Hydrological factors for phosphorus transfer from agricultural soils

Understanding and managing against phosphorus (P) transfer from agricultural soils to receiving waters is a multidisciplinary task in which the role of hydrology is particularly difficult to simplify. We review current knowledge to define the spatial and temporal controls on P transfer from agricultural soils via the various hydrological pathways. Rainfall intensity and duration and the interval between rainfall events are key temporal variables which influence the discharge (and hence P load) to receiving waters. In terms of understanding mechanisms, we postulate that levels of hydrological activity may be nominally classified at two levels. Level 1 activity occurs during light or little rainfall for a high proportion of time; in contrast, level 2 activity occurs less frequently but is more energetic and has a large capacity for P transfer over a small time period. The range of potential hydrological pathways of P transfer creates confusion because terminology varies. Often, process terms such as leaching or generic terms such as drainage are confused with hydrological pathways per se. Here we define the spatial variation in the hydrological pathways responsible for P transfer at two scales-soil profile and slope/fieldwhich subsequently can help the understanding of P transfers into the wider catchment, where problems occur. Our aim is to provide a simplified basis for classifying the otherwise complex hydrochemical regimes which result in P transfer