Selection of the best capable areas for construction of underground dam using Multi-criteria Decision Making (Case study: Marghmalek-Shahrekord Watershed)

Most part of Iran including Chaharmahal and Bakhtiari province is arid and semi-arid; thus in most parts of the region, groundwater is the only source of water. Thus, in the most parts of the region, groundwater is the only water resource, and is a major constraint on economic and social development. In arid regions, the underground dam is a new industrial site for supplying water for farming, especially in summer. An underground dam is a sub-surface barrier across the stream which retards the natural ground water flow of the system and stores water below ground surface to meet the demands during the period of need. The aim of this study was to prioritize the effective factors in the construction of underground dams and to develop and evaluate a multi-criteria decision making system based on GIS techniques in order to identify and prioritize potential areas for building underground dams in the watershed of Shahrekord. Marghmalek-Shahrekord watershed is located in north east of Chaharmahal and Bakhtiari province. This watershed is located in Sanandaj-Sirjan zone based on structural divisions and the atmospheric rainfall in the basin is mainly influenced by low-pressure Mediterranean systems. In order to accelerate the decision-making process, as well as avoiding the accumulation of excessive information for a surveyed problem, it is first necessary to eliminate inappropriate points by considering a number of key criteria and factors. At the first, suitable areas for establishing underground dams were determined with ommisive criteria and using Boolean logic and suitable areas for establishing underground dams were determined. The axes that do not cut the Qanat's gallery and are suitable from the aspect of length and area of the reservoir were identified. In the next stage, the proposed locations were prioritized using the analytic hierarchy process. Therefore, the indices that are more fit, matches the more suitable axes for the construction of underground dams in the study area. Five scenarios were considered for the prioritization. Quaternary formations are suitable for the construction of underground dam. Poor, medium and good rangelands and garden lands are suitable for underground dam. According to the first, second, third and fourth scenarios, axis number 46 and according to the five scenarios, axis number 41 have the first priority for groundwater dam construction. The results showed that the hydrologic factors and the socioeconomic factors were chosen as the most important factors with weights of 0.51 and 0.30, respectively. In determining the value of the indicators, the water quantity (runoff height) with a weight of 0.833 and also the depth of alluvium with a weight of 0.743 are more important than other indicators that this indicates the importance of the hydrologic criterion for experts. The best axes for the construction of an underground dam are in the alluvial river bed with high subsurface flow, the large reservoir capacity, high runoff volume and areas with high permeability and low slope. By integrating the maps of the Qanats axis of the study area and the map of possible gaps of underground dam construction, 15 axes were identified for the construction of underground dam in Marghmalek-Shahrekord watershed. The results of the reservoir analysis indicated that suitable subsurface reservoirs in these areas are matched with this fact that with decreasing slope, the length of the reservoir increase, that ultimately causes increases the volume of the reservoir. The large area of the watershed upstream of the dam has a higher priority and value because the high amount of runoff passes through the construction site. The depth of the alluvium has the most relative importance among the axis indices, because in the economy of an underground dam, length and depth of axis are most important. By investigating the results of the reservoir depth, it was found that the best region are alluvial bed with a depth of less than 10 meters, that is consistent with the results of Chezgi (2015). The water of these tables is very good in terms of quality because they are naturally refined by constituent lands. Also, they have a high subsurface discharge. These valleys are the best and most suitable place for underground water extraction. According to the obtained results about the slope, the relative importance of the slope increases with its lowering. The results of this study showed that most of waterways 3 and 4 were located at slopes less than 5 percent, which are suitable areas for building underground dam because the gradient factor has inverse relationship with reservoir volume and permeability and is consistent with results of Nilsson (1988); Rezaei et al. (2013) and Chezgi (2015). The proposed method can accurately be used to select the suitable sites for construction of underground dams through detailed and accurate surveys and by considering the effective factors.