4.1 Study Area
Iran (Fig. 2a) is a developing country in the southwest of Asia. It is the second-largest country in the Middle East, with an area of 1,648,195 km273. Iran is generally classified as a semi-arid and arid region (Fig. 2b), with a mean annual precipitation of about 250 mm year− 1. Mean annual temperature ranges from 10°C (in the west) to 35°C (in the center). There is no general prevailing climatic season in Iran. According to the Koppen climate classification, the south, east, and center of Iran have a desert and semi-arid climate. A hot or warm dry-summer continental climate is experienced in the west and northwest of Iran, and the coastal regions in the north of Iran have a Mediterranean climate. These highly variable climate characteristics are due to the location on the Caspian Sea, the Persian Gulf, and Sea of Oman and the mountain regions Albors and Zagros75.
Statistical Center of Iran (2016) reported that currently, more than 74% of Iran’s population live in cities and only 26% in rural areass76. The current urban population is much higher than in the 1950s and 1970s (27% and 44%, respectively)73. Statistics show that during 2011–2016 about 30,000 villages across the country were abandoned76. The population growth in the rural areas in Iran is negative, e.g., -0.73% during 2011-201676.
4.2 Climate Change
The World Meteorological Organization reported that Iran became warmer by about 1°C during 2001-201078. Due to the increasing trend in mean annual temperature and the decreasing trend in mean annual precipitation, aridity has been increasing during 1966-201577, indicating an evident change in climatic conditions in the country. It is also predicted that during 2020–2100, Iran will become even drier and hotter79.
4.3 Human Mal-Activities
Iran is considered one of the pioneer countries in the world in the construction of water resources and efficient water irrigation projects to successfully deal with limited water resources in a sustainable manner79,80. For instance, Qanats were widely used by ancient Iranians. However, by importing the first water pumps to Iran in the early 20th century, the importance of Qanats has become less due to the expansion of water wells. The number of wells increased from 45000 to 50000 in the 1970s to 764000 in 201181. This considerable increase leads to overexploitation of groundwater resources, changes in the hydrological cycle, land degradation, and desertification73.
The country is experiencing considerable challenges of environmental degradation, mostly related to water issues. As in the other countries in the Middle East, most of the water challenges are rooted in bad governance and mismanagement, to a large extent arising from an oil-based economy with remarkable social and economic changes such as urbanization, rising standard of living, high rates of consumption and infrastructure development82. Moreover, despite improving the social and physical assets in rural areas during the last decades, their economy still depends on agriculture and water. Thus, based on its "Hydraulic Mission" strategy, Iran over-invested in water storage and distribution infrastructures to meet the increasing demand of water during the last decades. Overexploitation of groundwater reserves and over-investment in dams and inter-basin water transfer projects with minimum concern about environmental consequences have led to drastic environmental changes in some parts of the country. Besides, the lack of appropriate socio-economic and political plans for dealing with these environmental degradations as well as the current economic, power, and services inequalities between rural and urban areas, increase the rate of RUM73.
Moreover, the construction of dams to provide water for agriculture and domestic purpose and generate electricity were the priority of the policymakers during 1955-196283. This aligned with a global movement during the 1930s-1970s that considered dam construction as a sign of modernization, development, and economic growth84. Currently, there are 316 dams in Iran, 100 dams are under construction, and a further 300 dams being considered for development79, while in the 1970s, there were only 12 dams in the country85. Notwithstanding their benefits, these dams have resulted in damages to the ecosystem, changes in land cover, human displacement, and more development and water use downstream due to the perception of water abundance73.
After the Islamic Revolution and due to international sanctions, the policy of Iran was to be self-sufficient. Thus, the policymakers decided for rapid development by constructing the major infrastructures without considering long-term consequences and the complexity of the human-natural system. It has resulted in many environmental challenges, including “drying lakes and rivers, declining groundwater resources, land subsidence, water contamination, water supply rationing and disruptions, forced migration, agricultural losses, salt and sand storms, and ecosystem damages”73. Additionally, climate change, prolonged droughts, and frequent floods have exacerbated environmental degradation in some regions. These factors together have been led to massive RUM.
These activities result in drying water bodies in many watersheds 73 , many other environmental challenges, and migration. In recent decades, the implementation of different development plans and overexploitation of biological resources have affected the eco-regions of Iran 86,87 . According to the last available report, the World Bank reported that in 2002, the annual cost of environmental degradation in Iran was about 8.4 billion US$ 88 .
