AbstractEnhanced levels of phosphorus (P) are often cited as the primary cause of eutrophication in flowing and standing water bodies in the UK. A legislation has restricted point source outputs of P in recent years, the importance of diffuse P inputs from agricultural land has been recognised. This research focuses on sediment and particulate-P (PP), and their delivery to the fluvial system from agricultural land via field drains.
Sub-surface field drains are widely installed across the UK. They reduce the period of time that land is waterlogged, allowing more flexible land use and longer access time for heavy machinery. Soil macropores allow rapid transit of surface sediment to field drains and beyond without significant alteration in character, a phenomenon known as bypass flow. Consequently, PP can be removed from the agricultural system via field drains, increasing nutrient levels in receiving waters.
The project had four aims. First, to identify and quantify the source of drain sediment within the soil profile. Secondly, to quantify PP loss via field drains at the catchment scale. Thirdly, to develop a replicable methodology to assess sediment and PP translocation at multiple locations. Finally, to map the risk of PP loss from field drains in England and Wales. The research was conducted in two parts. Stage one addressed the first two aims, using catchment scale monitoring, sediment fingerprinting and an unmixing model. The last two aims were addressed in stage two, which involved laboratory based rainfall simulation and national distribution of soil and drainage data from the SEISMIC database and sources in the literature.
The study was able to address all four scientific aims outlined. Field studies of four drain networks showed that upwards of 70% of sediment lost was topsoil derived, with spatial and temporal variations in source over the monitoring period. Within the context of the surrounding catchments, the drain networks were a major source of both sediment and PP to surface waters, losses far exceeding those from channel banks. Rainfall simulation experiments on a suite of soil series demonstrated variations in sediment loss both within and between series, which were largely attributed to differences in land use and CaCO3 content. Using the available data to identify key factors influencing sediment delivery to field drains a combined distribution map of these factors across England and Wales was produced at a 5 km2 resolution. This showed that the highest risk areas for sediment and PP loss were in Eastern England, East Anglia and the Welsh Borders. Low risk areas included much of Western England and Wales due to less widespread distribution of high-risk soils.
An original contribution to research was made by being able to identify spatial and temporal variations in sediment delivered to field drains, and by producing a national distribution map of risk areas that accounts for controlling factors as well as soil type. However, many issues surrounding the loss of sediment and PP through field drains remain unresolved. In particular, it is unclear from the data available whether drained land constitutes a greater risk in terms of sediment and PP loss than equivalent undrained land. Further work is suggested to refine the national risk distribution map presented, and to investigate ancillary issues highlighted during the progress of the experiments.
|Date of Award||2001|
|Supervisor||Ian Foster (Supervisor), Joan Lees (Supervisor) & Roz Jackson (Supervisor)|