Methodological approach

Nutrient inputs via atmospheric deposition

Atmospheric deposition on the water surface areas is calculated as a product of the water surface area and the area-specific atmospheric deposition rate of nitrogen and phosphorus.

The calculation of direct atmospheric deposition on water suface areas requires following input data:

  • area of all water surfaces, which are connected to the river system within the analytical unit.
  • Deposition rates for nitrogen and phosphorus.

Water fluxes

For subsequent calculation of the total water balance in an analytical unit, the precipitation on, and the potential evaporation from, water surface areas are balanced.

Nutrient fluxes

Deposition rates of N in European countries are available through the European Monitoring and Evaluation Programme (EMEP) program. The resolution of the raster grids for NHy and NOx Deposition (in kg/(ha·yr)) is 50 km. In certain cases, country-specific data with a higher resolution is available, as for example for Germany in 1 km raster (GRID after GAUGER et al., 2008).

Atmospheric N deposition is considered separately for different land uses as a mean value per analytical unit, whereas for P deposition, only a mean deposition rate per analytical unit is considered.

Nutrient inputs via overland flow

Nutrient emission via overland flow refers only to the dissolved fractions of nitrogen and phosphorus, and is calculated separately for different land uses (arable land, grassland, naturally covered areas, open land areas, wetlands, open pit mine areas, and snow and ice covered areas).

Following input data are necessary:
  • Land use map
  • Phosphorus content and phosphorus absorption capacity in topsoil
  • Phosphorus concentration in surface runoff from arable land [mg/l]
  • Nitrogen concentration in surface runoff from arable land [mg/l]
  • Precipitation and precipitation intensity

Water fluxes

Basis for the overland flow calculation is the specific runoff from these areas. The surface runoff is calculated as a function of the specific runoff of an analytical unit, following the approach by CARL and BEHRENDT (2006, 2008) and CARL et al. (2008).

Nutrient fluxes

The water soluble phosphorus concentration depends very strongly on the phosphorus saturation of the soil. Therefore the ratio between the mean phosphorus accumulation and the maximum phosphorus accumulation during the entire calculation period has to be considered on an administrative unit level. The nitrogen concentration in surface runoff from arable land is calculated on the approach by WERNER et al. (1991) which was modified by VENOHR et al. (2011). The emissions via surface runoff are calculated by multiplying the respective phosphorus and nitrogen concentrations with the surface runoff for each land use type. The sum of these emissions is the total emission via surface runoff.

Nutrient inputs via erosion

The pathway Erosion describes the particulate nutrient input from upland into surface waters. The quantification of N and P emissions depends of four parameters:

  • Soil loss
  • Slope
  • Nutrient content in surface soils
  • Land use
Since input data rely on long term values, yearly variation is accounted for by yearly precipitation, resulting in a precipitation ratio as correction factor. This approach bases on the Universal Soil Loss Equation.

Area

Input data is area dependent and derives from GIS calculations. Thus it is possible to consider areas underlying different land uses (basing on Corine Landcover) and slopes individually. Moneris differentiates five different slope classes for the land use arable land (from 0 to >8%), basing on a digital elevation map with 1000m grid cells. Erosion is also calculated for land uses grassland, open are and naturally covered areas.

Water fluxes

As already mentioned precipitation does not directly influence soil loss, but is considered by correcting long term data and thus influences erosion on a yearly base.

Nutrient fluxes

Not only nutrient content in top soil but also the calculated share of eroded sediment, which finally enters the surface waters, is crucial for the pathway erosion. Thereby it is assumed, that the share of small particles which contain more nutrients, is relatively increased with increasing duration of transport. This is accounted for by applying respective enrichment ratios for N and P to finally calculate the nutrient emission by the pathway erosion.

Nutrient inputs via groundwater

The emission pathway groundwater describes the total nutrient input to the surface waters after soil passage. Following input data are required:
  • Nitrogen surplus
  • Phosphorus content in topsoil
  • Land use information
  • Soil information
  • Hydro-geological information
The nutrient emissions to the surface waters via the pathway groundwater are the product of the nutrient concentration in groundwater and the calculated groundwater flow.

Area

To quantify nutrient emissions via the pathway groundwater, mainly areas are relevant whose subsurface has the ability for groundwater recharge. For this, area specific data about land use (e.g. Corine Landcover) and hydro-geological rock-types (e.g. hydro-geological map of Europe map of Europe, RIVM 2007) are required.

Water fluxes

The groundwater flow is calculated as a residual from the remaining flow components.

Nutrient fluxes

The nutrient surplus on agricultural areas, the phosphorus content in topsoil and the atmospheric deposition on unsealed areas are the significant influences of the nutrient concentration in seepage water. The subsequent nutrient retention during the soil passage driven by the hydro-geological rock-types, and the following nutrient retention during the groundwater residence time determine the nutrient emissions to the surface waters.
Grafische Darstellung der Moneris Methodik

Nutrient inputs via tile drainage

N and P emissions, via tile drainages into surface waters, are calculated for tile-drained arable land and grassland, respectively. Three parameters are considered in MONERIS (i) tile drain flow rate, (ii) size of tile drained areas, and (iii) mean nutrient concentration of the tile drain flow.

Area

To estimate the proportion of tile-drained areas for each analytical unit three types of input data can be used:
  • Maps of tile drained areas
  • Assessment of tile drained areas for soil types that are derived from representative areas with information about those (e.g. BEHRENDT et al., 2000; HIRT, 2005 and more)
  • Regional statistics for administrative areas

Water fluxes

The tile drainage discharge rate is calculated as 50% of the winter precipitation and 10% of the summer precipitation following KRETSCHMAR (1977, Eq. 17). These values were supported by analysis of data for tile drainage from monitoring stations and used for monthly modelling as individual monthly values (HIRT et al., 2011).

