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

► The APEX model provided adequate simulations for the annual volume of irrigation return flows and its N loads. ► APEX simulated that irrigation improvement was the best management option to reduce N loads in the IRF of the three watersheds. ► APEX simulations properly identified the main soil and crop N polluters within the watersheds. One of the main constraints of irrigated agriculture is off-site N pollution 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 best management practices for reducing off-site N 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 NO 3-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 irrigation management (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 irrigation management will increase IRF NO 3-N concentrations by 19% in La Violada and will decrease or will remain the same in the other two watersheds. APEX simulations 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 best management strategy to decrease off-site N pollution while maintaining or increasing crop yields in the three studied Mediterranean watersheds.