Mohamed Mostafa Mohamed, Saleh Qaran
Civil and Environmental Engineering Department
United Arab Emirates University
United Arab Emirates relied on groundwater as the main source of fresh water for several decades in the past. This resulted in the deterioration of the non-renewable groundwater aquifers; and thus, desalination plants have become the major source of fresh water supply in UAE. A major concern, therefore, is the vulnerability of these desalination plants to pollution and emergency conditions. In emergency conditions, the maximum amount of stored water in reservoirs and distribution systems will be enough for only 48 hours. Currently, production of these plants exceeds national water demand and the surplus is used to recharge groundwater in specific locations. While production of desalination plants is constant, demand is continuously increasing and soon will exceed production and then new plants will be needed. This would require investments of billions of Dirhams; not to mention the effect of these plants on the environment. In the meantime treated wastewater is normally unmanageably dumped into the desert or the sea. The main objective of this paper is to study the possibility to increase the national strategic groundwater storage through managed aquifer recharge of recycled water. In this study, we attempt to identify zones suitable for artificial-recharge in Abu Dhabi using (GIS). Data used includes hydraulic gradient and distribution map of source point pollutions.
In establishing an artificial recharge scheme, site selection is the prime prerequisite and its success depends on the collection and analysis of a great deal of geological and geomorphological data geographically (Ramezani Mehrian, , 2012). GIS, Geographic Information System has emerged as a useful tool for watershed characterization, conservation, planning and management in recent times. Geographic Information systems (GIS) can be employed to identify the regions of good potential for Artificial recharge in the region, some enterprises has been done in this regard utilizing GIS tools (Ramaswamy and Anbazhagan, 1997; Shankar and Mohan, 2005; Mahdavi, et al, 2013, Ramezani Mehrian, 2012). In this study, we are going to identify zones suitable for artificial-recharge in the eastern of Abu Dhabi Emirates using improved approach, which utilizes identification of the geologic, morphologic and climatic factors using Multi criteria of Fuzzy Overlay GIS technique and Analytical Hierarchy Process (AHP) in order to draw decisions towards optimum groundwater recovery system.
AHP is a technique of most common driver that scales the multi factors and stretches all its perspectives (trends) into one unique baseline for final comparisons. It converts individual preferences into ratio scale weights that can be combined into a linear additive weight w for each alternative (Ernest H. Forman and Saul I. Gass, 2001). The resultant w can be used to compare and rank the alternatives and, hence, assist the decision process for making a true choice where to be applied is most suitable for artificial recharge. The different units in each theme are assigned a knowledge-based hierarchy of ranking from 1 -- 5 (Table 1). These were assigned on the basis of their significance with reference to their influence on identification of groundwater artificial recharge sites. In this ranking 1 denotes poorly favourable zone, while 5 denotes to a very good zone.
Table 1. Ranks and their Influences on identifying artificial recharge sites
Results and Discussion
Various thematic maps of previous controlling factors are reclassified using GIS-AHP, and the weight influence and rank for each class is calculated and contoured in several maps. These maps show that groundwater level is scoring the highest because it is an indication that water level is harmed by declination due to over-exploited or low rainfall rates. The eastern part of Al Ain basin is showing low levels of GW where intensive water use is taking place, furthermore, is considered that morphological divide of the Basin where the subsurface discharge is starting towards the West and South-West in a common drainage direction. One thing, when looking to geology, we find that majority of the basin constitutes Alluvium deposits, accordingly our consideration in geology classes will be more sensible, but the other factors like slope that varies much between the East and the West will be more deciding for site selection process. GIS multi-criteria evaluation can combine a set of factors in order to resolve their differing perspectives into a single solution; this considered a resource allocation decision (Eastman, 2005) which is worthful approach in decision making process.
GIS-Fuzzy has been used for the integration of previous thematic factors to delineate the potential zones for artificial recharge. Each theme was assigned a weightage depending on its influence on groundwater recharge. Each class or unit in the map was assigned a knowledge based ranking from one to five depending on its significance in storage and transmittance of groundwater. The most common application for Overlay tools is suitability modelling, the factors were multiplied by the appropriate multiplier (Scores or Weights), and for each cell, the resulting values are added together. Weighted Overlay assumes that more favourable factors result in the higher values in the output raster, therefore identifying these locations as being the best. In the following is the resultant principal mapping for suitability to initializing ASR projects. The final map has been prepared showing different categories of land suitability for applications of artificial recharge.
This study presents improved approach of suitability mapping in Al Ain Basin in purpose for optimum methodology of site selection in applications of Artificial Storage and Recovery Systems (ASR). GIS has emerged as a useful tool for Representing, Weighting, Ranking and Combining multi-factors that are controlling the decisions of site selection.  Eastman, J.R., Van Fossen, M. E. and Solarzano, L.A., (2005) "Transition Potential Modeling for Land Cover Change", in GIS, Spatial Analysis and Modeling
 Mahdavi et al. (2011). Application of digital techniques to identify aquifer artificial recharge sites in GIS environment. International Journal of Digital Earth, 10.1080/17538947.2011.638937.
 Ramasamy, SM. and Anbazhagan, S. (1997) Criteria and Techniques of Detecting Site Specific Mechanisms for Artificial Recharge - A Case Study from Ayyar Basin, India. Jour. of Geol. Soci. of India. Vol. 50, No. (4). pp 449 - 456.
 Sukumar.S. and Sankar.K. (2010) Delineation of potential zones for artificial recharge using GIS in Theni district, Tamilnadu, India. International Journal of Geomatics and Geosciences. Volume 1, No 3.