APEX simulation of best irrigation and N management strategies for off-site N pollution control in three Mediterranean irrigated watersheds

One of the main constraints of irrigated agriculture is off-site Npollution 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-site N loads in the IRF of threeMediterraneanirrigatedwatersheds (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-site Npollution while maintaining or increasing crop yields in the three studied Mediterraneanwatersheds.Peer Reviewe