Assessing the impact of groundwater drop on the subsidence rate using the Sentinel-1 Radar images - Case study: Qorveh Plain

The land subsidence is the descending or collapse of the land under the influence of natural and human factors. The land subsidence is one of the issues that are being exacerbated by human factors, including excessive exploitation of groundwater. Subsidence can affect many constructions and facilities, causing problems for the industry, the environment, etc. This phenomenon is one of the most important environmental hazards that have been less considered than other natural phenomena due to the low human losses. The Qorveh plain is considered as one of the plains which have been introduced as a forbidden plain in the province of Kurdistan in recent years due to the over-exploitation of groundwater. Considering the amount of groundwater level drop and its direct impact on the subsidence level of the region, the present study evaluates the subsidence rate of the Qorveh plain during the period of 2017.12.19 to 2016.01.11. In this research, in order to evaluate the status of the groundwater drop, the statistical data from the Regional Water Organization of Kurdistan province has been used, and the Sentinel-1 images and the SBAS method were used (due to the unique capabilities of this method in terms of dimension, cost, time and accuracy compared to other remote sensing techniques) to estimate the subsidence rate of the region.

In this research, first, the status of the groundwater of the Qorveh plain and the drop rate of its level has been investigated. Then, the subsidence rate of the area and its relation with the groundwater drop has been investigated. Radar interferometry and SBAS were used to evaluate the subsidence of the study area. Radar interferometry method is one of the most powerful tools for monitoring the subsidence phenomenon. By comparing the phases of two radar images taken from a region at two different times, this method can determine the land surface changes at that time interval. The phase taken from a feature on the land surface is proportional to its distance to the radar sensor. Therefore, making any changes in this distance affects the measured phase. In this research, the Sentinel-1 images (2017.12.19 and 2016.01.11) have been used to perform the radar interferometry.

The hydrograph of the alluvial aquifer of the Qorveh plain has been provided for the water years of 1966-1676 to 2010-2011. During the 24 yeas, the groundwater level fluctuations in this plain are -13.29 meters, with an annual average of -0.55 meters. The least rate of dropping in the wells is in the wells located south of the Qorveh plain, and the rate increases toward the eastern and northeastern parts. In this research, the subsidence rate of the Qorveh Plain was estimated from 2017.12.19 to 2016.01.11 using the SBAS method. The final map indicates that during this period, the study area subsided between +61 and 216 cm, with the lowest subsidence occurring in the southern areas of the Qorveh plain, which corresponding to the sedimentary heights and slopes of Badr and Parishan and the rate has increased toward the east and west of the Qorveh plain.

The results of this study indicate that Qorveh Plain has witnessed a sharp drop in groundwater level over the recent years. Considering that the southern parts of the Qorveh plain corresponds to the heights and slopes of Badr and Parishan, and the rate of exploiting groundwater in these parts is lower, the rate of subsidence is less. The plain has also subsided further towards theeastern, western and northern parts and the outlet of the Shoor River, due to the growing increase of exploitation. The results indicate that the rate of subsidence is consistent with the rate of groundwater drop so that in the southern part which corresponds to the Badr and Parishan slopes, the rate was less than 10 millimeters during the period of 2017.12.19 to 2016.01.11. The results of the SBAS method indicate that the study area had subsidence of 216 mm during the 2 years and also a 61 mm uplift. Based on the final result, the highest rate of subsidence was related to the eastern and western parts of Qorveh plain and on the outskirts of the city of Dezaj and the villages of Ghasem-Abad, Shokuh-Abad, Avangan, Ganji, and others. A series of the aforementioned factors suggests that the Qorveh plain subsides about 20 centimeters per year. This is due to the over-exploitation of the groundwater. Unlike some areas where the displacement (subsidence and uplift) is due to the tectonic conditions, the results of this study have shown that in the Qorveh plain, the subsidence has a direct relationship to the drop of the groundwater. Therefore, it is necessary to monitor the use of groundwater, especially in the agricultural sector, and the rate of the exploitation should be proportional to the amount of recharge because in addition to the water shortage problems, the continuous use of the groundwater can lead to the irreversible risks of subsidence.