Phosphorus Loss from a Mixed Land Use Catchment in Northwest Spain

This work examined P loss from a mixed land use catchment in northwest Spain and how it was related to hydrological factors. A stream draining a mixed land use catchment of 16 km 2 in northwest Spain was monitored for 5 yr. Water samples were taken more frequently with increased stream flows. A total of 132 runoff events of different magnitudes were analyzed in this study. Mean annual P loss from the catchment was 11.5 kg km −2 , about 68% being transported during runoff events, which only contributed to 29% of the flow. The contribution of runoff events to particulate P and dissolved P losses was 76 and 46% of total losses during the study period, respectively. A high interannual variability in P loss was observed, the differences being related to rainfall amount and distribution. At a seasonal scale, the highest P loads and concentrations were observed during rainy seasons; the lowest P loads occurred in summer because they occurred with stream flow. Particulate P accounted for the greatest proportion (74%) of transported P. There was a high variability in P transported between runoff events, with maximum discharge and discharge increase being the most important variables explaining total P concentration in these events through their effect on suspended sediment concentration.