Document Type : Research Paper
Authors
1 PhD student of geomorphology of Hakim Sabzevari University
2 Assistant professor Hakim Sabzevari University
3 Professor Hakim Sabzevari University
Abstract
Introduction
Today, the phenomenon of land subsidence is one of the most important geomorphological hazards on a global scale, which causes great damage to urban and rural structures, development facilities. Most of the reports from around the world related to the phenomenon of land subsidence have been related to arid and low rainfall areas. According to the US Geological Survey, a subsidence phenomenon involves a collapse or subsidence that can have a small displacement vector. Land subsidence is a geological phenomenon that causes the earth's surface to descend slowly and horizontally.
In recent decades, most joints and fissures in agricultural areas have been reported due to over-abstraction of groundwater. Recently, urban areas have also been affected by this phenomenon. These seams and cracks cause a lot of annual damage to ground-level installations such as roads and bridges, buildings, power transmission lines, oil and gas transmission pipes, water pipes and sewage systems, wells wall pipes of exploitation wells. In addition, they provide a path for surface pollutants to move to groundwater sources and contaminate groundwater by infiltrating aquifers.
Among the innovations of this study, for the first time, radar interferometry technique was used to investigate the risk of subsidence of Nurabad plain aquifer subsidence. , Identification of vulnerable areas in the aquifer area.
Methodology
The method used in this research is applied analysis. Groundwater data including piezometer and exploitation wells were used to investigate groundwater level changes and Sentinel 1 images were used to calculate radar interferometry. Geographic weighted regression model (GWR) was used to investigate the relationship between subsidence and groundwater loss parameters.
By examining the changes in groundwater level of piezometer wells in the aquifer during a period of 17 years (1380-1397) showed that an average of 15 meters of groundwater drop has occurred in the plain. This decrease in agricultural and residential areas is a priority compared to other uses. By preparing the output of radar images during 4 statistical years, the results show that in 2015 the maximum amount of leakage was 7 cm and in 2016 it increased to 8 cm. In 2017 it is equal to 9 cm and in 2018 it is equal to 10 cm. In the end, by calculating the average of these four years, the amount of subsidence at the aquifer level increases significantly, with the regions in the central and eastern parts of the aquifer having the highest subsidence at the aquifer level. Which has been developed in residential and agricultural areas of the region so that the effects and evidence of these meetings are visible in residential houses and agricultural lands according to Table (2) in the analysis section to examine the status of subsidence rates in the study area using Radar images were taken during the mentioned years and show that in 2015 it is equal to 8 square kilometers, which is compared to 2014, in 2016 it is equal to 34 square kilometers, and in 2017 and 2018 it is equal to 40 and 86 square kilometers. According to the maps, we see in these 4 consecutive years that every year b The amount of subsidence at the plain level has increased and thus subsidence at the plain level has become a kind of hazard.
Results and discussion
High values of R2 have occurred in the central parts of the model aquifer, which shows a good estimate of the model in estimating the dependent variable and the predictive explanatory variable. Also, relatively lower values are located in the southern parts of the aquifer by examining the subsidence status in the plain. By creating a relationship with the groundwater drop layer, it showed that the observed values with the predicted values indicate a strong relationship. The layer shows the coefficient of determination R2, which is the highest coefficient of R2 in the central part, which is about -0.79 Is 0.53 Figure (7). The distribution of the remaining space of the GWR model errors shows that the model outputs are closer to the actual values. Second, the resulting map shows the low error values in the range.
Occlusion
The average subsidence rate is about 4 cm per year and the average 4-year average is 16 cm. The average area of the aquifer where the subsidence has taken place during 4 years is about 36 square kilometers, which includes about 5% of its area. The high concentration of wells and the greatest drop in groundwater and the concentration of existing cities and villages. In these areas, due to the water needs of the residential community, including the city of Nurabad and surrounding areas, as well as the water needs of the industrial sector, including factories and agricultural activities, access to water resources is more and groundwater extraction is very high. There has been a high decline that these factors with the intervention of the arid climate of this region has caused the high talent of these areas in the discussion of subsidence and has been identified as one of the critical areas of the city in the subsidence map. The results of radar interferometry show that uncontrolled extraction from the surface of underground reserves has caused the subsidence of the central part of the aquifer and also the presence of several aqueducts at the aquifer has caused damage to residential centers. One of the areas with the highest risk of subsidence in the plain, in the eastern, central and southern parts, the rate of groundwater loss in the aquifer within 14 statistical years is about 22 meters in the same well and the results of GWR method show local R2 in the central parts of the aquifer which is about 79% maximum and shows a high correlation between subsidence and groundwater loss Residual rate in the central part which is equal to 0.007 / 0007 which is a high coefficient of significance.
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