Nutrient fluxes

The N concentration in tile drain outlets, and the potential nitrate concentration in the seepage water, is calculated based on the approach of FREDE and DABBERT (1998) using the regionally differentiated N surplus. The nutrient concentration of seepage water is expected to correlate with those of the tile drainage flow. The soil boundary condition is that net mineralization and net immobilisation are both negligible. To consider denitrification in soils the nitrate concentration in the tile drainage flow is reduced by an exponent of 0.85 for arable land, and 0.7 for grassland (BEHRENDT et al., 2000). The calculated N concentrations in the tile drainage runoff, according to, correspond to BEHRENDT et al. (2000).

Nutrient inputs via point sources

Emissions from point sources are considered for municipal waste water treatment plants (WWTP) and industrial discharges.

Area

Nutrient emissions via point sources are taken from an inventory with information on individual waste water treatment plants (WWTP). Additionally, lumped discharges from industrial direct dischargers and remaining smaller point sources can be considered for each analytical unit.

Water and nutrient fluxes

For the WWTP inventory data on the discharge, the TN and TP concentrations and the size as inhabitant equivalents are needed. Temporal changes in discharges from waste water treatment plants can be considered by factors for individual years in each analytical unit. For the later calculation of retention in surface waters it can also be considered whether point sources discharge into tributaries or directly into the main river of an analytical unit.

In MONERIS usually all emissions are assumed to first reach the tributaries. In reality, WWTP´s are normally located at large streams and therefore discharge into these main rivers. In case that it is known whether the WWTP discharge into the tributaries or the main river, it can be separately considered for the calculations.

For monthly calculations it is assumed that emissions from point sources (waste water treatment plants and industrial direct dischargers), remain constant during the year and be equally distributed among the twelve months.

Nutrient inputs via urban systems

Nutrient emissions from urban systems are calculated for sealed urban areas connected or not connected to sewer systems, as well as for households not being connected to sewers systems or waste water treatment plants (WWTP). The calculation of nutrient emission via urban systems depends on the following input data:
  • Waste water statistics
  • Inhabitants (total, connected and not connected to sewer systems)
  • Precipitation
  • Land use

Area

In order to quantify nutrient emissions via urban systems, land use data from different sources (CORINE for Europe, ATKIS for Germany) is used. The sealed urban area is derived from these data sets (rasters or polygons) and areas are calculated for each analytical unit.

Water fluxes

The runoff rate from sealed urban areas that is generated from precipitation is calculated according to HEANEY et al. (1976. The higher the percentage of the area that is sealed, the larger is the runoff rate, and thus the larger the part of precipitation reaching the sewer systems.

Nutrients fluxes

Households connected to sewers systems and to WWTPs are not accounted for in the calculated emissions from urban systems as they should be considered in the WWTPs inventory. Storm water events generating high runoff from sealed areas are crucial for emissions from urban systems. Here, the increased discharges from combined sewers can often not handled by WWTP and have to be stored in the sewers systems. If the storage capacity in combined sewer systems is exceeded, the excessive water amount is bypassed directly, allowing raw sewage from households, commercial use, and streets to reach surface waters during overflow events. Beyond combined sewer systems, MONERIS considers four more pathways for nutrient emissions from urban areas.

Retention

Retention describes all nutrient transformation and loss processes and is thus an important element of the nutrient cycle.

MONERIS considers net nutrient retention rather than the contribution of individual chemical and biological transformations. At this, nitrification-denitrification, plant uptake, sedimentation and decomposition of organic matter are taken into account. The dominant retention process for nitrogen is denitrification. Nitrogen retention is calculated as a function of temperature and hydraulic load. Contrary to that, for phosphorus sedimentation is the most important retention process. In tributaries phosphorus retention is estimated by the mean of the functions of hydraulic load and specific runoff. For main rivers only the hydraulic load is considered. The retention of dissolved organic nitrogen is assumed to be negligible.

In the MONERIS retention calculation it is assumed that all nutrient emissions are distributed evenly throughout the catchment and enter the tributary before the main river as well as the tributary discharge into the main river at the outlet of the respective analytical unit. For this, nutrient loads from upstream analytical units are subject to retention in the main river of the downstream catchment.

Management Alternatives


Within the Management alternative setting a package of measures can be defined. The following measures are available.


Land use changes

  • Conversion of arable land to grassland
  • Restoration of tributaries
  • Reduction of tile drained areas
  • Connection of agricultural areas to surface waters
  • Retention ponds for drainage flows
  • Conversion of paved surfaces to unpaved surfaces.

Soil conservation practices on arable land
  • Soil conservation
  • Contour ploughing
  • Intercropping

Change of nitrogen surplus
  • Maximum of fertilizer and manure
  • Reduction by agri-environmental measures (AEM), e.g.:
              - Soil conservation
              - Intercropping
              - Extensified grassland
              - AEMx: others

Atmospheric deposition
  • Reduction of atmospheric NHy und NOx depositions

Sewer systems

Here, various parameters can be modified that affect nutrient emissions from urban systems, such as:
  • Increase of storage volume in combined sewer systems
  • Clearing basins for separate sewer discharges
  • Soil retention filters for separate sewer discharges
  • Inhabitants connected to sewer systems and WWTP´s
  • Phosphate-free laundry detergents
  • Phosphate-free dishwashing detergents

Small waste water treatment plants

Contains measures to reduce the nutrient emissions via decentralized treatment plants.


Waste water treatment plants (P and N concentrations)

Here the effluent concentrations for single WWTP's of a certain size (referring to the number of connected inhabitants) can be defined. The user can set concentrations in the range of suggested values.