02.02.2017: The Kerman Province of Iran is one of the main producers of pistachios in the world. In the Rafsanjan plain more than ninety percent of the agricultural land is used for production of this stone fruit. Groundwater is the main source of water supply for irrigation and industrial activities of the region. Since the plain is climatically extremely dry, and groundwater extraction exceeds recharge, the groundwater reservoir is draining. Erosion and subsidence of soil are the consequences.

From 1983 to today the groundwater table of the Rafsanjan plain lowered by approximately 17 meters. Based on long-term radar imagery data from the European satellites Envisat and Sentinel 1 as well as the Japanese satellite ALOS, a team of scientists led by Mahdi Motagh, GFZ section Remote Sensing, investigated the rate of the resulting soil subsidence for the years 2004 to 2016. In a study published in Engineering Geology the scientists show that in a large area of approximately 1000 square kilometers within their study area the soil subsided at rates greater than five centimeters per year, locally even by more than 30 centimeters per year.

Field investigations showed that locations with highest rates of subsidence were those where wells are installed. An unregulated extraction of groundwater for agricultural and industrial use therefore is the likely causes for soil subsidence. Within 30 years the agriculturally used area expanded by 50 percent, even though the rate of expansion has slowed down recently due to the impact of drought and increasing water scarcity. Decades of unrestrained exploitation led to unstable soils; soil erosion and subsidence became serious natural hazards for the region and require remediation measures.

Groundwater reservoir is irreversibly reduced

Groundwater reservoirs in the region receive their recharge from direct precipitation, basin-wide runoff from the surrounding mountain ranges, and seasonal river discharge. But since there is more water exploited than recharged, the volume of the groundwater reservoir is continuously reducing. The loss of volume was calculated to be about 300 million cubic meters per year. Desiccation causes an irreversible compaction of the sediment. Mahdi Motagh, main author of the study: “Even if in the future the recharge of water surpasses the extraction, the reservoir is not able to load more water. The loss in volume is permanent and a consequential damage of unrestrained groundwater exploitation. It is a challenge for future water supply.” The non-sustainable use of groundwater resources by pistachio culture therefore is a hazard for its own future. (ak)

Original study: Motagh, M., Shamshiri, R., Haghshenas Haghighi, M., Wetzel, H.U., Akbari, B., Nahavandchi, H., Roessner, S., Arabi, S., 2017. Quantifying groundwater exploitation induced subsidence in the Rafsanjan plain, southeastern Iran, using InSAR time-series and in situ measurements. Engineering Geology, available online 19 January 2017, DOI: 10.1016/j.enggeo.2017.01.011