4.1. Land use/cover change detection
Land use/cover change detection requires valid identification of the terrain phenomenon and classification methods (Yang and Lo 2002). After image classification and producing the land use/cover maps for the years 1984, 1992, 2002, and 2017 (Fig. 2), overall accuracy and Kappa coefficient were calculated using the error matrix (Table 4). Finally, land cover maps of the years 1984 and 2017 were compared by post-classification comparison.
One of the most considerable changes is the increase in cropland up to 12.38% (799918.02 ha) from 1984 to 2017 (Table 5), while rangeland decreased up to 14.29% (923570.46 ha) during the same period. It shows that rangeland class has responded to the needs of the community based on land use planning and was converted to cropland and urban classes during this period. The results of some studies have shown that sugarcane monoculture, in large scale, leads to soil degradation in tropical regions, whilst a wide area of Khuzestan plain is allocated to it (Behravan et al. 2013; Pourkeihan et al. 2018)
Results showed an increasing trend of cropland expansion in Khuzestan plain in three past decades (Fig 3), which are similar to the results of the studies of Madadi and Ashrafzadeh (2010), Faraji et al. (2016), and Roozbahani et al. (2017).
Two other significant changes happened in salty land and swampland with the decrease of about 3.89 % (251528.22 ha) and 1.44 % (93032.28 ha), respectively (Table 5).
Over the recent decades, urbanization has made significant land use/cover changes, such as decreased rural areas and increased built-up or urban areas (Mundia and Aniya 2005; Dewan and Yamaguchi 2009;). According to the results (Table 5), the built-up area had increased by up to 0.43% (27620.46 ha) from 1984 to 2017. The highest rate of urban expansion and cropland growth was observed from 1992 to 2002 in Khuzestan (Fig 3). Dam construction across Karkheh River in 1998 caused urban sprawl, population growth and agriculture flourishing in this period. Unfortunately, the urbanization growth and high pressure on land, in order to be inhabited, and the creation of other necessary structures such as dam construction has led to the destruction of natural resources. Results of Fig.4 showed that the main reason for Horelazim wetland reduction from 1992 to 2002 is the decreasing of water volume that enters into the pond because of Karkheh dam construction. Similar results were represented by Makrouni et al. in 2016. Karkheh River is one of the largest water sources that determins the water of Horolazim wetland (Fuladavand and Sayyad 2015). Also, the studies by Jones et al. (2005) showed that the Karkheh dam construction has caused severe reduction in Horolazim area. Zarasvandi et al. (2012) also indicated that 72% reduction in area of Horolazim wetland was induced by the increasing agricultural demands for water and human interferences.
In 1984 the wetland class had covered about 3.36% of the whole Khuzestan (216908.55 ha), but due to the anthropogenic forces, the amount of the wetland area decreased to 2.32% in 2017 (149809.5 ha) (Fig. 4). Unfortunately, the decreasing trend of the wetland area was observed from 1992 to 2002 (about 59878.8 ha). Cao et al. (2015) reported that Horolazim marshes are now subjecting to rapid land degradation due to natural and anthropogenic factors and might totally dry in the future. Land use and cover changes and intensive wetland desiccation occurred due to cropland expansion, high demand of water for irrigation systems, and dam and large projects construction (Ghobadi et al. 2015). Pourkhabbaz et al. 2015 showed that Shadegan international wetland area has decreased by 8.5% over the past two decades.
The decrease of 0.4% (25730.91ha) of water bodies is due to the increasing in water consumption and lengthening of dry periods. The discharge of Marun, Karun, and Karkheh rivers has been decreased by 40, 37, and 149 percent, respectively in 2000-2001 compared with the mean of the past 32 years in Khuzestan province (Mousavi 2005). Karkheh River basin (approximately 51,000 km2) stretches from the Zagros Mountains to the Horolazim swamp, which is a trans-boundary wetland located at the Iran-Iraq border. Karkheh river basin is one of the most productive cropland in Iran (Ashraf Vaghefi et al. 2014). The impacts of land-use change in river basins have been shown in previous studies (Masih et al. 2010; Tabari et al. 2011; Salajegheh et al. 2011). These changes indicate that the situation will get worse in the future.
4.2. Main local dust sources due to land use/cover change
According to the remotely sensed data classification results during the 35 year period (1984-2017),we assumed two main local zones were as dust sources in Khuzestan province, because of significant land use change. The southern part of the Horolazim wetland and the northern area of Shadegan wetland have been converted to dust storm sources as the result of converting the wetlands to low-density rangeland and sand with the area of approximately 29387.99 ha (Fig. 4).The dried areas of wetlands are sources of the dust storms formation, affecting on the surrounding areas (Heidarian et al. 2018; Rashki et al. 2021). Heidarian et al. (2018) reported that nine percent of Khuzestan province, equivalent to 349254 ha, are dust-generating sources. Human activities such as land use/cover change have been identified as dust centers in globale scale (Ginoux et al., 2012).
It is important to mention that the soil type of the western, southern, and southeastern of Khuzestan province is clay (Hamidi et al. 2017). Abyat et al. (2019) in their research showed that dust particles type was clay as well. Clay particles are light and can extend the distance of the dust movement. High ability of clay particles to attract organic and inorganic chemicals and their fine texture makes them more susceptible to be dangerous (Keramat et al. 2011).
