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|Desertification Indicator System for Mediterranean Europe|
in intensive irrigated agriculture
of reasons leading to increases in intensive irrigated agriculture and
why they are an issue in the context of desertification
Irrigation is very important for increasing crop yields in arid, semi-arid and sub-humid climates. In northern Mediterranean countries the association between scarce rainfall, mild winters and extensive markets triggers the increase of irrigation. Irrigation is frequently associated with over-exploitation of either surface water or groundwater resources. Over-exploitation of water resources leads to degradation of water quantity and quality, also affecting semi-natural ecosystems. Over-exploitation of groundwater produces exhaustion of the resource and generalized degradation, especially through salinisation. Imbalance between demand and supply usually produces offsite degradation, sometimes at long-range. The expansion of new irrigated lands is based on aggressive techniques provoking the destruction of soils, with no soil conservation measures are usually implemented. Low quality in water because of over-exploitation is associated with soil salinisation and loss of crop productivity.
Inner Alentejo, Portugal
The importance of irrigation is increasingly significant in the Lower Inner Alentejo, in order to feed cattle during periods of natural pasture deficit, or to diversify production in these marginal agricultural areas in order to increase the income of farmers. Therefore progressively, and as a consequence of the CAP incentives, there has been an increase in irrigated cultivated pastures as well as other cultures like almond trees, olive trees, sorghum, corn and linen. Before Portugal joined the European Union (in 1986), there was scarcely any irrigation in Mértola Municipality. In 1989, agricultural statistics described 254 hectares as having irrigation potential and by 1999 irrigation infrastructure was in place over 840 hectares.
Irrigated cultivation appears where topography permits, in areas such as flat valley bottoms or surfaces with very gentle slopes, and is generally based on drip irrigation systems from surface water reservoirs (small dams and ponds). Water is very rarely extracted from ground water, as the subterranean reserve is extremely small and limited as a consequence of the impermeable lithology (Palaeozoic metamorphic rocks).
Nonetheless, irrigation practices have always been used in the Inner Lower Alentejo and in the Mértola municipality, as is easily observable from the landscape in the number of wells and remains of traditional irrigation techniques in very small areas (such as mills or irrigation canals). Valley bottoms were good locations, as soils there have better quality and higher moisture content. These small spaces were intensively cultivated and cared for, as the products were fundamental for the local population. Horticulture and fruit trees (oranges, pears, apples, olives, figs) occupied these areas, and are still visible as small patches in the landscape.
Depopulation led to a loss of functionality of these areas (see the description of land abandonment). However, some are presently being used again with a different purpose, to produce EU subsidized crops (see Table below). This development has grave environmental consequences, as large amounts of water are used and the soil becomes increasingly contaminated with chemical products. The pollution of the aquifers, mostly by nitrates and heavy metals (from fertilizers and pesticides) is a reality, because present agriculture demands high unitary production and requires accordingly high amounts of nutrients.
Area of irrigated cultivation (ha) - Mértola Municipality
Source: INE, Recenseamento Agrícola
In the Guadalentín basin in southeast Spain, irrigated land has increased by more than 150% in the last 50 years. Local resources were already used extensively by the onset of twentieth century, and expansion has been based on the regulation and import of external water resources (from the Segura headwaters and Tajo basin) and over-exploitation of groundwater. In the main aquifer of the basin, the pumping rate is 4 times higher than the recharge rate. For the whole Segura basin, to which the Guadalentín belongs, 40% of the aquifers suffer from salinisation, affecting more than 60% of the resources used.
Agri Basin, Italy
The Agri Basin has a cool temperate Mediterranean climate, with strong differences between the coastline and the inland mountains. The inland countryside has a cooler climate and in the summertime the rainfall is above 150 mm; while on the coastline, for the same period, the rainfall is less than 100 mm. In addition, the summer is characterised by a strong drought and a mean monthly temperature of more than 23°C, in the warmest months.
Following the typical pattern of agricultural intensification in Mediterranean countries, the Agri Basin has experienced a relocation of profitable agricultural activities from the uplands to the plains, resulting in the marginalisation of agricultural land in the inland areas. Hence the lower parts of the Agri Basin have experienced a huge intensification in agriculture and expansion of irrigation, made possible by the construction of reservoirs that accumulate one third of the total surface water of the region. These investments have enabled the irrigation of great extensions of land and the realization of very profitable crops that have replaced the more traditional ones over large areas. The total irrigated area in the Agri Basin is 14,959 hectares (Istat, 2000), 35% of the entire irrigated surface of Basilicata, and is concentrated along the coast line (Lower Basin, about 80%) and in the remaining plain areas of the Basin. The further development of these high revenue crops, that also have an impact on employment, is currently hindered by the limited availability of water. This is accentuated by the fact that the period of maximum water consumption (July and August) coincides with high demand for water from other productive sectors (tourism, industry) and these are often favoured in comparison to the agricultural sector. Expansion of irrigation in the long term by increasing the number of reservoirs is not feasible because downstream aquifer recharge is already decreased by existing reservoirs, and coastal sediment aquifers are not being replenished.
In recent years, the considerable reduction of winter and autumn rainfall has caused a serious lack of water resources. As has been said, this problem especially affects the agricultural sector, which includes up to 70% demand for the total water resource. In the whole Basilicata Region it has been estimated that the lack of water for the agriculture sector is around 70 Mcms.
