Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Modelling Riverine Nutrient Input to the Baltic Sea and Water Quality
Measures in Sweden
Berit Arheimer
Swedish Meteorological and Hydrological Institute (SMHI), 601 76 Norrköping, Sweden. E-mail: Berit.Arheimer@smhi.se
1. Introduction
Eutrophication in the Baltic Sea and its coastal zone is
considered a serious environmental problem. The problems
are mainly caused by excessive load of nitrogen (N) and
phosphorus (P). The nations around the Baltic Sea report
their national load to the Helsinki Commission (HELCOM),
and at the latest load compilation it was also obliged to
specify the contribution from various sources. At present,
water management in Sweden is going through dramatic
changes related to the adoption of the EU Water Framework
Directive, a new Environmental Code and revised
Environmental Quality Objectives. New policies including
catchment-based management plans will be implemented,
which also demand catchment-based knowledge of nutrient
transport processes and appropriate tools for measure
planning. An integrated catchment model (Fig.1) has thus
been developed to be used for catchment characterisation,
international reporting and scenario estimates for more
efficient control strategies. The model may be applied at
different scales and with different resolution (Arheimer,
2003).
The catchment model is applied on the national scale within
a nested model system, called TRK (Brandt and Ejhed,
2003), which calculates flow-normalised annual average of
nutrient gross load, N retention and net transport, and source
apportionment of the N load reaching the sea. The system
consists of several submodels with different levels of
process descriptions that are linked together. Dynamic and
detailed models are included for arable leaching, water
balance, and N removal. Landscape information, leaching
rates and emissions are combined through GIS. The model is
validated against independent observations. The results are
spatially lumped on a subbasin level, and 20 years of daily
results are used when averaging to avoid weather-induced
bias. This presentation will be focused on: 1) model
evaluation at various scales, 2) analysis of impact from
various measures in one case-study, and, 3) model
applicability at the scale of the entire Baltic Sea basin.
Rain and snow
Soil and
Vegetation
(forest, arable, pasture,
Root zone
other open land)
leaching concentrations
Rural
Groundwater
households
(transformation)
HBV-NP
Surface
runoff,
Macro
pore flow
Point
sources
Upstream
subbasins
River
( transformation,
= water
= nutrients
Atm. deposition
on water surfaces
Lakes / Wetlands
(transformation,
erosion) deposition)
Figure 1. Schematic structure of the dynamic
catchment model HBV-NP.
outflow
2. Methodology
Root-zone leaching: Leaching concentrations from arable
land is calculated by the Swedish University of Agriculture
(SLU) with the SOILNDB model for different field
categories of Sweden, with consideration taken to crop
rotation. The procedure results in one normalised
concentration for each combination of region, soils and
crop. A similar approach is being developed for P based
on the ICECREAM model.
Water balance and discharge: The water balance at the
catchment-scale is estimated by using the conceptual
rainfall-runoff model HBV, which makes daily
calculations in coupled subbasins along the river network.
The HBV model consists of routines for snow melt and
accumulation, soil moisture, runoff response and routing
through lakes and streams. Driving variables are daily
precipitation and temperature, and monthly mean values of
potential evaporation. In the model, subbasins can be
disaggregated into elevation zones (for temperature
corrections) and land-cover types. Within each unit a
statistical distribution of soil moisture is assumed. The
application of Sweden includes 1000 subbasins, in the
range 200-700 km 2 . However, the model concept is also
applied for local management plans with higher resolution,
or for the entire Baltic Sea basin, only including some 30
subbasins.
Land cover, emissions and atmospheric deposition: For
each subbasin land cover is aggregated into the classes:
arable field-type (13 crops on 9 soils in 22 regions; i.e.,
2574 types on Sweden), forest type (3 types in Sweden),
clear-cut forest (additional leaching according to
atmospheric deposition rate), urban, and lakes (3 types
according to position in the catchment). Emissions are
classified as industrial point sources, municipal treatment
plants, and rural households. The first two are based on
empirical data, while the latter is based on population
statistics and coefficients considering average treatment
level in the region. Atmospheric deposition is calculated
for each lake surface by using seasonal results from the
MATCH model.
Nutrient transformation and erosion processes: The
HBV model calculates average storage (and residencetime)
of water and nutrients between root-zone and
stream, in streams and in lakes for each subbasin.
Leaching concentrations are assigned to the water
percolating from the unsaturated zone of the soil to the
groundwater reservoir. Different concentrations are used
for different land-covers and load from rural households is
added. Removal processes in groundwater are considered
before the water and nutrients enter the stream, where
additional load from industries and treatment-plants may
be added as well as river discharge from upstream
subbasins. Transformation processes may occur in the
stream and in lakes, and atmospheric deposition is added
to lake surfaces (for other land covers it is included in the
soil leaching). In the P routine (Andersson et al., 2003), he
root zone concentrations are separated into micro- and
macro-pore flow, and concentrations in surface runoff is
treated in a soil erosion GIS-submodel. Stream bank
erosion is included for the watercourses, as well as
diffusion from sediments and biological transformation.