Monitoring its land subsidence and its relation to groundwater harvesting Case study: Karaj Plain - Shahriar


Monitor and detection of displacement field due to changes in land surface are one of the practical and important studies in different topics of the Geological and geomorphical which have a significant role in trends and preventing natural disasters such as earthquake, subsidence and landslide. In the meantime, there are different methods for detection this displacement and geodetic measurement, that among them, Interferometric Synthetic Aperture Radar (InSAR) with a feature wide spatial coverage, fine spatial and time resolution and high accuracy has become one of the important and significant techniques. Spaceborne SAR Interferometry (InSAR) provides a unique tool for mapping the spatial and temporal evolution of subtle surface displacements and deformation over large areas . In this paper Stamps and Sbas methods are used.
 Stanford Method for Persistent Scatterers (StaMPS) is a new method for PS analysis that uses spatial correlation of interferogram phase to locate pixels with low-phase variance in all terrains, while no prior knowledge of temporal variations in the deformation rate for their identification is required .The Small Baseline Subset (SBAS) technique is a time series analysis approach, which uses interferograms with small baselines to minimize geometrical decorrelation at the expense of spatial resolution(Osmanoghloo ,2016 :93).Actually the SBAS analysis relies on a subset of interferogram pairs that were created with small temporal (over short time intervals) and geometrical baselines to limit decorrelation noise. A subset of 82  interferogram pairs were formed temporal and geometrical baselines less than 100 days and 1400 m. Radar images during 2003-2010 are processed by doris softwere. Based on this method the maximum land subsidence rate in KARAJ-SHAHRIAR Plain is 136 mm/year.
In recent years, rapid population growth coupled with agricultural expansion has tremendously increased pressure on the groundwater resources.Large-increases in water demand with little recharge, have strained the plain groundwater resources resulting in declines in water levels in last decades. Generally, alluvial basins of arid and semiarid zones are the places with excessive groundwater withdrawal(605 million cubic meters/years in karaj-shahriar), and also they have a high potential for land subsidence. Land subsidence due to groundwater extraction has been a common geohazards in many arid countries and districts of the world. In Iran, this is a serious challenge for many regions, particularly in the plains as like karaj - shahriar plain.
Given that the main cause of subsidence in this article extraction of underground water and drop in the water level can be assumed. So in this paper, water level changes in aquifer is analysed.The drop was confirmed through two models. GRACE mission data and wells  hydrogeology data processing show the The water level is decreasing. The changes during 2002-2012 through Grace data show anomalies between +5 (2002) to -15 (2012) cubic centimeters. This downward trend by processing piezometeric wells also prove to be. Chart trends in unit hydrograph show the 22 meters drop in level water during 12 years (2002 to 2014).
It is essential to note that karaj-shahriar aquifer consists of Quaternary sediments. These formations contain fin as silt. When the water gets out of the way, layers of clay and silt density and the   subsidence will happen gradually.There are some geomorphic effects on plain.