Irrigation return flows and phosphorus transport in the Middle Ebro River Valley (Spain)

Currently, there is an increased interest in the study of phosphorus (P) loss from soils aimed to understand and mitigate water eutrophication problems. The main objective of this study is to describe P losses in f ive irrigated agricultural watersheds considered as representative in terms of agricultural water management. Weekly water samples were collected during the 2007 hydrologic year (HY) at the watershed outlets and three P forms (total P, TP; total dissolved P, TDP; and particulate P, PP) were analyzed during irrigation (IS) and non-irrigation season (NIS). The P load per hectare was used to compare the study areas with other non-irrigated agricultural watersheds in Spain and Europe. Results indicate that most of the study areas showed increases in TP at higher flows. Annual TP concentrations were higher than the critical eutrophication threshold (0.02 mg L–1), with TDP being the dominant fraction. The TP was higher during the IS than during the NIS, except for the Arba River where seasonal TP concentration showed the highest values (0.237 and 0.275 mg L–1, respectively). Results also show that average TP yield (0.73 kg P ha–1 year–1) was higher than others reported on non-irrigated agricultural lands in Spain and Europe. This work is of great relevance and indispensable for guiding future research on P transfer aimed at establishing corrective measures to sustain irrigated agricultural productivity and surface water quality. Additional key words: agriculture; contamination; eutrophication; fertilization; water quality