Cao et al. (215) indicated that the shrinkage of Horolazim wetland, especially in the southern and southwest parts of the wetland areas is the result of phenomena such as climate change, low-water periods, as well as negative effects of human activities that a conclusion supported by Javadian et al.(2019) who found these wetlands to be one of the three most important dust centers affecting the other cities of Iran. Anthropogenic activities include extreme harvesting of water from wetland resources, wetland drying and converting into cropland, dams construction on the rivers, increasing human access to a wetland through construction roads inside wetland, an oil company drilling and undeniable impacts of Iran-Iraq war that have undesirable effect on the wetlands (Gerivani et al. 2011). Arkian (2017) highlights the impact of oil extraction around the Horolazim, one of the most important wetlands in southwestern Iran, as the primary reason for its declining water levels in recent decades. Cao et al. (2015) expressed that the dried area of Horolazim wetland that still keeps expanding is the main dust storm source in Iran, and have had a great effect to increase dust storm in the southwest part of Iran.
The results of Broomandi et al. (2020) indicated that Horolazim area acts as a source in the region. The susceptibility of southern area of Horolazim wetland to turn into a persistent dust centers has been mentioned in other studies (Broomandi et al., 2017; Cao et al., 2015; Heidarian et al., 2018).
The degradation process of Shadegan international wetland goes more quickly. When the population in the surrounding of wetland increases, it lead to demand for economic activities and development. Most of the economic activities and the development plans around the wetland have adverse effects on wetland due to the lack of environmental assessment of development plans.
The results indicated a decrease in the area of Shadegan international wetland, especially in the northern part of the wetland called the freshwater wetland. Most of the threats in this area can be caused by increased access to the wetland (road construction). Also, water-fed wetlands through streams in this area, are entered into the wetland and strongly effect on the salt, sediment, and water quality of the wetland. The cumulative effects of the threats have a high risk on this part of the wetland. Due to the construction of several dams and the implementation of irrigation network development projects in recent years, the drought has led to changes in the vegetation cover of these wetlands, which was identified as a source of dust (Javadian et al. 2019). Rahimi Blouchi and Malekmohammadi (2013) showed that instead of the change in natural habitats, change in hydrological patterns due to dam construction and also water pollution has been identified as the most important risks of the Shadegan international wetland.
4.3. Transport and dispersion modeling
After the detection of dust storm sources based on land use/cover change in Khuzestan province, HYSPLIT forward trajectory model was performed to find the possible paths of dust storm and their dispersion scale. This modeling was performed for the south of Horolazim (center 1) and north of Shadegan international wetland (center 2) in three elevation levels of 500, 1000, and 1500 meters for 365 days in 2016.
According to the HYSPLIT model results, dust particles in the hot period (spring and summer) of the year 2016, had tracks to the southwestern Khuzestan, southern Bushehr, Hormozgan, Persian Gulf, and even some days to Saudi Arabia (Fig. 5). Abdi Vishkaee et al., (2011 and 2012) mentioned that Shamal wind in summer is especially more severe than winter in Khuzestan province.
During the cold period of the year, western winds have a significant role in the occurrence of dust storm and its dispersion in Khuzestan province (local scale). According to the results, during the cold seasons (autumn and winter), dust storms moved to northeastern of Khuzestan province such as Chaharmahal and Bakhtiari, Isfahan, and southern Khorasan provinces in Iran and rarely to Iran’s neighbors such as Turkmenistan. The main factors in the formation and occurrence of dust storms in Khuzestan are migratory systems of westerlies (west winds) and polar front jet stream (PFJ) (Tavoosi et al. 2010; Raispour et al. 2016). It is important to mention that dust storms in Khuzestan occur mainly in the late spring (May) (Shahsavani et al. 2012) and in the summer (June-July) although some dust events in Khuzestan occur in the winter (Modarres and Sadeghi, 2018; Kamal et al. 2020). According to the results of forward Hysplit trajectory modeling from ineternal dust storm (center 1 and center 2), most neighbor provinces of Khuzestan subjected to dust storm with internal origine in cold season.
According to Figure 6, the MODIS color composite image was similar to MODIS AOD product of July 19th and 20th 2016. Analysis of the visibility data of AQM stations indicated that there were 30 dust-storm days in July 2016 (visibility <1 km) in Khuzestan. Figure 7 shows that the visibility value during the studied days has decreased in Khuzestan, Ilam, and Lorestan’s AQM stations. Based on the true color composite of MODIS sensor, dust storm in 17 July 2016 after passing through deserts of the north of Saudi Arabia and south of Iraq would extend to Khuzestan, Ilam, Hormozgan, and Bushehr located in the south and southwest of Iran. AOD product recorded a high value of particles in Khuzestan province (Fig 6). Khuzestan Province is the area that is heavily influenced by deserts of the north of Saudi Arabia and south of Iraq. The images clearly show that about 61% area of Khuzestan province covered with desert ecosystems (having low density vegetation) and western provinces are engaged with dust storm, except some of the northern and eastern mountainous areas. Rajaee et al. (2020) and Borna et al. (2021) used MODIS colore composite image and AOD product for HYSPLIT modeling assessment.