Reconnaissance to characterisation of land subsidence due to groundwater overdraft and oil extraction in and around Mehsana City, Gujarat, India by long-term hybrid differential interferometric SAR technique

We present a framework for studying reconnaissance to characterisation of land subsidence in the slowly sub-siding city of Mehsana, Gujarat, India, due to groundwater overdraft and oil extraction. Considering interfero-metric synthetic aperture radar (InSAR) data availability constraints, we first conducted a long-term conventional Differential InSAR (DInSAR) investigation followed by time-series advanced DInSAR analyses by Persistent Scatterer InSAR (PSInSAR). We identified land subsidence in different parts of the study area, with subsidence rates ranging from <-1 cm/yr to >-3 cm/yr. During the entire observation period (2004-2019), the core urban area has been consistently subsiding with differential subsidence rates of-1.7 to-2.1 cm/yr during 2004-2006,-1.6 to-1.7 cm/yr during 2007-2011 and-2.6 to-2.9 cm/yr during 2016-2018 in conventional DInSAR and-2.5 to-3.2 cm/yr in advanced DInSAR. The subsidence measurements by collateral DInSAR observations in multifrequency and/or multi-imaging modes during 2016-2017 agree well in terms of the spatial extent of subsiding areas and the rates of subsidence. We characterised land subsidence in four type areas due to the diverse causes based on the temporal behaviour of land subsidence: (a) densely-populated city centre, (b) dairy plants and other industries in urban/peri-urban areas, (c) emerging urban clusters inperiurban/rural areas, and (d) oilfields with agri-horticulture farms in the rural areas. We found a consistently increasing land subsidence trend in 'a', reducing land subsidence in 'b', newly emerging and rapidly growing land subsidence in 'c', and spatially-extensive subtle land subsidence in 'd'. From the temporal nature of groundwater level (GWL) decline and land subsidence, we inferred that the land subsidence has been occurring primarily due to groundwater overdraft associated with urban growth, industry and agri-horticultural farms and secondarily due to oil extraction in the southeastern part of the study area. A comparison of annual rainfall difference from the long-term average vs GWL change vs land subsidence shows that the rainfall deficit during 2-3 consecutive years and groundwater overdraft are primarily related to land subsidence in the study area.