trol structures may be installed on tile drains to manage the flow of water to both reduce runoff and help provide water for growth when needed by plants. Drainage ditches may be vegetated to help prevent erosion, catch eroded sediments and take up nutrients. Both drainage ditches and tile drains may be directed into constructed or re-established wetlands to  process  nutrients  and  agrochemicals.  However  such practices require a significant investment and to establish wetlands some land has to be taken out of production, pre­senting barriers to adoption of these mitigation measures.

3.1.1.4 Environmental consequences of irrigation
Although irrigation has had tremendously beneficial effects on crop yields, irrigation systems can have detrimental en­vironmental, economic and social effects upstream of the system, at the site of the irrigation system and downstream (Hillel and Vlek, 2005). Poorly managed irrigation can cause problems of salinization (buildup of salts), water-log­ging, erosion and soil crusting.
     Irrigation water applies water-borne salts to the soil sur­face and if there is not sufficient drainage, salts accumulate and they can markedly reduce the fertility of the soil. Irriga­tion in naturally saline soils, without careful management of drainage, may mobilize salts to the root zone, impairing plant growth. The water drained from agricultural fields where salinization is an issue, whether from the buildup of salts delivered by irrigation or through the mobilization of native salts, may have a high salt content which can cause environmental problems in the receiving waters and associ­ated wildlife, e.g., bird deformities resulting from selenium in the drainage water (Letey et al., 1986). Soil salinization affects an estimated 1 million ha in the EU, mainly in irri­gated fields of Mediterranean countries and is a major cause of desertification. Similarly, there are approximately 10 mil­lion ha in the western US affected by salinity-related yield reductions (Barrow, 1994; Kapur and Akca, 2002).
     In the last half of the last century, extensive work had been carried out in the US and globally, to research, diag­nose, improve and manage salt-affected soils on irrigated agricultural lands (Miles, 1977; Moore and Hefner, 1977; Ayers and Westcot, 1985; Hoffman et al., 1990; Rhoades, 1990ab; Tanji, 1990; Rhoades et al., 1992; Umali, 1993; Sinclair, 1994; Rangasamy, 1997; Rhoades, 1998, 1999; Gratan and Grieve, 1999). Modern management techniques are being deployed to improve water use efficiency to over­come these problems, by targeting the water more accurately and by using the most appropriate application technologies. Productivity can often be maintained in salt affected areas through careful application of appropriate practices (Miles, 1977; Hoffman et al., 1990).
     Soil crusting can be caused by the use of certain irriga­tion systems. For example, center-pivot sprinkling irrigation in the Coastal Plain area of the U.S has caused soil crusting arising from the sprinkler drop impact energy (Miller and Radcliffe, 1992). The water application rates of this high en­ergy impact irrigation system are often limited by low infil­tration rates due to crust formation. Changes in application practices can reduce this problem (Singer and Warrington, 1992; Rhoades, 2002). Erosion can also be caused by in­appropriate irrigation practices (e.g., Carter et al., 1985; Carter, 1986).

     Irrigation can create problems resulting from the re­moval of water from other locations. Abstraction of water from rivers can cause major reductions in water flow with consequent negative impacts on river and associated wet­land habitats. The drying and salination of the Aral Sea as a result of abstraction of water for irrigation from the main rivers feeding the sea is a particularly stark example of off-site impacts (Micklin, 1994, 2006). Similarly, abstraction of water for irrigation from boreholes can cause a lowering of the water table with adverse effects on neighboring natural wetland areas. Society needs to assess the overall impact of irrigations schemes, not just the agricultural cost and ben­efits (Lemly et al., 2000). Various strategies are needed to ensure long-term sustainability of irrigation including re­stricting irrigation to high value crops and using the best equipment and soundest management practices (Hillel and Vlek, 2005.

3.1.1.5  Environmental consequences of the adoption of genetically engineered crops
Transgenic crops are those created through the techniques of biotechnology to select a gene from one species and in­corporate it to the same or different species (also called genetically modified, GM, genetically modified organisms, GMOs, or genetically engineered, GE). These new cultivars will have new properties. Accordingly, the environmental effects of each new transgenic variety may differ and regula­tory systems have to evaluate each new variety individually. Current GE crops have to undergo an extensive environ­mental risk assessment throughout NAE (see e.g., Directive 2001/18/EC for EU requirements (www. europa.eu.int/eur-lex/pri/en/oj/dat/2001/l_106/l_10620010417en00010038. pdf) and http://usbiotechreg.nbii.gov/lawsregsguidance.asp for EU and US requirements).
     Currently, most transgenic crops are either classified as insect resistant (IR) or herbicide tolerant (HT). Cultivars with other characteristics have been approved for use in parts of NAE, or are in development, including disease re­sistance, pharmaceutical chemical production, abiotic stress tolerance (drought or salinity), nutritional characteristics (e.g., fatty acid composition) and storage characteristics (e.g., increased shelf life after harvest). (AGBIOS data base lists crops and traits that have been approved by nation: http://www.agbios.com/main.php).
     A general review of the 10-year history of cultivation and testing of the currently planted genetically engineered crops concludes that there is no scientific evidence that the com­mercial cultivation of GE crops has caused environmental harm (Sanvido et al., 2006) though they note that there are no requirements to monitor for potential effects where GE crop varieties have been approved for unregulated use. This conclusion is not accepted by some stakeholders. Because of the nature of the technology it has raised greater public and governmental concerns than "conventional" plant breeding, resulting in closer scrutiny of potential environmental ef­fects. Recommendations exist for further study of the envi­ronmental effects of transgenic crops (FAO, 2003).
     Insect resistant crops are based upon the inclusion of a gene derived form bacteria resulting in production of a pro­tein (Bt) that is toxic to certain groups of insects (moths and butterflies). As the toxicity is limited to particular groups