Simulating the effects of improving field drainage on environmental impacts of inorganic phosphorus pollution of waterbodies from livestock farming

A simulation modelling study was carried out of the effects of poor field drainage on raising losses to the environment of inorganic phosphorus during livestock grazing and slurry spreading. The study used the weather-driven colloid transport version of the MACRO model, as previously calibrated and tested using measurements of inorganic phosphorus in drainage water at experimental field sites. Results showed substantially higher losses with poor drainage compared to those with a properly functioning drainage system. Simulated mean losses from a single spread of slurry with poor drainage were typically at least double those with good drainage; for grazing in winter simulated mean losses were more than four times as high. These high losses arise because poor drainage raises the water table, leading to rapid movement of inorganic phosphorus attached to colloids through large water-filled soil pores. In contrast, with good drainage leading to a lower water table, most water movements are through small soil pores where colloids with attached phosphorus become trapped. Although losses were higher in winter months (when grazing and slurry spreading would still need to be restricted even with good drainage), the benefits of improved drainage were demonstrated for all months of the year, for three soil types and at two contrasting weather sites.