Development of catchment scale inorganic N model for Water quality part of Watershed Simulation and Forecasting System (WSFS-VEMALA)

Inese Huttunen, Markus Huttunen and Bertel Vehviläinen, SYKE

The WSFS-VEMALA model simulates hydrology and water quality for all river basins in Finland. The simulated water quality parameters are total phosphorus, total nitrogen and suspended solids. The model simulates on daily time step nutrient leaching from land areas, nutrient load balance for each lake, nutrient transport in rivers and finally loading into the Sea. The model consists of two main parts: hydrological and water quality part. Nutrient diffuse loading simulation was based on the daily relationship between nutrient concentration of runoff and the daily runoff, where the main dynamically changing variable is runoff and time independent parameters are calibrated for each sub-catchment, for each land use class and for each season of the year. We have an attempt to replace/compare this simplified approach of simulating runoff concentration with conceptual catchment scale inorganic N model (VEMALA-N). Simulation unit - subcatchment is divided into five land use/ crop classes – spring cereals, winter cereals, grassland, root crops and forest. The daily fluctuations of main soil N processes are simulated for each class. Main soil N processes included in the model are mineralization/nitrification, plant uptake, fertilizer decay and N leaching. The soil N balance is calculated for each day for each land use/crop class and subsequently the soil water N concentration is simulated. So far VEMALA-N model has been applied to 26 agricultural river catchments in South-West and Southern part of Finland to estimate inorganic N loading to the Baltic Sea.