Investigation of land subsidence risk areas due to groundwater level drop using FUZZY-AHP method (Case study: Shabestar-Soufian plain)

Groundwater depletion is one of the scenarios of climate change, which has become a global crisis today. Land use changes and land cover and rapid urbanization are the main causes. The decline of groundwater in recent years has caused land subsidence in many alluvial plains of the country. Uncontrolled and uncontrolled consumption of surface and groundwater resources, reduction of precipitation, concentration of consumption in some places (Imbalance between demand and water supply potential), inadequate cultivation pattern and lack of proper irrigation and digging of multiple wells and operation Irregularity of them in recent decades has caused a critical situation of groundwater resources in most plains of the country. To Groundwater level in most of the country's aquifers is constantly declining and the average annual decline over It has been 12 meters for the last 15 years. Decreasing the groundwater level of the plains increases the cost of water extraction and Increased energy consumption, reduced water quality and the emergence of land subsidence. As side effects, with decreasing volume The phenomenon of subsidence occurs in different spatial and temporal situations with different characteristics such as groundwater content, morphology, pedology and geology, which have different destructive effects. Shabestar-Sufian plain is one of the plains in the northwest of the country, which is facing many restrictions in terms of access to surface water and groundwater and is in a critical situation. In recent decades, the increasing trend of irrigated agricultural lands and overexploitation of groundwater resources has put the plain at risk of subsidence. Groundwater level drop is one of the most common causes of subsidence. Recognizing and examining ways to control it and prevent accelerating the process and expansion of subsidence phenomenon prevents the occurrence of hazards such as groundwater pollution, land slope change, well wall collapse, gradual sinking of masts and structures, changing the slope of rivers and roads Be. Risks that cause significant financial and human losses and on the other hand cause a waste of time and manpower, and pollution and destruction of the environment. In Shabestar-Sufian plain, there may be signs of subsidence in the form of sloping pipes, piping, shallow cavities, which is a warning for this fertile plain. If proper planning is not done regarding the exploitation of groundwater, we will soon see the destruction of lands, houses, water pollution, etc. In this regard,. The aim of this study was to identify areas prone to subsidence, for which the Fuzzy-AHP model was used. Information layers including land slope, altitude, geomorphological units, distance from the river, land use, lithology, groundwater level decline have been used as effective criteria. Expert Choice software was used to perform the calculations and after obtaining the final weights of each criterion, the obtained weight was applied on the layers according to the expert opinions in GIS software. Also, for the fuzzy model, fuzzy maps were combined using the fuzzy gamma operator. To modulate the very high sensitivity of the multiplication fuzzy operator and also the very low sensitivity of the sum fuzzy, the gamma fuzzy operator has been used and finally a map of subsidence prone areas has been prepared. Landslide risk areas are classified into five categories: very low (10.8%), low (14.2%), medium (7.22%), high (7.28%) and very high (2.23%). have became. About 52% of the southern, middle and western regions of the region are at very high risk of landslides due to excessive use of groundwater. In terms of spatial distribution, the highest risk of land subsidence is in the villages of Aq Kahriz, Dizaj Khalil, Shanglabad, Bagherabad, Kafi Malek, Kushk, Hallaji, Yousefabad, Ali Biglou, Haftcheshmeh and Qom Tappeh. Complete and common data were used between 2001 and 2017. Water loss map was prepared by idw interpolation method in gis environment. The results of groundwater status assessments in the region indicate that the study area in recent years has been faced with an increasing trend of groundwater level decline, so that the rate of water level decline has reached 4.7 meters in 18 years, a sharp drop in land subsidence in this region Has caused. . The calculated results are confirmed by examining the time series of water level drop in the wells of the region. Analysis of geomorphological parameters of the region has also shown that the same occurrence of subsidence has been in areas with a slope between zero to 5 valleys, have experienced less subsidence in the slope of more than 5 degrees or no subsidence has occurred in these slopes at all. It has a high subsidence in places with a height of less than 1500 meters, and with salt cover and alluvial materials, as well as areas with agricultural use. Areas close to the river were less prone to subsidence. Flat and smooth slope directions have been the main area of occurrence of this phenomenon. It can be concluded from the results that more than half of the 5% (51%) of the study are prone to high to very high subsidence, this means the critical situation in this plain.