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of sediment export shows there are ‘hotspots’ within the catchment which have
much higher sediment export rates up to 0.5 t/ha/year (Figure 1). For example, one
area in the western part of the catchment shows high concentrations of suspended
sediment in headwaters, and significant diatom activity in spring, associated with
dairy farming including forage maize on underdrained heavy clay soils. Mitigation
options here will need to focus on careful application of manures to avoid incidental
P ransfers, avoidance of poaching damage to grassland swards and improved
husbandry of the maize crop to prevent sediment mobilisation after harvest (such as
growing early harvesting varieties, establishment of winter cover crops and rotating
the crop around the farm on less risky fields).
In the headwaters of the Wensum catchment, problems of siltation are associated
with surface run-off from the light-textured and relatively erodible soils. Measures
here therefore need to be focussed on minimising surface run-off and erosion risk.
Heavy clay soils, of which a significant proportion would be expected to be drained,
occur in the centre of the catchment and estimated P export from this area was
therefore higher. Mitigation would require reduction in P inputs (for example by
reducing livestock manure inputs and by more careful adjustment of fertiliser inputs
for P derived from manures), and avoiding applications of P fertiliser or manures
during the winter. Although PSYCHIC indicated that a high proportion of the P input
was as pig and poultry manures, subsequent discussion indicated that a great deal
of the poultry manure was incinerated at a local power station, and it was thought
that pig numbers had fallen. Such local information is therefore necessary in the
formulation of the risk assessment as well as the estimated impact of any proposed
mitigation strategy.
Figure 2: Mobilisation of P by surface pathways in the Teme catchment
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