Determining areas of potential subsidence in Ravansar – Sanjabi plain using GIS and AHP

Ravansar plain is situated in the northern part of Qarasu basin and counts as one of the major, fertile plains in Kermanshah province.In the recent decades, rapid development of agriculture combined with fast population growth and increasing demand for water have tremendously reduced the level of groundwater in the plain. Extraction from underground aquifers of the plain has annuallydecreased level of groundwater by 2/2-meteralong with a deficit of 90/48 cubic meter in the reservoir. To study areas prone to subsidence, I listed effective factors on the identification and information layers including water withdrawal, water level drop and land use, etc. Next, it was weighted by AHP method and final weight for each criterion in level one and two was calculatedby EXPER CHOICE software.The weights obtained were converted into raster maps using GIS and then the areas prone to subsidence were finally mapped out by overlapping and necessary calculations was developed areas prone to subsidence.According to the map of the central part (east and west), southwest and northwest, the plain with total area of 060/07 square kilometer has been detectedmore prone to subsidence phenomenon than other areas. Moreover, the decline in groundwater levels and water withdrawal in these areas are more intense than other areas. The maximum withdrawal has been more than 9.4 million cubic meters as the map indicated that water level drop has amounted up to -45 to -56 meters over the 21 years leading up to 1392. On the other hand, to study correlations between the type of sediments and the areas with potential for subsidence, a thickness map of fine-grained sediments (clay and silt that are most prone to subsidence) was used. Comparing the mentioned map with potential subsidence map showed a weak correlation betweenthe depth of fine-grained sediments of wells and the possibility of future subsidence such thatthe maximum depth of fine-grained sediments in areas prone to subsidence at high risk, is about 12-0 m, while in some parts of the aquifer that is less prone to subsidence, fine-grained sediments take much more depth of about 29-22 meters. In addition, the highest amount of withdrawal of groundwaterhas been done in the wells that ran the risk of maximum subsidence such as Janjan and Nahrabywells from which between 9/4-4 million cubic meters of water have been extracted. The study concludes with offering some recommendations for preventing subsidence.