Seasonal variability of NO3− mobilization during flood events in a Mediterranean catchment: The influence of intensive agricultural irrigation

The temporal variability, hysteresis loops and various factors involved in the mobilization of nitrates (NO3−) have been studied for a 3-year period at the Flumen River basin. Multivariate techniques (cluster analysis and pearson correlation matrix) were used to establish the relationship between the water discharge and NO3− flushing, as well as to identify the agricultural and hydrometeorological parameters that influence its different mobilization trends. The relationship between changes in the NO3− concentration (ΔC) and the overall dynamic of each hysteresis loop (ΔR) was also analyzed in order to describe the NO3− trends according to the water discharge. A general dilution pattern of the NO3− concentration was noted in the Flumen River with respect to the degree of water discharge caused by irrigation return flows. While fertilization increased the NO3− concentration, the beginning of the irrigation season contributed to its dilution. However, in case of the NO3− load, the maximum values occurred during high flow periods in the irrigation period, which suggested the influence of the irrigation flow on the NO3− mass. The NO3− load increased to 2753t and 1059t during the first and second phases of the study period, respectively, with an average specific yield of 1.33tkm−2 y−1. The NO3− transport in the first phase of the study was 1722t during the irrigation season and 1031t during the non-irrigation period. Only 348t (13%) of NO3− was exported during the flood events. However, in the course of the second phase of the study, the NO3− load was 733t during the irrigation season and 326t during the non-irrigation period. In this case, 610t (57%) of nitrate was transported during the floods. These results revealed the clear influence of irrigation return flows on the NO3− response in Flumen River.