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The review analysis of twenty two irrigation efficiency (IE) studies carried out in the Ebro River Basin shows that IE is low (average IE)(avg)(= 53%) in surface-irrigated areas with high-permeable and shallow soils inadequate for this irrigation system, high (IE)(avg)(= 79%) in surface-irrigated areas with appropriate soils for this system, and very high (IE)(avg)(= 94%) in modern, automated and well managed sprinkler-irrigated areas. The unitary salt (total dissolved solids) and nitrate loads exported in the irrigation return flows (IRF) of seven districts vary, depending on soil salinity and on irrigation and N fertilization management, between 3-16 Mg salt/ha x year and 23-195 kg NO)(3) (-)-N/ha x year, respectively. The lower nitrate loads exported from high IE districts show that a proper irrigation design and management is a key factor to reduce off-site nitrogen pollution. Although high IE's also reduce off-site salt pollution, the presence of salts in the soil or subsoil may induce relatively high salt loads (>or=14 Mg/ha x year) even in high IE districts. Two important constrains identified in our revision were the short duration of most surveys and the lack of standards for conducting irrigation efficiency and mass balance studies at the irrigation district level. These limitations {emphasize the need for the establishment of a permanent and standardized network of drainage monitoring stations for the appropriate off-site pollution diagnosis and control of irrigated agriculture.

Under semiarid Mediterranean conditions irrigated maize (Zea mays L.) has been associated with nitrate pollution of surface water and groundwater. Cover crops grown during the intercrop period of maize could reduce N leaching. A 2-yr experiment was conducted in drainage lysimeters with three cover crops: barley (Hordeum vulgare L), winter rape (Brassica rapa L.), or common vetch (Vicia sativa L.). Bare soil was used as control treatment. Maize was fertilized with 300 kg ha −1 N in the control, and this amount was reduced aft er a cover crop according to the N content in the aboveground cover crop biomass. Barley and winter rape biomass had a higher N content than vetch (130-170 vs. 50 kg ha −1 ). Th e vetch treatment did not reduce N leaching or aff ect maize yield. Th e barley and winter rape treatments reduced N leaching by 80% compared to the control (25 kg ha −1 yr −1 ) mainly due to a reduction of NO 3 -N concentration in drainage. Maize yield was reduced by 2.7 Mg ha −1 aft er barley and winter rape but still high (≈14 Mg ha −1 ). Th is reduction was due to an N defi ciency caused by lower soil N in spring aft er the cover crop and insuffi cient N mineralization and/or lack of synchronization with maize N uptake. To use nonlegume winter cover crops to reduce N leaching in monoculture maize it is necessary to consider that N mineralization may not be suffi cient to fulfi ll maize N requirements and N fertilizer adjustment tools should be developed.