Watercourse pollution due to surface runoff following slurry spreading, part 2: Decision support to minimize pollution

Procedures for avoiding or minimizing pollution of watercourses by surface runoff(overland flow) during or after slurry spreading are explored. Output from simulations with the Swedish soil water model SOIL for fields with a range of conductivities and drainage system efficiencies las described in P art 1) have been analysed with particular reference to occurrence of surfac e runoff. The analytical procedure was linked to a simple model of the dura tion of pollution risk following slurry spreading. Development of this mode l, which simulates the decline in potency of slurry components due to micro bial degradation, volatilization and the effect of gentle rain assisting in filtration, is also described. Results show the greatest opportunities for reducing pollution from selecting fields for spreading slurry which are not susceptible to surface runoff because they have a high 'profile conductivi ty' (a combination of the hydraulic conductivities of the soil layers and t he efficiency of the field drainage system if present). It is impossible to avoid pollution if slurry is spread on a field with a low profile conducti vity, so slurry spreading on such fields should be avoided altogether, part icularly in winter. For fields with intermediate conductivities in the rang e 5-10 mm d(-1), there is a benefit from selecting spreading days which mee t certain weather and soil wetness conditions. (C) 2000 Silsoe Research Ins titute.