Leaflet Series: B Number: 3 Research Institutes: Salinisation Massimo Iannetta, Nicola Colonna
Introduction Salinisation is one of the key processes that can lead to desertification. It is a growing phenomenon all over the world and affects millions of hectares across Europe. Agriculture plays a major role in driving the phenomenon, by causing high water consumption and water chemical degradation, but at the same time it is the economic sectors that are facing the strongest impacts. Moreover current climate change scenarios could significantly increase salinity and result in the expansion of the affected areas. Salinisation processes The term salinisation is used for the process of salt accumulation in the soil. It occurs especially in arid and semiarid areas where soluble salts precipitate within or on the surface of the soil. Increasing salt levels in the top soil layers can negatively affect plant growth and productivity to the point of plant death. High concentrations of various salts (e.g., sodium chloride, magnesium and calcium sulphates and bicarbonates) affect plant growth both directly, through their toxicity, and indirectly, by increasing osmotic potential and lowering root water uptake. In dry climates continuous salt accumulation can lead to desertification, while in humid or subhumid climates moderate or severe salinisation may occur seasonally. Salt soil accumulation is the end product of several different processes. Thus, the term salinisation may include different processes driven by different causes that bring about the same result. Generally speaking, we can distinguish primary salinisation, due to natural soil characteristics, and secondary salinisation where human activities play a central role. Basically, salinisation occurs where, depending on the soil and groundwater table characteristics, the equilibrium between rainfall or irrigation and evaporation is moved towards evaporation. We can identify three main processes that can cause salinisation: the rising of the water table to, or close to, the ground surface: it occurs in non-irrigated drylands where salts accumulate by water evaporation in the top soil surface; the excessive use of water for irrigation in dry climates, with heavy soils, causes salt accumulation because they are not washed out by rainfall; the intrusion of saltwater: this occurs in coastal areas where seawater replaces groundwater that has been over-exploited. The first process occurs in alluvial plains or depressions in semiarid regions when groundwater levels are close to the soil surface. Capillarity sucks water to the surface where it evaporates due to the intense solar radiation, leaving behind deposits of salt. In those soil types we can often observe salt crusts. The second process occurs in cultivated areas where irrigation is associated with high evaporation rates and a clay texture of the soil. In this context salt leaching is scarce or absent and sodium magnesium and calcium ions accumulate in the soil surface layers. The last process occurs in coastal areas where excessive extraction of water, due to multiple demands, causes the lowering of the water table and the intrusion of sea water. In recent years, this process has spread dramatically throughout Mediterranean coastal areas. Increased salinity from groundwater affects productivity of irrigated crops and, in a medium- to longterm perspective, contributes to secondary soil salinisation. However, moderate soil salinisation is reported even in areas irrigated with good quality water depending on irrigation methods and aridity conditions, while salinisation may not occur in areas where farmers have relied on salt-rich water for years. These two examples clearly show that in each area potentially affected by salinisation a different and peculiar equilibrium between the different factors influences the salinisation process. How to measure Salinity? Whenever we collect data or read the literature about salinisation we face the difficulties of comparing different research results or data due to the use of different ways to measure salinity. Scientists and technicians are used to dealing with different units of measurement, but for the layman it is not immediately obvious how to compare the different measures. Salinity is a measure of the quantity of dissolved salts in water, and is traditionally measured in parts per thousand (ppt, or ) or as Total Dissolved Solids (TDS). TDS is the concentration of a solution as the total weight of dissolved solids. (1 ppm = 1 milligram/litre, and 1 ppt=1 gram/litre) More often salinity is calculated from the conductivity of the solution. As a general rule, the higher the salt concentration in a solution, the better is its ability to conduct electricity. Electrical conductivity of water (ECw) is nowadays expressed in units such as decisiemens per meter (ds/m). Rain water, for example, has a conductivity of 0.02 ds/m, while sea water has a conductivity of 50-60 ds/m. TDS and conductivity are not linearly related, two solutions with the same TDS could have a different ECw depending on the different types of ionic salts and their concentration. A generally accepted rule of thumb to convert TDS to conductivity is: TDS (ppm) = conductivity (ms/cm) x 0.67. It is common to find other measurement units such as mho/cm, or to meet submultiples such as ms (millisiemens) or µs (microsiemens). Table XX above will help to avoid confusion. The Siemens is the official unit for conductivity used in the Metric System while mho is an older unit commonly used in North America. Areas affected and prone to salinisation Salinisation is a worldwide issue. The FAO Unesco assessment in 1999 showed that saline and sodic soils are widespread and affect millions of hectares of land all over the world. Different estimates have been produced showing that a significant percentage of salt affected soils are human induced. In 1998, the second environmental assessment of the European Environmental Agency reported that about 4 million hectares of European soils were affected by salinisation, mainly in the Mediterranean countries. Four years later, in the third assessment, the total amount of soil affected was about 16 million of hectares. This new assessment, however, included countries, such as Russia, that were not considered in the previous report, so that the data are not directly comparable. The same source shows that in the Mediterranean area 25% of irrigated cropland is affected by moderate to high salinisation.
