Kirsti Granlund

Modelling diffuse emission and retention of nutrients in the Vantaanjoki watershed (Finland) using the SWAT model

Abstract The nitrogen and phosphorus natural removal in the Vantaanjoki basin (Finland) was modelled using the SWAT hydrological model, which simulates the water cycle and the movement and transformations of nutrients. The model was first calibrated and then validated. In a daily time step, the Nash–Sutcliffe coefficient for the simulations of flow, nitrogen and phosphorus loads ranges between 0.59 and 0.81 for calibration and from 0.43 to 0.57 for validation. The simulation of the whole Vantaanjoki basin over a period of 9 years (1989–1997) allowed an estimation of the annual average diffuse emissions and nutrient removal in the basin. Diffuse emissions predicted by SWAT were compared with those evaluated by traditional statistical methods, confirming the reasonable predictions of the model. The nitrogen and phosphorus load values measured at the final outlet were compared with the load reaching the surface waters, coming from both diffuse and point sources, obtaining an estimation of retention of 24% for total nitrogen and 51% for total phosphorus.

Ensemble modelling of nutrient loads and nutrient load partitioning in 17 European catchments

An ensemble of nutrient models was applied in 17 European catchments to analyse the variation that appears after simulation of net nutrient loads and partitioning of nutrient loads at catchment scale. Eight models for N and five models for P were applied in three core catchments covering European-wide gradients in climate, topography, soil types and land use (Vansjø-Hobøl (Norway), Ouse (Yorkshire, UK) and Enza (Italy)). Moreover, each of the models was applied in 3-14 other EUROHARP catchments in order to inter-compare the outcome of the nutrient load partitioning at a wider European scale. The results of the nutrient load partitioning show a variation in the computed average annual nitrogen and phosphorus loss from agricultural land within the 17 catchments between 19.1-34.6 kg N ha(-1) and 0.12-1.67 kg P ha(-1). All the applied nutrient models show that the catchment specific variation (range and standard deviation) in the model results is lowest when simulating the net nutrient load and becomes increasingly higher for simulation of the gross nutrient loss from agricultural land and highest for the simulations of the gross nutrient loss from other diffuse sources in the core catchments. The average coefficient of variation for the model simulations of gross P loss from agricultural land is nearly twice as high (67%) as for the model simulations of gross N loss from agricultural land (40%). The variation involved in model simulations of net nutrient load and gross nutrient losses in European catchments was due to regional factors and the presence or absence of large lakes within the catchment.

Influence of EU policy on agricultural nutrient losses and the state of receiving surface waters in Finland

In Finland, the first large-scale efforts to control nutrient loading from agriculture got under way with the introduction of the EU Agri-Environmental Program in 1995. We examined whether these efforts have decreased agricultural nutrient losses and improved the quality of receiving waters. To do so we used monitoring data on fluxes of nutrients and total suspended solids in agricultural catchments in 1990–2004 and on the water quality of agriculturally loaded rivers, lakes and estuaries in 1990–2005. No clear reduction in loading or improvement in water quality was detected. Hydrological fluctuations do not seem to have eclipsed the effects of the measures taken, since there was no systematic pattern in runoff in the period studied. The apparent inefficiency of the measures taken may be due to the large nutrient reserves of the soil, which slowed down nutrient reductions within the period studied. Simultaneous changes in agricultural production (e.g. regional specialisation) and in climate may also have counteracted the effects of agri-environmental measures. The actions to reduce agricultural loading might have been more successful had they focused specifically on the areas and actions that contribute most to the current loading.

Estimation of the impact of fertilisation rate on nitrate leaching in Finland using a mathematical simulation model

Agriculture comprises the largest single source of nitrogen (N) into watercourses in Finland. The emphasis of water protection policy is today on controlling the non-point nutrient losses from agriculture. In this study a mathematical simulation model was used as a management tool to estimate the changes in nitrate (NO3 -N) leaching resulting from changes in cultivation practices in Finnish agriculture caused by the Agri-Environmental Support Scheme as a part of the Common Agricultural Policy of the European Union. Detailed data were collected from about 400 farms by interviewing farmers in four study areas in different parts of the country. The potential impacts of these changes on nitrate losses were then assessed by a deterministic nitrogen leaching model and regional assessments were made by combining the results of model calculations with digital spatial data about soils, crops and fertilisation, using GIS-software. In general, the use of nitrogen fertilisers (inorganic fertilisers and manure) has decreased to meet the requirements of the Support Scheme. However, the estimated potential impacts on nitrate losses were rather small (3‐14% in different study areas). The model results showed that to achieve the targeted national and international reductions in agricultural nitrate leaching, fertilisation and particularly manure spreading should be reduced and adjusted better to the actual nitrogen requirements of crops.

