Ranking suitable sites for irrigation with reclaimed water in the Nabeul-Hammamet region (Tunisia) using GIS and AHP-multicriteria decision analysis
Highlights
► A new methodology is given to rank suitable sites for treated wastewater irrigation. ► Fuzzy-AHP based on GIS and easy-to-get spatial data, is the technique used. ► A Tunisian site is selected but the methodology is easily extendable worldwide. ► 31% of the total area is suitable and able to absorb all the wastewater produced. ► Best sites are located around the area already irrigated by treated wastewater.
Introduction
Tunisia, as well as several countries around the world, is facing the problem of water scarcity due to its arid and semi-arid climate, where the estimated available freshwater is only about 450 m3/citizen/year, which is relatively low compared to the international standards (1000 m3) (Louati et al., 2000). To tackle water penury and to fulfil growing population needs as well as agricultural, industrial and touristic sectors demand, Tunisia has made a remarkable investment in hydraulic resources mobilization. Thus, the available freshwater is mostly mobilized and is managed through various structures such as dams, channels, wells, hill dams and hill reservoirs and cisterns to harvest rainwater from houses roofs. In addition, water harvesting structures built on runoff watercourses and in the foothills areas (locally called jessours and tabias) are set up to meet olive trees and cereals water demand despite their location at Sahara limits (Ben Mechlia and Ouessar, 2004, Ben Mechlia et al., 2009).
Growing attention within the last 30 years has been paid to the treated wastewater (TWW) use, which has been considered as an important part of Tunisia's overall water resources balance (Bahri, 2002). Currently the Tunisian wastewater treatment office (ONAS) is running 106 urban wastewater treatment plants (WWTP) producing an annual total volume of 238 Mm3 (ONAS, 2009). This volume can irrigate up to 30,000 ha of cultivated lands, however only 9555 ha have been allocated (ONAS, 2009). Some irrigated districts failed in the start-up or in the long-term viability because of an inappropriate site selection.
In Tunisia, the irrigated land with TWW is commonly selected as the nearest agricultural area to the WWTP (CRDA, 2008). However, more criteria should be included to ensure a sound site selection where technical, economical, social and environmental characteristics interconnect. For instance, it is important to consider the negative impact of the practice on the environment (Ayni et al., 2011, Belaid et al., 2011, Drechsel et al., 2010) in addition to the farmers’ reticence, still considering treated wastewater as sewage (World Bank, 2010).
Considering the necessity of TWW use and its importance as a fast-growing practice (the allocated area is projected to reach 44,000 ha by 2021 (ONAS, 2009)), the establishment of a methodology to locate the best sites becomes a need. This methodology would be applied each time a WWTP's outflow is projected for use whatever the region. Thus, a better benefit of this non-conventional water is ensured along with the increasing availability of the national water and the reduction of the negative impact on environment.
On the other hand and according to a search made in scopus database, there is no scientific work published on site selection for TWW irrigation. However, site selection has been applied for many other applications such as aquifer recharge (Pedrero et al., 2011), solid waste disposal (Nouri et al., 2011), airport site selection (Ding et al., 2011), gas stations (Aslani and Alesheikh, 2011), water harvesting ponds (Al-Adamat et al., 2003), etc. Site selection techniques lead to the identification of the suitable sites and generate their suitability map through the integration of Multi-Criteria Decision Analysis (MCDA) into a Geographical Information System (GIS). MCDA combines technically feasible, economically viable, socially acceptable and environmental friendly criteria with respect to their importance to suitability. These criteria are then analysed using GIS and treated spatially to generate sites suitability maps.
Several MCDA techniques have been used in many fields for site selection and land allocation such as ELECTRE, PROMETHEE, AHP, TOPSIS, AIM, etc. (Behzadian et al., 2010, Conté et al., 2008, Gilliams et al., 2005, Reyhani-Khoram et al., 2007, Zhong-Wu et al., 2006). However, only few of them are integrated into GIS (Al-Adamat et al., 2010, Anane et al., 2008, Kallali et al., 2007, Marinoni, 2004), where Analytic Hierarchy Process (AHP) is the most applied (Anane et al., 2007, Tegou et al., 2010, Wang et al., 2009). AHP was established by Thomas Lorie Saaty in the 1970s (Saaty, 1980) and used to determine the priority for different decision alternatives via pair-wise comparisons with respect to common criteria.
The present study aims to establish an innovative methodology to map and rank suitable sites for irrigation with treated wastewater. It integrates a single-objective AHP method into a GIS model to select the most suitable sites for irrigation according to the available volume of TWW. The methodology uses easy-to-get data from Tunisian official institutions and available satellite images. The Nabeul-Hammamet aquifer watershed is selected as a study site to check the feasibility of the methodology.
Section snippets
Characterization of the study area
The study area corresponds to the Nabeul-Hammamet aquifer watershed. It is located at ‘Cap Bon’ peninsula at the North Eastern part of Tunisia and belongs to Nabeul District (Fig. 1). It covers 365 km2 of surface area, with a length of 34 km and a width ranging from 6 to 16 km. The climate is semi-arid with 400 mm as annual average precipitation at Nabeul city, and 19 °C as average temperature. The altitude varies between 0 m and 500 m. Geologically, the region is Pliocene and Quaternary, mainly
Suitable areas identification
The total suitable area for irrigation by TWW, obtained from the multiplication of the constraints, occupies 11,426 ha which corresponds to 31% of the total watershed area (Table 7). This constitutes quite a large area that can absorb all the amount of treated wastewater and contributes to increase the agricultural production of the region. The total suitable area is made by enclaves ranging from less than 1 ha to more than 1000 ha, sketched out mainly by the most restrictive constraints, namely
Conclusions and perspectives
In the present work, a single-objective AHP integrated with a GIS was carried out to identify and rank potential sites for irrigation with treated wastewater in the Nabeul-Hammamet aquifer watershed. Many criteria were selected, taking into account technical, environmental, social and economical aspects.
Constraints analysis shows that 31% of the Nabeul-Hammamet aquifer watershed is suitable for irrigation by TWW, which exceeds the required land for the reuse of the total SE3 and SE4 outflow.
Acknowledgements
We would like to thank the “Commissariat Regional de Development Agricole de Nabeul” and the “Groupement de Developpement Agricole” of Messaadi and Souhil for providing the needed data and information.
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