Fourth Study Conference on BALTEX Scala Cinema Gudhjem

Model parameters and coefficients: The catchment model
includes a number of free parameters, which should be
calibrated against time-series of daily observations.
Normally about 10 parameters are calibrated for the water
discharge, and 4-8 for the N and P transformation.
Calibration is made simultaneously for several observation
sites in a region to get robust parameter values, which are
then transposed to all subbasins in that region. This
procedure is made step-wise, starting with groundwater, then
rivers and finally lakes.
Up-scaling of results to coastal zones and national level:
Source apportionment for different coast segments or the
nation is achieved by adding sources for different categories
and all subbasins. This is made separately for gross and net
load to illustrate the influence of removal processes. Net
load is the remaining part of the gross load, which
eventually reaches the sea after N removal in groundwater,
rivers and lakes downstream a specific source and subbasin.
Scenarios of water-quality measure impact: Once the
model is set-up and validated for a catchment, it is possible
to make sensitivity studies, in order to evaluate possible
impact of various measures to improve the water quality.
Impact from several remedial-measure strategies has been
studied by scenario modelling of two Swedish river basins,
including costs estimates.
3. Results and Discussion
Time-series of modelled water discharge was compared to
observations at 307 sites in Sweden: at 188 sites the volume
error was less than 5%, 82 sites showed a volume error
between 5 and 10%, while it was more than 10% at 37 sites.
Hence, the model can be used for rather trustworthy
distributed mapping of national water discharge.
More than 100 independent time-series from different
observation sites were used for validation of the N of
Sweden. Annual transport and average concentrations show
good correlation to measured values, while daily
concentration fluctuations were more difficult to capture at
the national scale. The P routine of HBV-NP has only been
applied for two Swedish regions so far, and the results are of
similar quality as for N.
The Swedish environmental goal to combat eutrophication
includes a reduction of 20% for P load and 30% for N load.
In the case study of Rönne å in southern Sweden, the
potential of reducing the load to the sea varied a lot between
N and P for different measures. For P, it is still most
effective to approach emissions from rural households and
treatment plants. This has also been the Swedish policy so
far, although the control of rural households has been
insufficient and the treatment is not yet in accordance with
Swedish standards. Moreover, wetlands in agricultural areas
also seem to have a significant potential to reduce the P load
(by 5-8%). For N, on the other hand, the results show that it
is also necessary to combat diffuse leaching from arable land
to reach the environmental goal. The introduction of catch
crops, removal of autumn seeds and rubs, introduction of
spring crops and ploughing and fertilisation at springtime,
would remove 19% of the N load from the catchment at a
rather small annual cost (25 million SEK or 100 SEK/kgN).
This was found to be one of the most cost-effective
measures for this specific catchment. However, it must still
be combined with other measures to approach the goal. For
N, it seems doubtful that the goal is possible reach without
involving all sectors with emissions in the region.
- 187 -
When testing scale dependency and limits in the HBV-N
model application for the entire Baltic Sea region (Fig. 2),
it was obvious that one of the major obstacles is the still
high input-data demand. Although databases are now
available for the entire region, data quality is sometimes
poor and the spatial resolution not satisfactory for
distributed water quality modelling. It can also be
questioned if the basic concept is applicable on the
southern part of the basin, where residence times in
groundwater and transit time within soils is much longer,
due to different and deeper soils than in Scandinavia.
River behaviour is also much different with long residence
times in slow flowing waterbodies, which may not be
properly described in the present HBV-NP concept.
2
1
0
o n d j f m a m
Month
j j a s
40000 (m3/s) Q(computed)
20000
400 000
200 000
Tot-N
(mg/L)
0
1980 1982 1984 1986 1988 1990
0
Tot-N
(tonnes/
year)
Atm.dep.
on lakes
measured
computed
Q(recorded)
Industry Population Forest Open land
Figure 2. HBV-N modelling of the Baltic Sea basin
(Pettersson et al., 2000), recently expanded to
include also Skagerakk (not shown in the figure).
4. Conclusions
• The HBV-NP model seems to deliver trustworthy
results for daily water discharge and annual nutrient
load under Swedish conditions.
• The HBV-N model is a valuable tool for water
authorities in Sweden, when implementing the EU
Water Framework Directive and evaluating measures
for nutrient load reduction at the catchment scale.
• In applications of HBV-N to the entire Baltic Sea
basin, there is still a lack of distributed input-data,
and the model concept needs further development.
References
Andersson, L., Arheimer, B., Larsson, M., Olsson, J., Pers,
B.C., Rosberg, J., Tonderski, K., and B. Ulén. 2003.
HBV-P: a catchment model for phosphorus transport,
Proceedings of Quantifying the Agricultural
Contribution to Eutrophication, COST 832 Final
Meeting, 31 July - 2 August, Cambridge, U.K., 59-60.
Arheimer, B., 2003. Handling scales when estimating
Swedish nitrogen contribution from various sources to
the Baltic Sea. Lanschap 20(2):63-72.
Brandt, M. and Ejhed, H., 2003. TRK-Transport,
Retention, Källfördelning. Belastning på havet.
Swedish Environmental Protection Agency, Report
No. 5247.
Pettersson, A., Brandt, M. and Lindström, G., 2000.
Application of the HBV-N model to the Baltic Sea
Drainage Basin. Tidskriften Vatten 56:7-13.