Therefore, with regard to land usage, it is of fundamental importance to establish the reason for the perception, which persists even today, that soil irrigation poses a major problem in terms of sustainability. The nature of the damage must be analyzed with a view "to learning from the past". This paper will attempt to present a general overview of the topic, considering previous questions relating to our knowledge of soil characteristics in dry conditions, the impact of land systematization for irrigation, impacts of water application on soils and of drainage systems, and the hazard of further land degradation. Soil behaviour might constitute a cause of the non-sustainability of the system, but irrigation mismanagement could also account for soil degradation. Properly constructed irrigation systems, designed on the basis of a sound knowledge of the soils to be irrigated, followed up by effective management which takes into account soil behaviour, generally prevent possible adverse effects. The numerous technical solutions available for irrigation and drainage systems have to be adapted to prevailing soil conditions. However, all too often this kind of adaptation is carried out after the construction phase is completed and the irrigation system is in operation, rendering the process excessively empiric (Herrero 1992). Furthermore, while information on soils is essential, sociological and economic conditions cannot be ignored, hence the only viable approach is a holistic one (Rennes Workshop 1992).

The vulnerability of soils under irrigation is a classic topic. Nevertheless, the problem of salinity and sodicity in irrigated agriculture becomes more severe every year. Salinization appeared early in some 20 historic districts in which the problem exists. Worldwide, it is projected that, by the year 2000, 50% to 65% of currently irrigated cropland will suffer reduced productivity due to excess soil salinity (Buras 1992). Soil salinization is also related to land desertification processes. The sustainability of irrigated lands is sometimes questioned: For example, Heathcote (Heathcote 1983) stated that, owing to the salinity hazard, in arid zones fmancial budgets would be better employed in the development of industries than in the irrigation of lands. More to the point, given that these arid areas of the world need to produce food, the vulnerability of soil under irrigation must be taken into account in order to avoid land degradation. Moreover, in many countries, the growth of cities occurs at the expense of the irrigated lands which surround them, in which long-established and working irrigation systems exist. Consequently, agricultural expansion must consider the use of new lands where irrigation systems will be required, even though mismanagement of such systems frequently induces land degradation.

The Research on which this work is based was carried out with the support of the Dirección General de Investigación Científica y Técnica (DGICYT), Project number PB 90-0090-C02-02.

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Peer reviewed