Iran’s pistachio growers get that sinking feeling
Thirsty crops are draining groundwater faster than it is being replenished – with disastrous results. Kate Ravilious reports.
Iran’s insatiable demand for water, which is being drawn from aquifers far faster than it can be replenished, is causing large chunks of farmland to sink and buildings to crack, according to a new study.
Using 12 years of satellite observations, a team led by Mahdi Motagh from the GFZ German Research Centre for Geosciences in Potsdam has revealed widespread sinking across 1,000 square kilometres. The damage is irreversible – and the loss in aquifer volume is permanent.
The work, published in Engineering Geology, sounds a warning for other areas reliant on aquifer water, including parts of Australia.
The Rafsanjan plain in southern Iran is a major pistachio growing region, with more than 90% of its agricultural land devoted to the profitable export crop. Its climate is exceedingly dry, so farmers rely heavily on groundwater to irrigate. In recent decades water extraction has significantly outstripped recharge rates from rainfall and surface run-off, so the groundwater table has lowered by some 17 metres.
To see how the land surface has been affected by this depletion, Motagh’s team used radar imagery from European and Japanese satellites.
Analysing data from 2004 to 2016, the scientists identified a 1,000 square kilometre region in which the ground had sunk an average of five centimetres per year – with some spots dropping more than 30 centimetres per year.
Field investigations revealed that the areas with the greatest rates of subsidence were those with the deepest wells.
“In one region I met people developing new methods to extract water from 300 metres’ depth. It is not a good situation,” Motagh says.
Much of the damage has occurred in recent times as farming expanded – the area devoted to agriculture has increased by 50% during the past 30 years. But the future looks bleak for the region's farmers.
The researchers estimate that the aquifer has been losing 300 million cubic metres of storage capacity every year for the past decade.
“We show that this subsidence process is inelastic, meaning it is not recoverable and the damage to the aquifer below is irreversible,” Motagh explains. Rock that was once like a porous sponge has been compressed, leaving few spaces in which water can collect.
And loss of groundwater is not the only problem for Rafsanjan. Today, the landscape is riven by a network of huge fissures, up to four metres deep and three kilometres long, the result of the subsidence. This process of collapse is playing havoc with buildings and infrastructure such as roads and bridges.
This sinking is by no means limited to Iran. In 2015, geoscientists reported deformations in the ground overlying south-east Australia’s Gippsland Basin. The three worst areas, which sunk around eight centimetres per year, were near mine sites.
Across the world, excessive groundwater extraction is an increasing problem. Bangkok, Beijing, Mexico City, the Antelope Valley in California, and the Las Vegas Valley in Nevada are among the regions that have been affected similarly in the past, and today must import water.