Runoff and nutrient losses during winter periods in cold climates—requirements to nutrient simulation models

Large areas in Europe may experience frozen soils during winter periods which pose special challenges to modelling. Extensive data are collected in small agricultural catchments in Nordic and Baltic countries. An analysis on measurements, carried out in four small agricultural catchments has shown that a considerable amount of the yearly nutrient loss occurs during the freezing period. A freezing period was defined as the time period indicated by the maximum and minimum points on the cumulative degree-day curve. On average 6-32% of the yearly runoff was generated during this period while N-loss varied from 5-35% and P loss varied from 3-33%. The results indicate that infiltration into frozen soils might occur during the freezing period and that the runoff generating processes, at least during a considerable part of the freezing period, are rather similar compared to the processes outside the freezing period. Freeze-thaw cycles affect the infiltration capacity and aggregate stability, thereby the erosion and nutrient losses. The Norwegian catchment had a high P loss during the freezing period compared to the other catchments, most likely caused by catchment characteristics such as slope, soil types, tillage methods and fertiliser application. It is proposed to use data, collected on small agricultural dominated catchments, in the calibration and validation of watershed management models and to take into account runoff and nutrient loss processes which are representative for cold climates, thereby obtaining reliable results.

Basin characteristics and nutrient losses: the EUROHARP catchment network perspective.

The EC-funded EUROHARP project studies the harmonisation of modelling tools to quantify nutrient losses from diffuse sources. This paper describes a set of study areas used in the project from geographical conditions, to land use and land management, geological and hydro-geological perspectives. The status of data availability throughout Europe in relation to the modelling requirements is presented. The relationships between the catchment characteristics and the nutrient export are investigated, using simple data available for all the catchments. In addition, this study also analyses the hydrological representativity of the time series utilised in the EUROHARP project.

Integrated Nitrogen Modeling in a Boreal Forestry Dominated River Basin: N Fluxes and Retention in Lakes and Peatlands

Two models, N_EXRET and INCA, were applied to the Simojoki river basin (3160 km2) in northern Finland in order to assess nitrogen retention in wetlands and lakes. N_EXRET is a spatial, export coefficient-based N export and retention model developed for large river basins. It utilizes remote sensing-based land use and forest classification, evaluated export coefficients, and data on areal N deposition and point sources of N. A new version (v1.7) of the Integrated Nitrogen in CAtchments model (INCA) is a semi-distributed, dynamic nitrogen process model, which simulates and predicts nitrogen transport and processes within catchments. Average retention of the gross total N load of 700 t a-1 to the river system was estimated using N_EXRET model as 17 t N a-1 to the wetlands and 77 t N a-1 to the lakes. A good fit was found between modeled and measured values along the river. Inorganic N fluxes simulated by the INCA model were compared with measured fluxes along the river Simojoki, with a good fit between modeled and measured NH4+-N fluxes, and an adequate fit for NO3--N fluxes. Both fluxes were overestimated at the first reach, below Lake Simojärvi. High percentage of peatlands led to high NH4+-N/NO3--N ratios derived from data, indicating negligible nitrification in large river subbasins and particularly in small research catchments.

Calibration of the INCA-N Model in the Pyhäjoki and Yläneenjoki Catchments in Finland

Models serve as a method of gaining an understanding of nutrient processes occurring in natural systems and testing scenarios on how to cost effectively reduce nutrient loading. The Integrated Catchment model for Nitrogen (INCA-N) was applied to the catchments of the River Ylaneenjoki (233 km2) and the River Pyhajoki (78 km2) to improve the estimations of nitrogen load reaching Lake Pyhajarvi in south-western Finland. The model was calibrated for flow, nitrate concentrations, and ammonium concentrations for each catchment for the years 2003-2008. The simulated hydrograph was similar to the observed hydrograph in each catchment with the major difference being the flow peaks were lower in the simulated results (R2=0.71-0.72). The dynamics of nitrate concentrations were reasonably represented in each catchment, but the extreme observed values in summer and winter were not reproduced in the simulations (R2=0.37-0.46). The simulated ammonium concentrations followed the seasonal trends of the observed data based on visual inspection, but statistically were not as good as the flow or nitrate calibrations (R2=0.20-0.25). In the Ylaneenjoki catchment, the simulated ammonium concentrations are much lower than the observed concentrations in the summer. For the Pyhajoki catchment, the simulated spring and winter peaks in ammonium were higher than the observed data in all years except 2003. The results support the need for accurate inputs to the model, especially fertilizer application rates. The calibrations also showed that continuous nutrient monitoring data is beneficial for producing accurate watershed models.