Nutrient retention in riparian floodplains on landscape scale, the necessity for a monthly retention approach

This study analyses the computed nitrogen retention, the distribution and the extent of riparian floodplains of three German rivers, as input data and application of the retention model has not been carried out on landscape scale so far. The Software FLYS 2.1.3 was used for the calculation of the floodplain extent and depth at certain discharges. Thus a first empirical approach is suggested to quantify the share of load that enters the floodplain (incoming load) and the extent of floodplain as variables depending on discharge ratios. Measured loads have subsequently been applied to the presented approach to calculate incoming loads on a monthly and yearly basis for the years 1999 and 2002. Finally, linear and exponential yearly retention models were applied, obtained from the literature. Large variations in the retention results were found between the years and the models and between monthly and yearly calculations. In hydrologically average years, calculated retention rates are in the range of reported values (440-670 kg N ha⁻¹ yr⁻¹), whereas for wet years, retention values account for 1,400 kg N ha⁻¹ yr⁻¹. Consequently, this approach needs to be improved to reduce overestimation by considering more complex characteristics of the floodplain, but generally its application is possible on the landscape scale.