Hyderabad: Land subsidence, or the sinking of land, is an emerging concern for coastal areas amid rising sea levels and climate change. In Machilipatnam, Andhra Pradesh, located six kilometers from the east coast and 40 kilometers from the Krishna River, land has been sinking at a concerning rate of about 10 mm per year since 2017, according to a study by the National Geophysical Research Institute (NGRI).
Unlike most cases, where subsidence is attributed to groundwater extraction, the NGRI study identifies increased groundwater levels as the primary cause in Machilipatnam. Published in the Current Science journal, the study notes that this is largely an agricultural and urban region, free from hydrocarbon extraction and significant geological changes. Data from local wells show groundwater levels are either stable or rising (up to 17 mm per year) over the past two decades.
Rainfall patterns over the past 40 years, particularly in the last six years, indicate increased precipitation, which may explain the land sinking. Seasonal rainfall typically causes temporary subsidence of 20-30 mm during monsoons, followed by recovery. However, sustained rainfall increases over several years can result in cumulative land sinking. GPS and satellite data confirm a correlation between recent rainfall and subsidence in Machilipatnam.
Dr Vineet K. Gahalaut, chief scientist at NGRI and a lead researcher of the study, explained that subsidence can result from hydrological processes, soil properties and tectonic forces. "In southern India, where seismic activity is rare, we must consider other factors," he said. "In regions like northwest India, groundwater extraction has sometimes caused land uplift rather than subsidence, due to minimal compaction."
In Machilipatnam, subsidence persists despite stable or rising groundwater levels, challenging the assumption that excessive extraction is the cause. Concerns about sea-level rise influencing subsidence are not supported by observations from the past five years in the region. Instead, increased rainfall and its impact on groundwater infiltration appear to be the key drivers of subsidence. The findings underscore the complexity of land subsidence, cautioning against oversimplified links to groundwater withdrawal or climate change.
