l cultivated land, about 8.1 million ha are under irrigated agriculture (ICID, 2002).
Surface irrigation techniques are used on 98.7 per cent of the area equipped for irrigation and 1.3 per cent benefits from the pressurised irrigation systems. Until recently, more than 50000 wells of various types have been dug and used for groundwater extraction. More than 60 per cent of the water available at the farm gate comes from groundwater. The databases on average annual water withdrawals reveal that irrigation uses about 93 per cent, followed by the 5 per cent share for domestic purposes, and 2 per cent is used in industrial sector (FAO, 2006). Land degradation is a major impediment to sustainable crop production in many arid and semi-arid regions of the world (Dregne et al., 1991; Bossio et al., 2007).
Salinity-induced land degradation is a major issue in Iran. In addition, sodicity-induced land degradation and microelement salinity such as boron salinity have been developed in some of its areas. The problems of salt-affected soils have serious implications in the irrigated areas where about half of the land is prone to different levels of salinity (Cheraghi, 2004) leading to national economic loss of more than US$ 1 billion. Consequently, the dependency on irrigated agriculture is at stake in areas where land and water resources degradation has increased over time. Although salt-affected soils exist throughout Iran, slightly and moderately salt-affected soils are mostly found in the northern part, while soils with high salinity levels are prevalent in the central part (Figure 2.2).
2.11 Sources of salts and causes of salinisation in Iranian soils
The salinisation of land resources in Iran has been the consequence of both naturally occurring phenomena (causing primary or fossil salinity and/or sodicity) and anthropogenic activities (causing secondary salinity and/or sodicity) (Siadat et al., 1997; FAO, 2000). The following factors have contributed to primary salinity and/or sodicity of the land resources in the country.
Geological composition of the parent material of the soils. Iran is rich in the distribution of naturally occurring materials such as halite and gypsum forming layers with marls, which occur in many parts of the Country. The best known deposits are in the Zagros and Persian Gulf region and in Central Plateau.
Fig. 2.2 Soil Map of Iran indicating area under salt-affected soils and other types of landscape. Modified from Banie (2001).
Stream salinity causing salinisation of surface water resources, mainly due to natural conditions. It is one of the main causes of salt accumulation in soils of Central Plateau.
Wind-borne salinity resulting from strong winds, blowing most part of the year in the Central Plateau. It contributes to the expansion of soil salinity by distributing the salts accumulated at the soil surface to a wider area.
Seawater intrusion, which occurs mostly in coastal areas where seawater enters the inland channels or inundates coastal lowlands by tidal waves. Seawater intrusion into the shallow groundwater in the coastal areas has led to salinisation of soils in Caspian Coastal Plain, and in Khuzestan and Southern Coastal Plains in south western Iran.
Low rainfall and high potential evapotranspiration as a consequence of extreme temperatures. In some cases, such as Zabol Irrigation Project in the Sistan Province, the average annual rainfall is 55mm against the potential evaporation of about 4800 mm, which is 87 times greater than the rainfall.
The human-induced salinisation has occurred mostly in unique topographic conditions of semi-closed or closed intermountain basins where irrigated agriculture has been practiced for centuries. The factors contributing to secondary salinisation of land resources in Iran include (FAO, 2000):
Irrigation with saline and/or sodic waters without adequate management practices in areas of extreme water scarcity.
Lack of drainage facilities, which are the key to appropriate disposal and reuse of saline drainage water generated by irrigated agriculture.
Unsustainable pumping of groundwater through over-exploitation of saline aquifers.
Inadequate irrigation management practices with freshwater such as over-irrigation, particularly in areas with no or limited drainage, resulting in rising water tables and waterlogging problems.
Over-grazing of the pastures and other vegetation resulting in exposure of soils to greater risks of salinisation.
The effects of different combinations of the above factors have caused large-scale salinisation of land and water resources. Development of secondary salinisation in the irrigated areas of Khuzestan, Sistan, Moghan, Zarrineh-Rud, Doroudzan, Saveh and Zayandeh-Rud are important examples (Ghassemi et al., 1995).
