The execution of the Sigmaplan can have a severe negative impact on ecologically valuable areas as marshbanks and mudflats along the Schelde. Therefore within the frame of the research project OMES an ecosystem model is developed to estimate the losses of valuable areas and to evaluate effects of alternative ways to carry out the Sigmaplan. This ecosystem model focuses on the role of macrophyte dominated intertidal areas in C and N fluxes through the estuary. An important question is: Do macrophytes like common reed and bulrush contribute significantly to the removal of ammonium from the system? The influence of macrophytes on the coupled nitrification-denitrification in the rhizosphere depends on the phenological development of the plant. Therefore growth models of reed and bulrush are needed. A bulrush growth model is developed within the scope of this research as a modification of a reed growth model (SUCREED). Most important modifications of the reed growth model are: * Changing partitioning parameters because bulrush has no leaves; * Implementation of a new method to calculate LAI (Leaf Area Index). LAI is a state variable in the reed growth model, in the bulrush model calculates the LAI based on the assumption of conical shaped sterns;* Changing the assumed spherical leaf-angle distribution of reed to a distribution using three leaf-angle classes for bulrush;* The leaves of reed are assumed to reflect and transmit bath 10% of the radiation. Due to the thickness of the stems (act as leaves) of bulrush the calculation of coefficients of reflection and extinction coefficients are based on stems that reflect 10% of radiation, transmittance is assumed to be zero;*Incorporation of subroutine TIDE that accounts for temporarily floods due to tide on the Schelde.The data used for calibration and validation were gathered during 1997 at a marsh bank near Appels (Belgium). The sensitivity analysis revealed that the model is sensitive for the EFF parameter (light use efficiency of individual leaves), but this parameter is known frorn literature. The model was found to be sensitive for parameters that determine the remobilization process and no accurate data could be found. Therefore these parameters were used to calibrate the model. The model performance after calibration is good, Goodness of Fit of 0.134. Validation of the model was done using a limited amount of data gathered during 1996 at the same marsh bank near Appels (Belgium) and showed an acceptable result (Goodness of Fit 0.619). It also revealed that the method used in the model to describe phenological development of the plant is debatable. The method is based on summing daily temperatures from the calibration year (1997), because of temperature differences between 1997 and 1996 the phenological development is not accurately described.
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