Table 1. Average rates of nitrogen fertilizer used 
on arable and grassland in England and Wales. 
Source: Mattikalli (1993). 
  
  
Year N (kg/ha) 
Arable Grassland 
1. 1943 19 4 
2. 1962 74 23 
3. 1970 90 53 
4. 1984 200 60 
5. 1987 200 60 
6. 1989 200 60 
  
2. THE EXPORT COEFFICIENT MODEL 
The export coefficient model estimates 
nitrogen loading in the surface water at the outlet 
of a watershed using areal extents of different 
types of land-use within the watershed and the 
associated fertilizer application rates (Reckhow 
and Simpson, 1980; Beaulac and Reckhow, 1982; 
Johnes and O'Sullivan, 1989). The model follows 
from that developed by Jorgensen (1980) which 
calculated the areal external nutrient load upon a 
water body by summing outputs from all 
potential sources. Using this concept, the nitrogen 
losses from a watershed to surface water are 
estimated for all possible sources of nitrogen, and 
by defining appropriate export coefficients. As 
the present study is focused on a predominantly 
rural watershed, only losses from arable, 
grassland, and woodland are considered. 
Sediment losses are not addressed owing to the 
lack of data in the literature on sediment export 
rates. 
The nitrogen loss, L (in kg/year), at the 
outlet of a watershed is given by: 
n m 
Zt Yn]: a 
i=1 £5) 
= area of a discrete land-use patch (ha) 
E = export coefficient for a specific land-use 
type (kg/ha) 
m = number of discrete patches under one land- 
use class 
n = number of land-use classes 
a = artificial inputs (e.g. sewage, industrial etc.) 
(kg/year) 
134 
An export coefficient (E) indicates the . 
proportion of the total nitrogen added to a given 
land-use patch that is lost to surface water. This is 
the key parameter, which determines the amount 
of nitrogen loading from the various land-use 
types, and is obtained from the literature. 
Individual coefficient (such as Ewoodland: 
Egrassland, Earable €tc) is deemed to allow for 
biological, atmospheric and geological 
(weathering) contributions to nitrogen loss from 
each land-use type. This is a modification 
compared to the original structure of the model; 
similar modifications were employed in the 
version constructed for Slapton Ley by Johnes 
and O'Sullivan (1989). The parameter values and 
export coefficients used in this application of the 
model were compiled from the literature (Table 
2). Nitrogen input and loss from the human and 
livestock population are considered here because 
urban area constitutes a very small proportion in 
the study area and the emphasis of the this study 
is only on the agricultural inputs. 
Table 2. Export coefficients for nitrogen. Source: 
Mattikalli (1993). 
  
  
Land-use N fixed N loss 
(%) (kg/ha) 
Temporary grass 5 30 
Permanent grass S 50 
Cereals 12 4 
Root crops 20 10 
Oilseed rape 30 4 
Woodland 13 20 
Rough grazing and Set-aside 2 10 
  
3. STUDY AREA 
AND DATA DESCRIPTION 
The River Glen watershed in eastern 
England is selected. The geographical co- 
ordinates of the area are: longitude 09 20' - 09 
40' W; latitude 529 40' - 539 00' N, and it extends 
between Grantham, Bourne and Stamford in 
South Lincolnshire. This region is characterized 
by major changes in agricultural activity with 
increased arable farming in the past few decades. 
The choice of study area was influenced by the 
existence of considerable quantities of land-use 
data required by the export coefficient model. 
The study area is mainly used for agricultural 
purposes, and other land-uses such as urban areas 
form a small proportion. Crops such as wheat, 
barley and sugar beet dominate the watershed. In 
addition to arable farming, grassland cultivation is 
also important within the watershed, although 
much of the grassland in the region has changed 
  
  
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