Investigation of Kabudrahang Plain Subsidence Based on Interferometric Synthetic Aperture Radar Technique and Groundwater level Changes

Iran is located in an arid and semi-arid climate with an average rainfall of about 239.3 mm during five years ago. There are many challenges for decision-makers in terms of supplying water for agriculture, industry, and drinking. On the other hand, lack of comprehensive decision-makers, along with lack of awareness of beneficiaries of the consequences of excessive use of groundwater resources, has led to land subsidence in most plains of the country. This issue, due to the nature of land cognition and its relation to the use of groundwater, has been considered by researchers in the fields of water resources management and geology. Land subsidence of Iran has been studied worldwide and the effects of groundwater abstraction have been determined at a high level compared to other factors. Hamedan province in the western part of Iran is not far from overuse of water and the effects of land subsidence, due to the extensive agricultural lands and agricultural hubs. In this regard, studying Kaboudrahang aquifer subsidence with its sinkholes (18 massive sinkholes) was selected as the study site in the northwest of Hamadan province. The hydrograph of the plain aquifer unit shows a profound drop in the groundwater level of the aquifer (40 meters) between the years 1988 and 2018.

Regarding plain subsidence, some researches have been done. Accordingly, the radar interference technique in ENVI software was used to determine the amount of land subsidence and finally its output expressed in GIS software. To use this method, 11 Sentinel satellite images from 2014 to 2019 were selected and groundwater changes in 49 observation wells were studied. To investigate land subsidence and groundwater changes, the relation between land subsidence rate and groundwater level drop was examined through a regression analysis, as well as several observation wells that have a reduction in the date of the last satellite image relative to the water level. A regression analysis was selected between the water level drop and land subsidence rate and between the water level drop and the cumulative land subsidence obtained by radar interferometry during the period. To investigate in more detail, the effect of groundwater abstraction in each observation well and the relationship between land subsidence and its changes over time have also been investigated. On the other hand, land subsidence is delayed according to some research on groundwater exploitation, which varies due to the different hydraulic conductivity of the aquifer along with the different geological structures of the plain. Lag regression was used as a simple tool to determine the lag time between groundwater exploitation and land subsidence.

The study results showed that during the years 2014 to 2019, the land subsidence of the plain was 124.7 mm and an average rate was 13 mm every six months. Positive and negative values were observed in the land subsidence rate map in which positive values indicate upward radar visibility and negative values are related to radar visibility or subsidence. In general, across the plain, there is no equal correlation between subsidence rate and groundwater level decline. Its main reason can be found in the difference of geological structures and diverse soil texture throughout the plain, which has led to different responses to groundwater exploitation. Wells that had a drop in water level between the first and last satellite image show a decreasing trend of land subsidence rate and cumulative land subsidence. This indicates a decrease in the power of land subsidence in the utilization of water over time. The reason is that groundwater passed through subsidence-sensitive layers, after which the effect on land subsidence is reduced. The lag time of subsidence and operation of groundwater was determined as an average of 1.5 years.

Based on the analysis of this research, more significant impact of land subsidence on agricultural lands can be identified due to the use of groundwater. On the other hand, there is need for proposals and solutions to reduce the rate of land subsidence, and so it is recommended to change the cultivation pattern, to control surface water with artificial recharge, and to create deep plowing in agricultural areas to increase permeability.