Dust storms are a big problem in Iran, especially in the southeast and southwest. In the Sistan area in the southeast of Iran, part of the Hirmand River Basin (HRB) (see Fig. 2.b), shared by Iran and Afghanistan, human-made infrastructures in Afghanistan and debilitating dust storms have brought life to a standstill. The Lake Hamoun in HRB (Fig. 2.b) has dried up due to conflict between Iran and Afghanistan over the utilization of the Hirmand River89. All research on dust storms in the Sistan area believes that the dry lake bed is the main cause90.
Salt storms also occur, for example, in the northwest of Iran from Lake Urmia (Fig. 2.b). Lake Urmia, the largest lake in the Middle East, has shrunk significantly due to upstream human mal-activities as well as frequent droughts91,92. In addition to the direct consequences for the lake’s ecosystem and regional economy, shrinkage of the lake also leads to salt storms in the surrounding cities and villages73. Both dust and salt storms cause many physical and mental health problems, as well as the loss of agricultural lands, forcing, in some cases, many people to leave their homes.
Groundwater is the primary source of water used for domestic, industrial, and agricultural consumption in Iran93. Increased water demand has led to overexploitation of groundwater in Iran94, such that the country is one of the top groundwater miners in the world95,96.
4.4 Hydro-Climatic Disasters
Iran is one of the disaster-prone countries in the world. However, flood and drought are the biggest concerns of the Iranian governments and play a significant role in migration.
Iran has several large rivers that flood in spring, causing considerable damages to agriculture. For example, in March and April 2019, precipitation with a 200-year return period occurred in most parts of Iran, especially in the west and north. During 16 days (17 March to 1 April), the amount of precipitation was about 72 mm, almost 29% of the average yearly precipitation, resulting in a runoff of 119 MCM 97 . This flood affected 3,899 villages and more than 2,100,000 people and in about IRR 38,528 billion (US$ 321 million) of damages to the agriculture sector.
In Iran, severe and long-lasting droughts occur regularly: in the last 50 years, the country experienced 27 droughts 98–100 . One of the severest droughts since the 1940s occurred from 1998 to 2000. In 1989–1999 alone, about 4.2 million head of livestock died and about 12 million hectares of agricultural lands were damaged. The damage to agricultural products during 1989–1999 was about US$ 4.3 billion 99 . Approximately 11 km 3 of groundwater was abstracted and many wells and Qanats dried up 99 . In 2000–2001, rangeland production was reduced by about 70%. About 80,000 tons and 40,000 tons of rangeland production have been lost in Kordestan and Guilan provinces, respectively 99 . OCHA (2001) reported that “based on the official estimates, this year's drought  is directly affecting more than 2.6 million hectares of irrigated farms, 4 million hectare[s] of rainfed agriculture, 1.1 million hectare[s] of orchards and more than 75 million animals 101 ”. After a brief spell, the most severe, prolonged and extensive drought over the last 30 years has occurred since 2003, with undesirable consequences for many rural communities in the center, east and south of Iran 98 . Drought has caused many rural people in Iran to lose their jobs and income, especially people whose lives depended on water and agriculture, and forced them to migrate.
Accordingly, drought is a common phenomenon and a part of normal life in Iran; however, it is also predicted that the country would be hotter and drier due to climate change with more pressure on water resources 102 and agriculture 103 . Agricultural activities in Iran are susceptible to droughts: one mm of rainfall below the historical average leads to approximately US$ 90 million losses in the agriculture sector 79 .
4.5 Agricultural Consequences
Agriculture in Iran depends mostly on groundwater: in about a third of the country, more than 80% of the land is irrigated with groundwater104. During 1993–2007, the area irrigated with groundwater increased by 39%, while that irrigated with surface water declined by 15%105. Reduction in groundwater levels (with an average of 0.4 m year− 1 across the country104) threatens the agriculture sector in the country. Moreover, over-abstraction of groundwater has led to environmental degradation and water salinization93. Irrigating the crops with saline water results in the concentration of salts in the soil and decrease in grain yields105,106.
