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Total P inputs in fertilisers and manures were very similar in the Avon, Wensum and
Teme catchments (13–15 kg/ha/year), but dropped to 6 kg/ha/year in Bassenthwaite
due to the high proportion of rough grazing. In all catchments, > 70% of the total
P inputs to land were derived from spread manure and excreta deposited during
grazing. The manure derived largely from pig and poultry enterprises in the Wensum
catchment, and from cattle in the other catchments.
The predicted estimates of total sediment and P export at the outlet of each catchment
varied from 0.03–0.32 t/ha/year and from 0.59–1.34 kg/ha/year, respectively
(Table 1). Of the total P export, 13–73% was predicted to be derived from diffuse
agricultural sources. The very high diffuse P contributions (c. 70%) in the Teme and
Bassenthwaite catchments were associated with larger amounts of P attached to
eroding soil particles and mobilised from fresh applications of manures (incidental
P). Hence sediment exports were at least two- to three-fold greater in these two
catchments than in the Avon and Wensum catchments. Farmyards contributed most
P in the Teme catchment. In the Wensum and Bassenthwaite catchments, most P
is delivered by land underdrainge systems, whilst in the Avon and Teme there is a
significant surface run-off component (Table 1). Knowledge of the relative proportions
of surface and sub-surface export helps identify where sediment and P mitigation
options will be most cost-effectively targeted.
Figure 1: Spatial distribution of sediment export in the Hampshire Avon
catchment
Total catchment export represents only the average of a range in 1 km 2 export
values and can be misleading without an analysis of the spatial distribution of export
rates across the catchment. A spatial distribution analysis also provides a means of
targeting measures to areas where the risk of sediment and P loss risk is greatest.
For example, in the Avon catchment the total export of sediment is very low (30 kg/
ha/year) due to low stream density (Table 1). However, an analysis of the pattern
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