November 29, 2017, by Tim Radford
Granada’s Alhambra – at risk? Image: By Fjvm90, via Wikimedia Commons
When the heat is on, ground shrinkage means subsiding soils. Sustained drought could cause structural problems in parts of Spain.
LONDON, 29 November, 2017 – Some citizens of the Granada region of south-eastern Spain have that sinking feeling, thanks to their subsiding soils. They are going down in the world.
Researchers who have checked data from three satellites confirm that the land on which they live is subsiding at the rate of a centimetre a year in times of drought.
At the same time, the scientists have monitored extraction of groundwater from the aquifer beneath the Granada basin – the Vega de Granada – to find that water uptake could be matched with the slow collapse of the clays and soils in parts of the river basin.
And the finding has been confirmed by study of 50 years of ground explorations and the presence of cracks in the streets and pavements in at least one municipality. The region suffered sustained drought from 2003 to 2009. During the rainiest period between 2011 and 2014, the evidence of subsidence vanished.
“These data are of enormous interest for an adequate management of the aquifer, especially during periods of drought, such as the one we are currently experiencing”
The study, led by José Miguel Azañón of the University of Granada and Rosa Maria Mateos from the Spanish Geological Survey, published in the Journal of Hydrology, is based on data that the authors say “are not alarming, and do not imply any risk at the moment.
“These data are of enormous interest for an adequate management of the aquifer, especially during periods of drought, such as the one we are currently experiencing.”
The authors present the study as a test of Europe’s earth observation satellite Sentinel-1 and its ability to deliver radar measurements to accuracies of millimetres.
But there is also an implication that sustained drought could one day deliver hazard.
That is because heat waves and drought could become the new normal for southern Spain in a world in which climate change follows global warming driven by profligate consumption of fossil fuels.
Researchers have warned that parts of Europe could by 2020 experience twice the number of extremes of heat, and that by 2040 these could have quadrupled. Longer-term studies suggest that temperatures could become dangerously high for southern Europe, in effect bringing the Sahara across the Mediterranean.
And there have already been concerns about the impact of heat and drought upon agriculture in the Iberian peninsula, including the cork oak forests of the region.
So the response of local water-bearing rocks and clays – hydrologists call this the phreatic level – to extremes of flood or drought is a matter of intense practical interest to local authorities, town planners, home buyers, builders and the insurance industry.
If a whole village sinks at a uniform rate, nobody will notice. If subsidence happens in some streets or zones but not others, cracks begin to appear.
“In very vulnerable areas, with high content of clays, decreases of just a couple of metres in the phreatic level could cause subsidence. An accumulation of displacements could lead to a subsidence of several centimetres per decade, which would present a long-term danger for the villages located in the Vega de Granada area,” the authors say. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.