The drought effects are strongly aggravated by inefficient water management that has greatly reduced water availability. In fact the irrigation infrastructures are obsolete and the prevalent use of traditional irrigation techniques has caused an excessive consumption of water. Furthermore the water distribution system suffers from poor maintenance and a lack of modernization, with the result that the distribution losses amount to 30%, sometimes even 50% of the total. For this reason an increasing number of farms use forms of private storage. Data available on the provenance of utilized water show that in southern Italy 62% of the farms use wells (V. Gallerani - D. Viaggi, The state of irrigation in Italy). This increase in independent extraction is provoking serious negative effects on the water table, which is lowering and suffering intrusion of brackish water, and resulting in salinization. The combined effect of an elevated quantity of salts in the irrigation water, the high seasonal irrigated volumes and the elevated environmental water deficit increases the soil degradation risk.
Efficient use of irrigation methods (drop irrigation and sub-irrigation used on a regular daily basis with low volumes of irrigation water equal to the demands of the crop) is hindered by the type of water allocation to the farm, which is still available on a specific weekly basis. These point methods help to avoid the pollution of surface and underground water bodies and to conserve soil organic matter; because of the reduction of evaporating wet surfaces there is a reduction in mineralisation processes.
The situation in the water sector
is complicated at the administrative and institutional levels. First of
all, the water is also partially used by neighbouring regions, causing
an ongoing political debate about the appropriate exploitation of the
water itself. Secondly, the recently completed privatisation of the distribution
system has increased the uncertainty about the availability and price
of water resources.
Recently, in 2001, the Basin Authority of Basilicata has been appointed as the local government body to regulate water management, and one of its priorities has been the creation of an integrated plan for each local river basin, including the Agri.
The irrigated land in Greece presently
extends to more than 1,327,000 hectares or 32% of the national farmland.
Within this area about 37% is irrigated by public irrigation and drainage
systems, the rest by private systems. The country's food situation had
already substantially improved before its entrance in the European Union
in 1981. After that, agricultural development resulted in intensive arable
cropping on all fertile, irrigable lands. Water is applied by various
methods such as surface irrigation or by drip or sprinkler. Considering
that irrigation efficiency is in this order: drip >sprinkler>surface
irrigation, considerable water conservation is expected from the shift
from sprinkler and surface irrigation to drip irrigation.
When water is applied at high rates by sprinkler irrigation systems, soils can be affected by crusting due to the beating and dispersing action of the water drops. When dry soil is suddenly wetted, water moves into the soil peds from all directions, compressing the air in the pore spaces. The pressure of the entrapped air causes the peds to fall apart. The surface aggregates which collapse and slake down during wetting may form a slick layer of dispersed mud, sometimes several centimetres thick, which clogs the surface macropores and thus tends to inhibit the infiltration of water into the soil and the exchange gases between soil and the atmosphere. Such a layer is often called a surface seal. Upon drying, this dispersed layer shrinks to become a dense, hard crust that inhibits seedling emergence by its hardness and tears seedling roots as it cracks, forming characteristics polygonal patterns. Drip irrigation is considered more efficient in protecting soil aggregates and conserving water.
Irrigation on the island of Lesvos is largely limited to the plain areas of Kaloni, Gera, Eressos, Petra, Polyxnotos, and Thermi. Water is pumped from the existing aquifers or supplied from the Agiaso springs. It is mainly applied by surface or sprinkler irrigation causing problems of soil crusting. Salinization problems are mainly found in the Kaloni and Eressos plains.
Salinization is a major problem for the island of Lesvos as it is for the whole country. It is estimated that about 15% of the lowlands in Greece has been affected by salts or is sensitive to salinization. In the last four decades favourable soil and climatic conditions and the availability of ground or surface water has resulted in intensive farming of the lowlands. The development of high input agriculture in the plains provided much higher net outputs than those obtained from hilly areas or terraced agriculture.
Furthermore, the development of fast means of transportation and the availability of cheap holiday offers have encouraged the expansion of domestic and international mass tourism over the last 30 years. The increasing tourism exerts a significant impact on the environment and in particular on the land use patterns and the allocation of water resources. The most immediate changes in land use are (a) the shift in crop production to meet the tourist requirements, (b) the replacement of some traditional crops, and (c) the abandonment of low quality lands. The high demands for water consumption or other economic activities have increased the price of water, forcing up the cost of agricultural production. In addition, in many cases, low quality (with high electrical conductivity) water is used for irrigation. The need for intensification of agriculture to meet the high cost of production, the use of poor quality of water, the lack of drainage systems are in many cases responsible for soil degradation resulting from waterlogging, salinization, alkalinization, and soil erosion.
The transport and distribution of salts within a landscape and in a soil profile reflect the prevailing water balance conditions, and the depth of the ground water. Therefore, precipitation and evapotranspiration together with soil profile characteristics are important for the distribution of salts in a landscape and in a soil profile. A general decrease in precipitation and/or an increase in evapotranspiration will cause an increase in the area of soils affected by saline or sodic conditions around the country. This is because in those regions with high evaporation rates, capillary rise is accelerated and salts accumulated residually, where drainage is nearly absent. The extent to which this will happen at a local scale will depend on various factors controlled by the water balance, soil type, aspect and slope, and by the total salt and sodium inputs. Salinity problems are most severe in areas in Greece receiving rainfall between 300 and 600 mm.
Furthermore, the increasing rate of fertiliser application and the disposal of sewage waters and wastes of various origins (with different chemical composition and a hazard to the environment) result in an additional water-soluble salt load on the agricultural lands.
of how the indicators inter-relate
The Aridity index is, probably, the best indicator of irrigation expansion in terms of natural potential to increase crop productivity. The difference in Net farm income between irrigated and non-irrigated land explains why farmers decide to change crops and invest in new irrigation, although usually an institutional envelope in the form of water use policy/law is necessary to enable this. Water resources in form of local groundwater or external water resources that are related to Expenditure on water and expenditure on energy. The degradation that is induced is clearly manifested in changes in Groundwater depth, pumping/recharge ratio and the extent of salinized area, as well as Irrigation potential realised, as progressively there are more potential irrigable area that can not effectively irrigated.