Groundwater recharge study in arid region: An approach using GIS techniques and numerical modeling
Introduction
Groundwater is available in some quantity almost everywhere. The depletion of groundwater levels is not a new story in Tunisia, but one that has continued for at least a decade, whilst annual water demand has increased for agricultural and industrial activities. Added to the depletion of groundwater resources, the arid climate aggravates the situation. Even though these resources are scarce, the hydrogeological system characterization and the artificial recharging of aquifers might help to alleviate the problem to some extent. The practice of artificial recharging is increasingly emerging as a powerful tool in water resources management (Ma and Spalding, 1997). By constructing suitable types of artificial recharge structures, groundwater resources can be augmented (Başağaoğlu and Mariňo, 1999). Many agencies have produced papers and reports on the depletion of groundwater levels in Tunisia and the need for artificial recharge of the groundwater aquifers (Besbes et al., 1978; Mamou and Kassah, 2000).
Surface and sub-surface hydrological features such as lithology, geological structure, drainage density, groundwater flow and boundary conditions of the aquifer system play an important role in groundwater replenishment. But using conventional methods, it is not an easy task to study the hydrogeological basin parameters to identify the suitable sites for artificial recharge, since many controlling parameters must be independently derived and integrated, which involves additional cost, time and manpower.
Groundwater recharge mapping depends on several variables and needs a computationally consistent manipulation of geo-referenced information at different spatial scales. A Geographic Information System (GIS) is a widely used software system for storing, managing, analyzing, and visual expressing of geographic information (Apel, 2006; Yoo and Kim, 2007). GIS’s powerful spatial analysis functions have made it a practical application in the field of hydrogeology, including analysis of stream network, geometry of the studied aquifer and hydrodynamic analysis based on the piezometric map.
Many assessments of groundwater conditions made with remote sensing techniques have been reported (Krishnamurthy and Srinivas, 1995; Bastiaansen et al., 1998). Geographic Information System techniques have many advantages over older and improved geo-referenced thematic map analysis and interpretations. In addition, unlike conventional methods, GIS methods are able to take into account the diversity of factors that control groundwater recharge. Thematic maps integrate various features derived from data in a GIS environment (Krishnamurthy et al., 1996; Saraf and Choudhury, 1998; Murthy, 2000; Baker et al., 2001; Henry et al., 2007). GIS utilities can be used to create a hydrogeological database (Krishnamurthy et al., 2000), while hydrogeological modeling is essentially used to estimate the aquifer state in the future under entropic conditions (Boronina et al., 2003). However, only a limited number of studies have taken the approach of specifically locating the potential artificial recharge zones, and as such there is no integrating of the hydrogeological modeling to estimate the efficiency of the proposed recharge.
GIS techniques and the numerical modeling create a unique opportunity to improve the groundwater management. The integration of such techniques can provide preliminary spatial distribution of the recharge zone. The numerical model can check the efficiency and the reliability of the artificial groundwater recharge.
The scope of the study was, therefore, limited to the following: (1) available data, supported by the field observations, was used to generate a hydrogeological GIS model, (2) thematic maps were followed for the conceptualization and the characterization of the aquifer system in order to generate a map showing the artificial recharge zones of the study area, and (3) the finite-difference code, MODFLOW (McDonald and Harbaugh, 1988; Romero and Silver, 2006), was used for numerical modeling. The simplified two-dimensional numerical model of the hydrogeological system was designed to test the efficiency of the refill practice. This model is useful as a computational management tool and for the development of data gathering strategies.
Section snippets
Methodology
Because the present study was aimed at the generation of a package to demonstrate the potential of GIS techniques for the selection of suitable areas for artificial recharge of groundwater and the capacity of numerical modeling for testing the efficiency of the groundwater replenishment, it was decided to use a study area in an arid climate having problems with groundwater availability, thus requiring a suitable plan of action for groundwater development. The process has been designed to make
Aquifer properties
The hydrogeology of the study area is essentially controlled by the geological setting, the morphogenesis of the terrain, the distribution of rainfall and the movement of groundwater through the interconnected formations of the aquifer system. The field geological observations allied to the boring cuts surveys reveal that the district can be subdivided into three hydrogeological units, namely a porous and two fissured formations. The porous formation consists of mio-plio-quaternary sands and
Conclusion
A major issue in the development of groundwater resource management is that of effectively storing and manipulating the vast array of data that may be available in various formats. In order to demarcate the artificial recharge zones, a methodology coupling a GIS and groundwater modeling was used. By integrating the different thematic layers using a GIS-based model developed specifically for this purpose, an artificial recharge zones map of the study area was established.
Groundwater flow
Acknowledgement
The authors thank Zammouri M. (FSB, Tunisia) for essential contributions and suggestions during the numerical modeling.
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