Irrigation-induced nitrate losses assessed in a Mediterranean irrigation district

Irrigated agriculture is crucial for productivity of major crops (mainly cereals) grown in Mediterranean countries, where extended and prolonged drought conditions adversely impact productivity. Under such conditions, irrigation and rainfall events combined with nitrogen (N) fertilization can induce nitrate (NO3) losses in irrigation return flows (IRFs). Such water-induced NO3 losses in IRFs were assessed during the 2007–2010 hydrological years in the 9495ha of the Akarsu Irrigation District (AID) of southern Turkey, with daily monitoring at three drainage gauging stations to quantify flow rates, NO3 concentrations and loads. Climatic data, soil characteristics, fertilizer N application rates to major crops, cropping patterns, and irrigation and rainfall depths were also recorded. Nitrate concentrations were higher in IRFs during winter months, ranging between 37 and 44mgNO3L−1 on average, compared to the concentrations in the irrigation season (10–23mgNO3L−1). Since most of the fertilizer N was applied in winter and early spring to wheat (2/3 of 195kgNha−1) and first crop corn (1/3 of 340kgNha−1) as preplant and surface applications; NO3 concentrations were high during these seasons because of the limited N consumption of these crops in their early growth stages. However, the NO3 load distributions in winter and summer months were similar. Annual loads of 39.7, 29.3, 55.3 and 55.2kg NO3–Nha−1 were measured in the 2007–2010 IRFs, respectively, with 45 to 57% occurring during the irrigation seasons. The consistent high NO3 over 4 years point to the potential to reduce losses and associated N pollution through better crop, irrigation and N fertilizer management. Well-established fertilizer and irrigation water management plans are critical to reduce NO3 pollution risks in Mediterranean irrigated lands.