Changes of nitrogen and phosphorus loads to European seas
Bruna Grizzetti, Fayçal Bouraoui, Alberto Aloe
Supplementary material
In this supplementary material we describe more extensively the modelling approach used and we provide more details on the methodology developed for estimating point and diffuse input of nutrients at European scale.
[1] The modelling approach
The quantification of nitrogen and phosphorus loads to surface waters at the European scale and over a long time is constrained by the chronic shortage of water quality and quantity measurements throughout Europe, the uneven spatial and temporal covering of monitoring networks, and the partial availability of information on nutrient inputs from agriculture and point sources. This lack of data combined with the complexity of the processes of transport and transformation of nutrients within the river basin strongly influenced the choice of the modelling approach used in the study, calling for a simplification of the processes representation.
River basins were considered as the spatial units of analysis, representing the physical boundaries of nutrient processes and consequently the coherent perspective for nutrient management. Each river basin was divided into a number of sub-basins according to its physiographic characteristics (the description of the European database is provided in the following paragraph), and the sub-basins were used as the elementary spatial units for the model application.
The annual total nitrogen and total phosphorus loads reaching the surface water were estimated by the model GREEN (Geospatial Regression Equation for European Nutrient losses). GREEN is a
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statistical model developed to estimate nutrient fluxes to surface water in large river basins
(Grizzetti, 2006). The model was developed and used in European basins with different climatic conditions and nutrient pressures (Grizzetti et al., 2005) and was already successfully applied to the whole Europe (Grizzetti et al., 2008; Bouraoui et al., 2009). GREEN contains a simplified representation of the processes of nutrient transport and retention in the river basin and a geospatial representation of the nutrient sources and physical characteristics that influence the nutrient processes. To apply the model, the area of study is divided into a number of sub-basins that are connected according to the river network structure. Similarly to other modelling approach, such as
SPARROW (Smith et al., 1997; Alexander et al., 2002), GREEN distinguishes between diffuse and point sources of nutrients. The first include mineral fertilizers, manure applications, atmospheric deposition (only for nitrogen), crop fixation (only for nitrogen), and scattered dwellings, while the latter consist of industrial and waste water treatment discharges. For each sub-basin the model considers the input of nutrient diffuse sources and point sources and estimates the nutrient fraction retained during the transport from land to surface water (Basin Retention) and the nutrient fraction retained in the river segment (River Retention).
In the model the nutrient retention is computed on annual basis and includes permanent and temporal removal. Diffuse sources are reduced both by the processes occurring in the land (crop uptake, denitrification, and soil storage), and those occurring in the aquatic system (aquatic plant and microorganism uptake, sedimentation and denitrification), while point sources are considered to reach directly the surface waters and therefore are affected only by the river retention. For each subbasin i the annual nutrient load estimated at the sub-basin outlet (L i
, ton N/yr or P/yr) is expressed as following:
L i
=(DS i
*(1-BR i
)+PS i
+U i
)*(1-RR i
) Equation s.1
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where DS i
(ton N/yr or P/yr) is the sum of diffuse sources, PS i
(ton N/yr or P/yr) is the sum of point sources, U i
(ton N/yr or P/yr) is the nutrient load received from upstream sub-catchments, and BR i and RR i
(fraction, dimensionless) are the estimated nutrient Basin Retention and River Retention, respectively.
In the model, BR i
is estimated as a function of rainfall while RR i
depends on the river length. These physical characteristics were selected, among metrics that can be easily calculated at European scale from readily available data, according to a correlation and a principal component analysis
(Grizzetti, 2006). Basin retention and river retention for the sub-basin i are modelled as following:
BR i
=exp (-α
P
* X
P,i
)
RR i
= exp (-α
L
* X
L,i
)
Equation s.2
Equation s.3
Where X
P,i
and X
L,i
are the inverse of rainfall and the river length, respectively, for the sub-basin i , and α
P
and α
L
are the model parameters for rainfall and river length , respectively. X
P,i
, X
L,i
, α
P
and
α
L
are all dimensionless, because the inverse of rainfall and the river length were scaled by maximum scaling (Frank and Todeschini, 1994) before being entered in the model. For more details on model parameterisation see also Grizzetti (2006), Grizzetti et al. (2008) and Bouraoui et al.
(2009).
Although simple in its structure the model GREEN is able to provide spatially distributed estimates of nutrient River and Basin Retention at large scale.
In addition to the retention in rivers, the removal of nitrogen and phosphorus in lakes was taken into account. In particular the retention occurring in lakes with surface larger than 50 km
2
was computed according to Kronvang et al. (2004). The retention for nitrogen was calculated as follows:
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Equation s.4 where R
N
if the fraction of retained load of incoming nitrogen in the lake, z is the average lake depth (m), and RT is the hydraulic residence time (yr). The average lake depth and hydraulic residence time were obtained from Pistocchi and Pennington (2006). A similar approach was used for phosphorus (Kronvang et al., 2004):
Equation s.5 where R
P
is the fraction of retained load of incoming phosphorus in the lake.
Nitrogen and phosphorus natural background losses originated from forests, which are particularly relevant in Nordic regions, were included in the modelling based mainly on the literature. 38% of the nitrogen atmospheric deposition on forest was considered to reach the river system as reported by HELCOM (2003). For phosphorus a fixed value of 0.15 kg P/ha·yr was used also in line with the values reported by HELCOM (2003).
Finally, nutrients originated from scattered dwellings were considered to be halved before reaching the water system. This simplification was introduced because, although sources from scattered dwellings are diffuse they do not undergo exactly the same pathway of agricultural or soil sources and therefore they are not subject to the same basin retention.
For the analysis of impacts of nutrients on the aquatic ecosystem the nutrient concentrations were estimated at the sub-basin outlets, dividing the nutrient load, provided by the model GREEN, by the water flow estimated by modelling. The water flow was modelled for each sub-basin as the difference between the annual rainfall and the actual evapotranspiration. The latter was computed according to a Budyko type of curve (Pike, 1964):
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E

RAIN

1



RAIN b PET

 n


1 n
Equation s.6 b and n are calibration parameters, E is the annual actual evapotranspiration (mm), RAIN is the annual rainfall (mm) and PET the potential evapotranspiration (mm) estimated following the formula of Turc (1961). More information on the methodology is provided in Bouraoui et al., 2009.
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