Validation and modification of the phosphorus loss index as applied to a small catchment

Environmental legislation in many countries requires the development of phosphorus indices to determine the risk of P loss from soils to water. The concentrations of total phosphorus (TP), molybdate‐reactive phosphorus (MRP) and particulate phosphorus (PP) and the movement of these P forms were determined in a small catchment in Galicia (north–west Spain) with the aim of establishing an index of P loss. The discharges and concentrations of the different P forms were measured at different points in the river basin. The information was used to estimate the accuracy of a P index derived from the model described by Gburek et al. (2000). The amount of P lost over the 5‐month period of study was ca. 60 g/ha; in essence, the overall loss was low. The P losses differed throughout the study period and between sampling points, and some areas of the catchment acted as P sinks. This was probably associated with the transport of deposited material by the rivers. Of the two components included in the index, the source factor was consistent with the source areas in the catchment as identified by water analysis. However, the transport factor was not very consistent with the field data and must be reconsidered.