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 (as described in Part 1) have been analysed with particular reference to occurrence of surface runoff. The analytical procedure was linked to a simple model of the duration of pollution risk following slurry spreading. Development of this model, which simulates the decline in potency of slurry components due to microbial degradation, volatilization and the effect of gentle rain assisting infiltration, 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 conductivityʹ (a combination of the hydraulic conductivities of the soil layers and the 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 conductivity, so slurry spreading on such fields should be avoided altogether, particularly in winter. For fields with intermediate conductivities in the range 510 mm d-1, there is a benefit from selecting spreading days which meet certain weather and soil wetness conditions.