“An annual 31 centimeters of land subsidence is reported in Tehran Province. Over three billion cubic meters of water is extracted from groundwater sources in the capital every year,” ILNA quoted Hassan Razavi as saying.
Land subsidence is defined by scientific references as a gradual settling or sudden sinking of the earth's surface owing to subsurface movement of earth materials.
Principal causes are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, gas and oil extraction as well as earthquake, Financial Tribune reported.
In more humid areas underlain by soluble rocks such as limestone, gypsum or salt, land subsidence is an often overlooked environmental consequence of land - and water-use practices.
Groundwater Use
However, the main reason of land subsidence in Iran is injudicious groundwater extraction that has been causing serious concern among conservationists, environmental/economic experts and academia.
The disturbing trend has gone way beyond the global norms of annually 4mm of land settling. This is while “more than 200mm of land subsidence per year is considered critical,” Razavi said,
Although reversing land subsidence is almost impossible, slowing its progress is doable provided the government and public cooperate.
Water consumption and reforming farming systems are the key to addressing the worsening problems.
Surface water streams can be controlled through dam construction so that it can help vegetation in drier lands and replenish groundwater reserves.
Besides land subsidence, groundwater overdraft may lead to destruction of vegetation, increasing the possibility of dust storms, holes in the plains and higher salt content in groundwater.
Curbing the spread of land subsidence is of primary importance. When the ground surface moves lower, the entire city sinks with it, which in turn affects the stability of buildings and the functionality of infrastructure.
Tehran Dams
“Last water year (September 2018-2019), the share of groundwater for drinking purposes in Tehran Province was 100 million cubic meters less than before,” Razavi was quoted as saying by ILNA.
The less use of groundwater was due to unprecedented rainfall in March and April that significantly raised water levels in dams.
However, heavy rainfall brought along its own set of problems – lost lives, overflowing rivers, washed away bridges, inundated homes and destroyed infrastructure. Flooding damaged and in some regions destroyed gas, electricity and water infrastructure. Tens of thousands of people were displaced.
The five dams supplying water to Tehran, namely Taleqan, Latyan, Mamlou, Karaj and Lar, now have 290 million cubic meters of water more than previous years.
Tehran dams together can hold 1.93 billion cubic meters of water. Recent rains plus melting snow in the catchment area of dams helped increase the volume of water in the huge structures. Dams now hold almost 1,000 mcm of water.
To prepare for possible floods and prevent natural calamities like in the past year, “a part of the water stored in Latyan, Karaj and Mamlou dams will be released,” Razavi said. The released water will be used for irrigation.