Another updated source of data is Europe's water: an indicator based assessment, published in 2003, where saltwater intrusion is evidenced in different European countries, particularly Spain, Italy, Greece and Turkey. Maps clearly shows a strong connection between the water exploitation index and areas affected by salinisation. It is clear that many of the areas where an intense littoralisation process has occurred are the same as where we are now facing salinisation. Cause and effect relationships Several different indicators have been proposed to assess and monitor salinisation throughout Europe and to measure how the process evolves over time and space. No single indicator is able, by itself, to provide enough information about the process. At the same time we have to deal with an heterogeneous availability of data between countries and inside countries, so we do not yet have a clear picture of the situation and of its dynamics. DPSIR conceptual framework for salinisation Driving forces Industry, Agriculture, Tourism, Households, Human settlement growth Pressures High water consumption, Groundwater over-exploitation State Surface water quality and quantity, Soil chemical quality, Soil structure, Groundwater status Impacts Groundwater levels, Saltwater intrusion, Soil salinisation, Low crop yields, Abandonment of non tolerant crops, Farmer income reduction, Land abandonment, Desertification Responses Alternative supplies (dams, pipelines), Water use restrictions, Regulations, Water contracts agreements, Desalinisation, Crop tolerance improvements, Water saving techniques Impacts on agriculture Productivity is not affected by low level salinity, but a sudden drop in productivity is observed after a speciesspecific threshold is crossed. Cereals are generally more tolerant to salinity than horticultural or fruit-tree species. The economic impact of salinisation is not easy to evaluate because of the nonlinear relation between salinisation and productivity. Thus, salinisation may remain undetected for years at moderate levels of salinity, while a further increase may cause land abandonment in a few years. Tolerance of crops to salinity Sensitive Bean, Onion, Clover, Potato, Pepper Moderately tolerant Corn, Soybean, Tomato, Oats, Wheat Tolerant Barley, Cotton, Olive, Rye Mitigation and adaptation measures Agriculture plays a double role as both the first and the last element of the salinisation causal chain. On the one hand, it increases pressures on soil and water resources, while on the other hand it has to deal, by mitigation and adaptation strategies. Farmers are adapting to increased soil and water conductivity by a mixture of strategies that include better choice of crops and cultivars, rotation, irrigation methods, water storage, water mixing, water reuse, and desalinisation. Case studies in Spain The Vélez River Coastal Aquifer The scarcity of rainfall in the first half of the 1990s and the increased groundwater extractions, led to a considerable decrease of the piezometric levels in the aquifer. A significant advance of the salt water wedge was registered and the water shortage due to the severe Vélez River Coastal Aquifer drought was aggravated by deterioration in the water quality. During the last few years rainfall increased and the reservoir has supplied water, thus the pumping of freshwater from the aquifer has almost stopped. At present, the aquifer presents no evidence of significant marine intrusion, but an outflow from the dam sufficient to maintain a subterranean discharge to the sea is necessary in order to get a long-term control of this problem. Case studies in Cyprus: the Akrotiri peninsula By the early 1990s, with a water average exploitation rate of 18 million m3/year, the salinisation of the irrigated land had become alarming, causing the farmers to modify their agricultural practices and abandoning coastal crops and several wells. More lately, the aquifer has been declared a conservation area by the authorities and exploitation has been reduced to 7 million m3/year.
Nowadays intensive farming is the main economic activity in the area, while planned tourism development could increase water consumption. A permanent multi-parametric probe to collect conductivity data in a specific point of the Salso-Imera river has been wirelessly connected to the Agricultural Extension Service Office (SOAT) of Licata in the framework of the research RIADE project on desertification. This information is used to alert the farmers of critical situations and give them the possibility to adopt the best solution to reduce the salinity reaching their fields. Conclusion The challenges we are facing today, to provide solutions to the problems of salinisation, include the need to mobilise the scientific community to mount an integrated and a multi-disciplinary program for methods, standards, data bases and research networks for assessment and monitoring of water and soil salinisation. It is, however, not enough to research and understand the problems. We have to ensure that the knowledge we have gained is applied in practical ways, for the benefit of mankind. Akrotiri peninsula At present, to enable the efficient and sustainable management of the aquifer only limited extraction is permitted. Irrigation demand for citrus plantations and seasonal crops mainly relies upon a very costly irrigation system which conveys surface water from the Kouris river dam. In additon, in order to mitigate the adverse effects of reduced water availability and deteriorating water quality in the aquifer, there is artificial recharge with water from the upstream dam or from a sewage plant. Case studies in Italy: the Licata Plain area The Licata plain is included in the Imera river basin that crosses the central part of Sicily where naturally salty soils are widespread. Greenhouse expansion to produce high value vegetables for the national market strongly increased groundwater abstraction from the 1960s (more than 2000 legal and illegal wells) to reach critical levels (average ECw was 5,9 ms/cm). Sicily, the Southern Imera Basin
Contributing projects: UNIVERSIDADE LUSÓFONA de Humanidades e Tecnologias Humani nihil alienum Integrated Research for Application of innovative technologies and processes to combat Desertification (2002-2006)