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

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.