44 Pag., 3 Tabl., 3 Fig.\ud \ud The definitive version is available at:\ud http://www.sciencedirect.com/science/journal/03783774This study analyzes the effects of irrigation modernization on water conservation, using the Riegos del Alto Aragón (RAA) irrigation project (NE Spain, 123354 ha) as a case study. A conceptual approach, based on water accounting and water productivity, has been used. Traditional surface irrigation systems and modern sprinkler systems currently occupy 73% and 27% of the irrigated area, respectively. Virtually all the irrigated area is devoted to field crops. Nowadays, farmers are investing on irrigation modernization by switching from surface to sprinkler irrigation because of the lack of labour and the reduction of net incomes as a consequence of reduction in European subsidies, among other factors. At the RAA project, modern sprinkler systems present higher crop yields and more intense cropping patterns than traditional surface irrigation systems. Crop evapotranspiration and non-beneficial evapotranspiration (mainly wind drift and evaporation loses, WDEL) per unit area are higher in sprinkler irrigated than in surface irrigated areas. Our results indicate that irrigation modernization will increase water depletion and water use. Farmers will achieve higher productivity and better working conditions. Likewise, the expected decreases in RAA irrigation return flows will lead to improvements in the quality of the receiving water bodies. However, water productivity computed over water depletion will not vary with irrigation modernization due to the typical linear relationship between yield and evapotranspiration and to the effect of WDEL on the regional water balance. Future variations in crop and energy prices might change the conclusions on economic productivity.Peer reviewe

42 Pags., 9 Tabls., 5 Figs. The definitive version is available at: http://www.sciencedirect.com/science/journal/03783774One of the main constraints of irrigated agriculture is off-siteNpollution due to export of nitrate in irrigation return flows (IRF). Models capable of simulating the growth of crops and the N loads in IRF as affected by irrigation and N fertilization may be valuable tools in watershed studies. The Agricultural Policy Environmental eXtender (APEX) model was used to assess bestmanagement practices for reducing off-siteN loads in the IRF of three Mediterranean irrigated watersheds (Akarsu in Turkey, La Violada in Spain and Sidi Rached in Algeria). The watersheds (ranging from 4013 to 10,971 ha) were monitored along three hydrological years to determine the volume of IRF and the NO3-N concentrations and loads in IRF. APEX was calibrated with the data of the first two years and validated with the last year's data. APEX adequately simulated crop evapotranspiration and the volume of IRF and N loads in the IRF (errors < 20%). Simulated annual values were in general more accurate than simulated monthly values. APEX predicted that improving irrigationmanagement (change of irrigation system and/or scheduling) will decrease N loads in IRF over current values by 45% (Akarsu), 40% (La Violada), and 8% (Sidi Rached). However, improved N fertilization only will reduce N loads in IRF by 17% (Akarsu) or below 5% (La Violada and Sidi Rached). Improving irrigationmanagement will increase IRF NO3-N concentrations by 19% in La Violada and will decrease or will remain the same in the other two watersheds. APEXsimulations identified the main soils (shallow and low water holding capacity soils) and crops (heavily fertilized or shallow-root crops) N polluters within the studied watersheds. Overall, APEX simulated that the improvement of irrigation performance was the bestmanagementstrategy to decrease off-siteNpollution while maintaining or increasing crop yields in the three studied Mediterranean watersheds.This study was supported by the European Commission research project (INCO-CT-2005-015031).Peer reviewe

30 Pags., 3 Tabls., 9 Figs. The definitive version is available at: http://www.sciencedirect.com/science/journal/01678809Irrigation district salt balances identify the main sources and sinks of salts and quantify salt loads in irrigation return flows. Salt balances were performed in La Violada Irrigation District during the 80s (1982–1984), 90s (1995–1998) and 00s (2006–2008) decades. Total Dissolved Solids (TDS) and loads in irrigation return flows were related with changes in irrigation performance and infrastructures during these decades. TDS increased linearly to increases in Irrigation Consumptive Use Coefficient (ICUC) (P < 0.01), and decreased with increases in Drainage Fraction (DRF) (P < 0.001). The salt balances revealed that 82% of the exported salts originated from soil gypsum dissolution. Annual salt yields in return flows were high and similar in the 80s and 90s (about 19.3 Mg/ha year), and halved in the 00s (9.9 Mg/ha year) due to comparable reductions in irrigation and drainage volumes derived from a better irrigation performance. Irrigation season salt yields were high and steady around 14.1 Mg/ha for ICUC < 51% and DRF > 66%, and decreased exponentially for values above and below these thresholds, respectively. Therefore, the key management strategy to reduce salt discharge to downstream areas is to decrease drainage volumes by improving irrigation management.This work was sponsored by the Spanish Ministry of Science and Education project AGL2006-11860/AGR, the European Regional Development Fund (FEDER) and the European Union project INCO CT-2005-015031.Peer reviewe