Irrigation can be the most expensive operation both in monetary terms and in terms of energy input. The cost of a cubic meter of water in the Mediterranean area, for instance, can exceed €0.50 (desalinized sea water costs in Cyprus €0.70/m3, Y. Papadopoulos 2002, personal communication) while the direct energy input to lift and pressurize water from deep wells can exceed 4 MJ/m3, with a corresponding cost, just for electric energy, of over €0.2/m3. Furthermore, water availability is ever decreasing and competition is mounting among the various uses - domestic, agricultural and industrial - with agriculture taking the lion share, namely, up to 90%, and using it rather inefficiently on the average. As a consequence a careful assessment of real crop water requirements, an enhancement of conveyance and application efficiencies, a better management and whenever possible the adoption of a deficit irrigation schedule are needed: again, human inputs for plant water status monitoring, correct irrigation management, irrigation system maintenance, participatory irrigation management and capacity building are called in substitution of physical inputs.
More research and demonstration activity should be devoted to water harvesting, which can be considerably useful not only in reducing irrigation requirements but also in the reduction of overland flow and consequently in the protection of soils from water erosion, as well as in leaching soils from salts accumulated with irrigation water. The solution of tied ridges, or diked furrows, to be obtained either by animal energy or when possible with the use of mechanical equipment, has been demonstrated to be very useful under a variety of conditions (e.g. Rizzo et al. 1994; Shapiro and Sanders 1997).
The attitude of 'alternative agriculture' movements to irrigation ranges from outright refusal 'not to alter natural conditions', to prohibition of using plastic pipes, maybe in the belief that metal or asbestos-cement pipes are less polluting, to reject of re-using treated domestic wastewaters, to acceptance, in a more realistic mood.
The selection and sizing of the most appropriate irrigation system, as a function of specific human, climatic, economic, agronomic conditions are critical in the process of optimizing the resources. Energy requirement, resulting from the sum of direct energy to lift and pressurize water plus indirect energy for manufacturing and installing the irrigation system, is a generally overlooked, yet important factor in the selection of irrigation methods (Sardo 1982). To fully appreciate the data in Fig. 4 it is useful to consider (a) that water is considered available without any need for lifting, e.g. from a well, and (b) as a reference, that in the UK the overall energy input for beet production ranges between 15.72 and 25.94 GJ/ha (Tzilivakis et al. 2005).
The negative water balance of hydrological basins in many areas is a factor inducing to manage water more carefully and take advantage whenever possible of the available non-conventional water resources. In particular, irrigating with domestic wastewaters after a primary or secondary treatment can offer several advantages, including the availability of nutrient-rich water, generally free of pollutants dangerous to crops, unlike industrial wastewaters, and the savings linked to the elimination of the expensive tertiary treatment (Hamdy and Karajeh 2001).
Also irrigation with brackish and saline waters is actively explored, with teams studying plant response to irrigation at various salinity levels and implications on the soil and the environment (e.g. INCO-DC 2001; DRC 2002). Results so far achieved show that unsuspected possibilities are open for the use of large, till now neglected unconventional water resources and that traditional guidelines based on crop salinity energy GJ
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tolerance are often exceedingly restrictive. An accurate management when using brackish waters is required to make sure that a correct salt balance is maintained in order to protect soil fertility (Hamdy 1999). This is particularly true with supplemental irrigation, when reduced volumes of irrigation water are applied while the bulk of incoming water is provided by rains, securing a sufficient salt leaching.
Wallender (2007) gives a very interesting example of an integrated model permitting a simulation linking hydrologic, agronomic and economic aspects of irrigation in San Joaquin Valley in California, taking into account soil and water salinity. In his words the agricultural production model simulates agricultural production decisions at the water district level. It is assumed that growers maximize profits subject to the pertinent resource and environmental constraints. Given initial conditions on surface water allocation and soil, surface water, and groundwater salinity, the agricultural production model simulates agricultural production on an annual basis and produces spatially distributed information on cropping patterns, water applications, groundwater pumping, irrigation efficiencies, and crop yields. The output from the agricultural production model is subsequently used by the hydrologic model to simulate the impacts of these management decisions on the natural system. His optimistic conclusion is of particular interest: [T]his decade long effort to develop an integrated, scale dependent analysis is the start of an effort to define sustainability of irrigated agro ecosystems in terms of the quantity and quality of the soil, deep vadose zone, groundwater, and surface water; the agronomic and ecosystem productivity; and, finally, the economic viability.
• Economic: in order to achieve the highest net income, when designing an irrigation system a trade-off is required between application uniformity, labour requirement and system cost: a higher uniformity and a lower labour requirement are in fact linked to higher capital costs but permit subsequent savings in terms of water and costs for labour and energy. One further aspect to be considered is the cost for pressurizing the irrigation system, which may be not relevant in those regions where only supplemental irrigation is practised, or whenever pressure is obtained by gravity, but influences heavily the budget when volumes of about 5,000 m3/ha/year or more must be lifted and pressurized. Third, evaporation losses depending on the selected system can be of importance, particularly in those arid or semiarid regions where they can account for 30% or more. A typically 'win-win' solution in many cases can be to reduce to the possible extent water pressure at the nozzles, thus saving energy and money while reducing evaporation; however, the risk is enhanced of large water drops splashing and forming a crust on the soil surface and higher precipitation intensity due to the reduced jet radius determining some overland flow. When carefully managed, irrigation can be economically useful even in humid areas since it determines a reduction in production risk.
• Environmental: all those considerations applying to economic impact apply to energy input as well, since savings in water quantities or in required pressure automatically translate into energy savings. Furthermore, savings in water volumes alleviate the burden on the often negative balance of water resources and reduce the risk of water logging and salinization: it is not necessarily true that excess water is beneficial for salt leaching - it is beneficial only under some conditions while it can magnify the risk of salinization whenever a high water table or a low-permeability soil horizon is present. When associated with fertilizer application irrigation permits to increase fertilizers efficiency, provided that application uniformity is sufficient, thus reducing the applied quantities and avoiding/reducing leached amounts.
• Social: irrigation is a powerful tool in the improvement of farmer's social conditions, not only in increasing their income and productivity but also in reducing the risk depending on climate vagaries. It also adds to social stability by enhancing the employment and food security, and concurs to enhance the cultural level of irrigators. Further aspects refer to the quality of aquifers and watercourses, which can be protected or impaired by an appropriate/inappropriate irrigation management.
Synthesis of Subsection 3.2.3 - Irrigation is necessary for achieving high yields in arid or semiarid areas and reducing risk in humid areas; however, it is very demanding in terms of economic and energetic costs. It is necessary to find a trade-off between capital investment and management costs depending on local conditions. A sustainable irrigation management requires to consider salt balance and soil erosion. Water harvesting is very useful to reduce irrigation requirements and erosion risk. A reassessment of water quality for irrigation is needed, particularly when applied volumes of irrigation water are modest compared to rainfall.
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