NUSWALite

SWQN

As a hydraulical basis for NuswaLite SWQN can be used, an application of the SurfaceWater.dll. The SurfaceWater.dll is based on the description of 1-dimensional flow in linear surface watercourses. The model uses a network of based on nodes with connections between them. The nodes contain a certain volume of water based on the actual water level and the dimensions (e.g. length, width and slope) of the canals connected to it. The connections can be defined as open watercourses with a certain resistance, or as a structure (e.g. a weir, a pump) with specific parameters. The specifications of structures can be changed in time by providing structure control time series. Water flow between the nodes is calculated as a linear function of the water level difference during the distinguished time steps and the calculated resistance of the connections.

SWQN supplies data to the SurfaceWater.dll redirects the results to several result files. Optionally SWQN can produce network layout and water balance files for the next step in the model chain: NuswaLite.

NuswaLite

NuswaLite (Jeuken and Groenendijk, 2003) calculates the retention and the ecological effects of nutrients in a river basin. The model is a simplification of the NUSWA (NUtrient modeling in Surface Waters) model (Van der Kolk et al., 1995). The model describes the dissolved organic and mineral fractions of nitrogen and phosphorus concentrations in a network of nodes. Also two fractions of living biomass are considered: a floating fraction, which can be transported with water flow, and an immovable fraction having roots in the sediment. Biomass is considered to have a fixed nutrient ratio, so no separate pools of nitrogen and phosphorus in biomass are defined. Besides inflow, outflow (not for immobile biomass) and nutrient loading, the following processes are taken into account:

• Growth of biomass with linked uptake of nutrients and limited by solar radiation and nutrient availability
• Death of biomass which adds to the organic nutrient pools
• Degradation of organic nutrients to their mineral forms
• Denitrification of mineral nitrogen
• Linear sorption of mineral nutrients to the sediment

The set of equations describing these processes is solved using a numerical finite difference solution technique. This technique enables the use of large time steps (usually limited to one day by boundary conditions).