fertilizer application rates for 1931 are not
available, data from 1943 were considered to be
approximations for those existed in 1931. These
attributes, along with polygon area parameters,
were employed to derive the nutrient load from
every discrete land-use polygon. Such loadings
were accumulated over a sub-catchment to obtain
total loading. This involves a large number of
operations to be repeated for each sub-catchment
and for all six land-use coverages. Therefore,
special routines using the Arc Macro Language
(AML) were developed to carryout these
operations automatically.
Validation of the model was carried out by
comparing predicted nitrogen loadings with
observed loadings. Since the export coefficient
model predicts annual loading, it was necessary to
extract both runoff and water quality data to
compute annual observed loading. Both water
quality and runoff data were examined and
records of specific dates having both water
quality and runoff data were extracted in order
that nutrient ratings could be established. The
TON concentrations and discharge data were
available only for Burton Coggles and Holywell
Brook during 1984-1987, and for Kates Bridge
during 1984, 1987 and 1989. These data were
analyzed to develop rating relationships between
nitrogen loading and runoff, since daily records
of the former were not available. Rating
relationships developed for 1984, 1987 and 1989
are presented in Table 3. It is clear from this table
that nitrogen loading have strong positive
relationships with discharge, and the high r2
values indicate that loading can be calculated with
a fair degree of accuracy. It was possible to
employ these relationships, along with the daily
runoff records, to compute the annual loading.
This restricted the validation of the model to these
specific sub-catchments for 1984, 1987 and
1989.
5. RESULTS AND DISCUSSION
Nitrogen loadings in the surface water were
calculated using the export coefficient model.
Results were validated by comparing estimated
loads with observed loads recorded at water
quality sampling stations. Since the export
coefficient model estimates annual loadings, the
observed loads were also calculated for a period
of one year. Daily discharge data for an entire
year were used in conjunction with the rating
relationships presented in Table 3 to calculate the
observed yearly nitrogen load. Observed nitrogen
loads were compared with those estimated by the
export coefficient model, and Table 4 shows that
they are of a similar order.
Table 3. Relationships between nitrogen loading
and discharge for Burton Coggles, Holywell
Brook, and Kates Bridge. Models are of the form:
Y = Bo + B1 X where Y is the calculated nitrogen
load in kg/day; X is the observed discharge in
m?/sec; Bo and B4 are the regression coefficients.
Sampling Year Model
station Bo B1 r2
Burton Coggles 1984 - 34.724 1883.5 97.6
1987 .- 1.534 1130.5 97.6
Holywell Brook 1984 - 14.224 914.3 95.3
1987 - 14.334 1059.8 94.3
Kates Bridge 1984 -141.0+ 1267.0 93.8
1987 -204.0+ 1152.0 85.3
1989 -34.6+ 873.0 95.1
Table 4. Comparison of nitrogen loading
estimated by the export coefficient model with
the observed loads.
Sampling Year Nitrogen loss
station (tonnes/year)
Observed Predicted
Burton Coggles 1984 106.35 128.33
1987 83.76 124.36
Holywell Brook 1984 84.36 92.39
1987 91:61 84.87
Kates Bridge 1984 612.87 533.68
1987 336.73 470.50
1989 862.07 603.80
This demonstrated a generally successful
application of the model, and the export
coefficients used, as a basis to estimate nitrogen
loading. However, some variations can be seen
between the observed and estimated values which
may be attributed to the use of rating
relationships to calculate the observed loads and
the application of export coefficients selected
from the literature (Mattikalli et al., 19953).
Nevertheless, the results demonstrate that the
model estimates nitrogen loading with a fair
degree of accuracy.
The predicted losses of nitrogen for the
entire study area for all six years are presented in
figure 2. It is clear from this figure that nitrogen
loading has increased during 1931-89. Nitrogen
loading increased from 67.80 tonnes/year in
1931 to 603.80 tonnes/year in 1989. This
increase i
due to th
land and
nitrogen
nitrogen
attributec
fertilizer
compared
388888
Nitrogen loading, tonnes/year
3
1
©
Figure 2.
the entire
In 1
loading i
fertilizer :
of nitroge
combined
fertilizer
identify t
and fertili
achieved
different
historical
The result
were use:
loading (f
Using
100 4
80 4
Nitrogen loading, tonnes/year