It is estimated that soil salinization in Iran has led to an economic loss of more than US$1 billion 93 . Overgrazing and deforestation directly cause soil degradation, leading to both a decline in agricultural production and a loss of agricultural land. Soil degradation indirectly–through desertification–results in loss of agricultural land. In the case of climate change, it is predicted that with 2.7–4.7°C increase in temperature in Iran, the average yield reduction in rain-fed wheat crops will be about 18% by 2025 and 24% by 2050107. Currently, the total cultivated area in Iran is approximately 16,477,000 ha, of which 46% is irrigated 76 . However, due to environmental changes, it is becoming less, with considerable economic consequences in the rural areas.
4.6 Economic Consequences
The economic consequences such as unemployment, decline in income, and food insecurity have not been investigated in detail in Iran. However, few studies have provided some information about these consequences. According to the interviews conducted by Khavarian-Garmsir et al. (2019), especially dust storms have contributed to the economic downturn and unemployment in Khuzestan province, southwest of Iran (Fig. 2.b). The interviewees from Masjed Soleiman, a city in this province, indicated that unemployment is the oldest reason for migration. In Urmia Lake Basin in the northwest of Iran, environmental changes and recent drought cycles have led to a reduction in agricultural productivities, higher rate of unemployment, economic problems and migration109. In Sistan area in the southeast of Iran, people also believed that the economic problems arise from the water scarcity and prolonged drought in the area110.
4.7 Fears, Hopes and Pull Factors
The majority of rural people in Iran are employed in the agricultural sector. Agricultural degradation and economic problems increase uncertainty in the life of rural people. Many rural people are in doubt whether to leave their home places and emigrate to the cities76. For example, according to the interviews conducted in Khuzestan, some unemployed people out-migrate to find a job in other cities and, some employed people are looking for better economic situations and indeed for a better future108. Studies in Khuzestan province show that 35–40% of rural migrants changed their jobs over the last four census periods due to environmental and economic factors111. Seeking better economic situations in combination with cities’ attraction and social network in the cities increase the rate of RUM. This migration leads to communities’ population decrease, security threat, and consequently frustration and dissatisfaction.
The findings of Khavarian-Garmsir et al. (2019) in Khuzestan showed that in addition to environmental changes and economic decline, the market downturn is also affected by political factors. They also indicated that in some areas, basic facilities are not available, and uneven economic development and inequity in job opportunity and life quality may lead to mass RUM. One more critical issue in Khuzestan province is the effect of Iran-Iraq war and the government’s weakness in rebuilding the damaged areas. This makes many people to be dissatisfied with their current situations. Accordingly, Khuzestan province is known as the emigrant-pole in Iran such that more than 50% of rural-urban migrants come from this province76. Such situations are also observed in Sistan area in the southeast of Iran, where 55% of the rural population depend directly on agricultural activities for their income112. During the drought in the early of 2000s, 124 villages were abandoned, and unemployment rose considerably 113.
Mass RUM in Iran shows that hopes are not so strong to keep people in villages. This could be due mainly to a lack of wise management and preparedness plans for dealing with the consequences of environmental changes and governmental supports, such as providing water, jobs, investment, and amenities, allowing border markets, and engaging local people, especially young people, in managing local challenges. Additionally, Mianabadi et al. (2021) showed that sense of belonging, or current place attachment, and thrustworthy social network in the villages in the Sistan area are key factors that encourage people to stay in their homelands110. However, unpleasant situations make rural livelihoods increasingly at risk and threaten the communities and households that are more dependent on natural resources100.
Among the leading causes of RUM, climate change cannot be mitigated at the local scale, and hydro-climatic disasters cannot be avoided completely. The only manageable leading cause is human mal-activities; thus governmental policies to deal with the migration should focus on this issue. Satisfactory, reliable economic conditions encourage people to stay in their villages. These conditions can be provided in different stages of environmentally-induced migration. Factors encouraging people to stay in villages include investment in small businesses, supporting economic activities other than agricultural ones, such as greenhouse production and border trade and markets in rural areas near the borders, and engagement of all stakeholders in local governance. Policymakers can provide direct financial supports before and after the consequences occur. They can also focus on coping and adaptation strategies to avoid unpleasant consequences of environmental changes by increasing resilience and decreasing the vulnerability of individuals and communities. These strategies include sustainable development of the region through land-use planning, using the water diplomacy tools to meet environmental demands, especially for transboundary wetlands and lakes, considering the socio-political aspects of water management and development projects and sustainable participation of local communities in policy and decision making. However, migration is a complex and nuanced problem that requires a complex solution. Thus, there is no single solution for dealing with mass RUM. Therefore, depending on their unique and special characteristics, a unique and special solution might be helpful for each region.