Erosion and Sediment Transport: Finding Simplicity in a Complicated Erosion Model - Environmental Modelling: Finding Simplicity in Complexity - page 250

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Figure 15.3 Locations of the SSEW/MAFF
overflight surveys 1982-1984 and the subsequent
(modelled) MIRSED study sites throughout
England and Wales.

is subject to areal sediment yields of 1-2 t ha − 1 year − 1 ,
significantly, less than 2% is subject to yields in excess
of 2 t ha − 1 year − 1 (but see Parsons et al ., 2004, 2006 for
difficulties in evaluating and comparing erosion rates in
this way). Therefore, despite high average values of soil
erosion, only a small proportionof the transect exceeds the
'tolerable' levels of soil loss (Morgan, 1980; Evans, 1981).
This was true also at Gwent and Sussex west, though
slightly larger areas (approximately 3.5%) experienced
rates in excess of 2 t ha − 1 year − 1 on the South Downs in
Sussex, perhaps reflecting the steep local relief and the
dominance of winter wheat in the area (Boardman, 1993).
In contrast, the Herefordshire site evidenced high average
and maximum predicted soil-erosion rates coupled with
extensive distribution of this soil loss with at least 8%
of the transect above the 2 t ha − 1 year − 1 tolerance level.
Consequently, it is apparent that whilst high predicted
soil-erosion rates occur locally at all of the latter four sites,
significant predicted rates are more commonly found on
the soils within the Herefordshire transect.

The lowest erosion rates were predicted for the soils
within the Bedfordshire and Cambridgeshire, Notting-
hamshire and Norfolk East transects. The Bedfordshire
and Cambridgeshire site were characterized by relatively
flat topography (slope gradients rarely exceed 12.5%), and
dominated by clay and clay loam soils. Average soil loss
rates of approximately 0.4 t ha − 1 year − 1 were predicted;
maximum rates were similarly low and only marginally
exceeded the higher soil tolerance level of 2 t ha − 1 year − 1 .
The distribution of areas exceeding the higher tolerance
level was limited to between 1.0 and 2.8% of the total area
of the 112 km 2 transect.
The Norfolk east transect was also relatively flat with
maximum slope gradients of 15%. Sandy loam soils
dominated mostly under winter and spring cereals, along
with a combination of grassland, horticulture and row
crops. Average results were amongst the lowest modelled
for any of the sites, the predominance of low slope
gradients within the transect having a modifying effect
upon the soil loss. Significant rates of soil loss (above