2.12 Extent of salt-affected soils in Iran
Scattered publications provide information about the extent and characterisation of salt-affected soils (Dewan and Famouri, 1964; Abtahi, 1977; Abtahi et al., 1979; Mahjoory, 1979; Matsumoto and Cho, 1985; Banie, 2001; Figure 1). The estimates in the 1960s reveal a figure of 15.5 million ha of salt-affected soils, a value that is almost 10 per cent of surface area of Iran (Dewan and Famouri, 1964). In the 1980s, the estimates indicated an additional 2.5 million ha (total1/418 million ha) affected by various levels of salinity (Soil and Water Research Institute, 1987). In the 1990s, the area affected by salinity ranged between 16 and 23 million ha (Siadat et al., 1997). These figures included both cultivated and barren lands, including 7 million ha of salt marsh in Dasht-e-Kavir and Dasht-e-Lut. Another estimate revealed 25 million ha under saline and/or sodic soils (Sayyari and Mahmoodi, 2002). Based on the data extracted fromthe Soil Map of Iran (in digital format), slightly and moderately salt-affected soils cover about 25.5 million ha, while soils having high salinity levels occupy 8.5 million ha (FAO, 2000). These estimates reveal that the magnitude of the salt-affected area in Iran is much larger than initially estimated in the 1960s.The slightly and moderately salt-affected soils are mostly formed on the piedmonts at the foot of the Elburz (Alborz) Mountains in the northern part of the country. The soils having severe to extreme salinity are found in the Central Plateau, the Khuzestan and Southern Coastal Plains and the Caspian Coastal Plain. Salt-affected soils cover about 17 per cent of the area of the Khuzestan Province, and 16.5 per cent area of the Central Province. The Gilan, Kordestan and Kermanshah Provinces have the smallest percentage of area under salt-affected soils (Koocheki and Moghaddam, 2004).
Despite widespread salinisation of land and water resources in Iran, no comprehensive study has been undertaken to assess the extent of irrigation-induced salinity and sodicity. The databases on the quantification of salt-affected soils are fragmented because of the lack of established networks for the monitoring of spatial and temporal changes in soil salinity. Usually, patchy crop stand, retarded crop growth, leaf burn and feedback from local farmers in the form of complaints for soil degradation are used as indicators to establish the evidence of changes in salinity problems (Siadat et al., 1997). In addition, the water quality monitoring programmes are inadequately equipped to collect datasets from areas having the potential to develop secondary salinity and sodicity. These monitoring programmes are important as they act as warning systems for potential areas.
Considering size of the country, particularly the area that is already under salt-affected soils and that with the potential of developing secondary salinity and sodicity, there is a need to use modern approaches such as Geographical Information Systems (GIS) and Remote Sensing (RS) as rapid survey and monitoring tools to track pertinent changes in these soils. These approaches have become an organic component of land evaluation in large countries-Australia, China, USA and India-confornted with the problems of salinity and waterlogging (Qadir et al., 2008). Integrations of remotely sensed data, GIS and spatial statistics assists in modeling large-scale variability to predict the presence and distribution pattern of plant species as well as soil characteristics. Therefore, progress of the projects addressing the management and amelioration of saline or sodic soils can be monitored by tracking timely changes in vegetation and soil characteristics.
2.13 Factors in the formation of salt-affected soils
Salt-affected soils have attracted global attention in research, yielding a considerable body (general information) of data on their properties and management. Although substantial information is available on these soils, it remains challenging to optimize their use and management.
In general, management of salt-affected soils is site-specific and requires an understanding of degradation and regeneration processes to optimize management strategies. Basic pedological research is needed to understand some of the unresolved edaphological aspects of the soils to develop optimal management practices.
The soil-forming factors are the most relevant and appropriate factors explaining salt-affected soils formation. They are interdependent and highly variable and therefore influence the properties of the soils in multiple ways.
The characteristics of salt-affected soils are related to overall climate; however, other factors such as texture, clay mineralogy, cation saturation, and the amount of exchangeable