The HBV-NP simulates nitrogen (N) and phosphorus (P) transport and transformation at the catchment scale (from 1 km2 to > 1 000 000 km2). The objectives are usually to estimate transport, retention and source apportionment, to separate human impact from anthropogenic, and to evaluate climate and management scenarios. It is based on the hydrological HBV model, which gradually has been equipped with a N routine (Bergström et al. 1987, Brandt 1990, Arheimer and Wittgren 1994, Arheimer and Brandt, 1998). The P routine has recently been developed within VASTRA - the Swedish Water Management Research Programme.
HBV-NP is a dynamic mass-balance model, which is run at a daily time-step, including all sources in the catchment coupled to the water balance
c = concentration of nutrient fraction
V = water volume of groundwater, river or active part of lake
in = inflow (e.g. for groundwater: soil leakage from various land uses; for lakes/wetlands: upstream rivers and local discharge, precipitation on the surface)
out = outflow to river, lake or downstream subbasin, evaporation
D = atmospheric deposition on water surfaces
P = emissions from point sources or rural households
F = retention (removal or release), see Table 1.
The spatial resolution of the model depends on the subbasin division in each application. The HBV-N has been applied in large-scale studies, covering southern Sweden (145 000 km2 divided into 3700 catchments; Arheimer and Brandt, 1998), the country of Sweden (450 000 km2 divided into 1000 subbasins; the TRK project), and the Baltic Sea drainage basin (~1 720 000 km2 divided into 30 subbasins; Pettersson et al., 2000). The model has also been used for more detailed studies, as for the Genevadsån River (200 km2 divided into 70 subbasins; Arheimer and Wittgren, 2002; Arheimer et al, 2003). Additionally, the model has been applied in Matsalu River in Estonia (Lidén et al., 1999), and in XX and XX Rivers in Germany (Fogelberg, 2003).
When applying the model the river basin may be divided into several coupled subbasins, for which the calculations are made separately, and this gives the spatial distribution of the model results. The hydrological part (i.e. HBV-96) consists of routines for accumulation and melt of snow, accounting of soil moisture, lake routing and runoff response. The model includes a number of free parameters, which are calibrated against observed time-series of river discharge and riverine nutrient concentrations. For large-scale catchment applications, the calibration procedure is made step-wise for surface runoff, tile drains and groundwater, rivers and lakes, with simultaneous consideration to several monitoring sites in a region.
In the nutrient routine, soil leaching concentrations are assigned to the water percolating from the unsaturated zone to the response reservoir of the hydrological HBV model (Fig. 1).
Different concentrations are applied to water originating from different combinations of land use and soils. The arable land may be further divided into a variety of crops and management practices, for which the nutrient leaching is achieved by using field-scale models, e.g., SOILN (Johnsson et al., 1987); or ICECREAM (Tattari et al., 2001) extended with macropore flow. For P, also soil surface erosion and water transport is considered, using a GIS-based model component, e.g. DelPi (Hellström, 2003).
In addition to the diffuse soil-leaching, nutrient load is also added from point-sources, such as rural households, industries, and wastewater treatment plants. Atmospheric deposition is added to lake surfaces, while deposition on land is implicitly included in the soil-leaching. The model simulates residence, transformation and transport of N and P in groundwater, rivers, wetlands and lakes. The model considers that stream bank erosion, as well as sedimentation and suspension processes in the rivers may have an impact on the river load.
The equations used to account for the nutrient turnover processes are mainly based on empirical relations between physical parameters and concentration dynamics. The fractions modelled are: dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), particulate phosphorus (PP), and soluble reactive phosphorus (SRP). Calculations are made with a daily time-step. Simultaneous calibration of water balance and nutrient concentrations may be performed (Pettersson et al., 2001).