Phosphorus transport into a stream draining from a mixed land use catchment in Galicia (NW Spain): Significance of runoff events

In this study the water draining from a mixed land use catchment of 16 km2 in NW Spain was monitored over a 5-year period (2004/2005–2008/2009) to determine the temporal variability in phosphorus (P) transport, to examine the P fluxes and controlling factors, and to analyse the relationships between discharge and P (PP: particulate phosphorus and DP: dissolved phosphorus) at event scale. A total of 132 runoff events was analysed, providing an extensive dataset on P values under different hydrological conditions. Mean annual P export from the catchment amounted to 184 kg equivalent to 11.5 kg km−2, a value relatively lower than those obtained in other mixed land use catchment. The PP accounted for 74% of the total phosphorus (TP). The intra- and inter-annual variability in P transport was high due to the fact that most of P was discharged during a short-time period: 35% of PP load and 19% of DP load occurred in only 2% of the time. Runoff events contributed to 68% of P exported during the period of study. A high variability in P transport during different runoff events was observed. Phosphorus export increased with the magnitude of runoff events and rainfall amount, suggesting that these could be the key factors controlling P yield. Erosion was also an important driving force behind P export in the catchment. The relationships between PP and DP and discharge resulted in different hysteresis patterns. The relationship PP-discharge was mainly characterised by clockwise behaviour, linked to sediment in the surface runoff, while DP-discharge was predominately anticlockwise due to transport in subsurface flow, showing different pathways of transport between PP and DP.