used to determine it. The APWP of Africa and South America are fairly well defined for the 330–260 Ma interval. However, this study pointed out a moderate shift between these two curves, and an incoherency of the South American data, contrary to the African ones, which are homogeneous. A number of South American pole positions were re-evaluated in an effort to better constrain the APWP for the entire continent. Most of discarded poles correspond to sites at the area of the junction of Cordillera with the stable craton. That could have structural implications for the evolution of the western margin of the Gondwana. A new criterion for the evaluation of paleomagnetic poles reliability for APWP is presented. Based on comparison of data from different continents and labeled “coherence” criterion, it is independent from Van der Voo’s ones.     

Geophysical approach for assessment of seawater intrusion in the coastal aquifer of Wadi Nador (Tipaza,Algeria)

The aquifer of Nador has suffered significant salinization due to seawater intrusion. It was strongly exploited during the 1980s and 1990s. A piezometric analysis in April 2012 showed the piezometric level to lie at 0 m a.s.l. over the plain; as a result, this aquifer is highly sensitive to the marine intrusion with an electrical conductivity of the groundwater in of exceeds 2500 μS/cm and so there are no abstractions for irrigation or drinking purpose from these sectors. The geoelectric study also showed the lateral variation in the electrical resistivity for two moments separated in time by more than 45 years. The fall in resistivity may be due to the encroachment of seawater into previously freshwater zones and/or infiltration during the era of pumped abstractions downstream. The resistivity surveys reveal two distinct sectors: the saturated aquifer in brackish and saltwater having resistivity values to 36-10 Ωm, which extends nearly 1600 m inland.     

Climate change impact on rainfall spatio-temporal variability (Macta watershed case,Algeria)

This work describes the climate change impact study on rainfall patterns in Macta watershed, located in the northwest of Algeria. The monthly rainfall data collection, verification and validation have built a database with 42 stations, each with 42 years of observations from 1970 to 2011. The study of annual total rainfall has identified a downward trend and quantifies the deficits that are within the observation time series. The division of the annual rainfall series into four periods allowed to highlighting the increase in inter-year temporal variability with the coefficient of variation increases from 17 to 27%. The study shows an annual rainfall deficit increment from 13 to 25%. The standard deviation values decrease significantly for the last two periods showing a spatial variability. Multivariate statistical study by the hierarchical clustering method resulted in the formation of station groups indicating the unification of monthly rainfall patterns.